Download Studies of Nanomagnetism Using Synchrotron-based X

Studies of nanomagnetism using synchrotron-based x-ray
photoemission electron microscopy (X-PEEM)
X M Cheng1 and D J Keavney2*
1 Department of Physics, Bryn Mawr College, Bryn Mawr, PA USA
2 Argonne National Laboratory, Argonne, IL USA
*E-mail: [email protected]
Abstract. As interest in magnetic devices has increased over the last 20 years, research into
nanomagnetism has experienced a corresponding growth. Device applications from magnetic storage to
magnetic logic have compelled interest in the influence of geometry and finite size on magnetism and
magnetic excitations, in particular where the smallest dimensions reach the important magnetic
interaction length scales. The dynamical behavior of nanoscale magnets is an especially important subset
of research, as these phenomena are both critical for device physics and profoundly influenced by finite
size. At the same time, nanoscale systems offer unique geometries to promote and study model systems,
such as magnetic vortices, leading to new fundamental insights into magnetization dynamics. A wide
array of experimental and computational techniques have been applied to these problems. Among these,
imaging techniques that provide real space information on the magnetic order are particularly useful. Xray microscopy offers several advantages over scanning probe or optical techniques, such as high spatial
resolution, element specificity, and the possibility for high time resolution. Here, we review recent
contributions using static and time resolved x-ray photoemission electron microscopy to nanomagnetism
research.
PACS numbers: 75.78.Fg, 75.78.Jp
Contents
1. Introduction .................................................................................................................................... 2
2. Overview of Nanomagnetism......................................................................................................... 3
2.1. Vortex dynamics in quasi-0D nanodots ......................................................................................... 4
2.2. Domain wall motion in quasi-1D nanowires.................................................................................. 5
2.3. Magnetic coupling in quasi-2D multilayers ................................................................................... 6
2.3.1. Exchange bias in ferromagnet / antiferromagnet hybrid structures ............................................ 6
2.3.2. Magnetic anisotropy and spin reorientation transition in ultrathin magnetic multilayers ........... 7
2.3.3. Electric field control of ferromagnetism in ferromagnet / multiferroic heterostructures ............ 8
2.4. Magnetization reversal in complex nanostructures ........................................................................ 9
3. Photoemission electron microscopy (PEEM) ................................................................................ 9
3.1. The instrumentation of PEEM........................................................................................................ 9
3.2 The chemical sensitivity of PEEM ................................................................................................ 10
3.3 The magnetic sensitivity of PEEM ............................................................................................... 10
3.2 Time-resolved X-PEEM................................................................................................................ 12
4. Recent PEEM Experiments in Nanomagnetism........................................................................... 13
4.1. Time-resolved studies of vortex dynamics in magnetic disks ...................................................... 13
4.1.1. Demonstration of gyrotropic vortex motion in Co squares ....................................................... 13
4.1.2. Observation of non-gyrotropic motion in NiFe squares ............................................................ 14
4.1.3. Investigation of the free vortex core motion in circular FeNi disks .......................................... 15
4.1.4. Nonlinear effects in vortex dynamics........................................................................................ 16
1
4.2. Studies of domain wall dynamics in nanowires ........................................................................... 17
4.3. Studies of exchange bias in ferromagnet-antiferromagnet interfaces .......................................... 18
4.3.1. The first X-PEEM observation of interfacial magnetic coupling in exchange biased layers .... 19
4.3.2. Importance of experimental geometry in interpreting the XMLD signal.................................. 19
4.3.3. Studies of uncompensated spins in exchange-biased and coupled bilayers .............................. 20
4.4. Studies of spin reorientation transitions in coupled magnetic multilayers ................................... 21
4.5. Study of magnetization switching by electric field in ferromagnet-multiferroic structures......... 22
4.6. Magnetization reversal in complex nanostructures ...................................................................... 23
4.6.1. Studies of ferromagnet - multiferroic nanocomposit films ....................................................... 23
4.6.2. Studies of multilayered nanodisks............................................................................................. 24
4.6.3. Studies of an artificial spin ice system ...................................................................................... 24
5. Future Prospects ........................................................................................................................... 25
5.1. Aberration-corrected optics for improved spatial resolution ....................................................... 25
5.2. Short pulse sources at storage rings for improved time resolution .............................................. 26
5.3. Fourth generation x-ray sources ................................................................................................... 27
6. Summary ...................................................................................................................................... 28
References: .......................................................................................................................................... 30
1. Introduction
Nanomagnetism, a discipline of studying
magnetic phenomena unique to structures with
dimensions in the submicron scale, is a
particularly exciting area of research due to its
fundamental role in physics as well as its potential
technological applications [1,2]. A great example
is the discovery of the giant magnetoresistance
(GMR) effect [3,4], an effect of spin-dependent
scattering in a nanostructured layered composite.
The discovery of the GMR effect, which was
awarded the 2007 Nobel Prize for Physics [5] lead
the way to an explosion of interest in nanoscale
magnetic systems that exploit it. What followed
was the development of an extraordinary effort to
understand the physics of spin in electronic
materials and to develop new devices around spindependent transport effects. This effort, known as
“spintronics”, envisions replacing standard charge
electronics with faster, denser, lower-power
versions based on control of spin carrier
populations. Manipulation of spin in electronics
would also make possible new functions not easily
achievable with charge electronics. These include
non-volatile memory and, in principle, logic
devices that may be reconfigured or
reprogrammed on the fly. While much of the
research into nanomagnetism and Spintronics
remains at a very fundamental level, it has resulted
in at least 3,200 publications in the last ten years
[ 6 ], and has had a major impact on the
development of new materials, deposition and
lithography
techniques,
and
magnetic
characterization techniques.
Essential
to
the
advancement
of
nanomagnetism is the development and
application of a remarkable number of direct, real
space magnetic imaging methods [ 7 ], among
which synchrotron-based soft x-ray magnetic
microscopy is extremely powerful. Very high
chemical and magnetic contrast is available in this
x-ray energy range resulting from strong core level
absorption resonances and associated x-ray
magnetic circular dichroism (XMCD) effects [8,
9 ]. Consequently, numerous imaging techniques
using the XMCD effect have been developed,
often with achievable spatial resolution from 15 to
100 nm. The use of synchrotron radiation as the
source for these microscopes also confers time
resolution limited by the x-ray source pulse length
(typically 50-100 ps at most 3rd generation
sources), when used in a pump-probe mode. In this
article we focus primarily on x-ray photoemission
electron microscopy (X-PEEM), in which the local
secondary photoelectron yield is electrostatically
imaged, yielding a full-field image with a spatial
resolution limited usually by aberrations in the
electron optics. There have been significant
contributions to our understanding of both the
static and dynamic properties of nanomagnets
through magnetic imaging techniques such as XPEEM.
In this article, the contributions to
nanomagnetism research using synchrotron-based
X-PEEM is reviewed with the emphasis on the
2
role of magnetic imaging, in particular timeresolved X-PEEM imaging, in nanomagnetism.
Following a brief review of the development of the
nanomagnetism field over the previous 10-15
years (Section 2) and an introduction to the PEEM
technique (Section 3), in section 4 several recent
PEEM experiments in nanomagnetism are
covered. Here we review experimental results
touching on major aspects of nanomagnetism,
including interfacial coupling, finite size and
shape effects, and magnetization dynamics. In
Section 5 we outline a few current developments
to the state of the art optics and sources and their
potential impact to magnetic imaging and
nanomagnetism, followed by a summary in section
6. Several comprehensive reviews dedicated to
specific topics on nanomagnetism and magnetic
imaging are available in the literature: Bader [1],
(opportunities in nanomagnetism); Prinz [10], and
Wolf et al [11] (Spintronics); Chien et al [12]
(magnetic vortices); Vaz et al. [13], (ultrathin film
structures); Stamps [14], and Nogués and Schuller
[15], (exchange bias); Johnson et al. [16], and
Sander, [17] (magnetic anisotropy); Eerenstein et
al [18], Ramesh et al [19] and Spaldin et al [20],
(multiferroics); Freeman and Choi, [7], (magnetic
microscopy); Ade and Stoll, [21, and refs therein]
(a comprehensive review of x-ray microscopy
techniques); Schönhense [ 22 ], Kinoschita [ 23 ]
Locatelli and Bauer [24], and Imada et al, [25]
(PEEM); Schönhense and Elmers [ 26 ] (timeresolved PEEM).
2. Overview of Nanomagnetism
Research into nanomagnetism encompasses a
wide array of materials and types of nanoscaled
structures. The development of advanced
nanotechnologies, from chemically-driven self
assembly of nanoscaled particles to the top-down
lithographic approaches, enables the manipulation
of materials at the nanoscale to realize new
functionality. Nanoscaled magnetic materials with
one or more dimensions at the nanoscale (10-7-10-9
meters), can be loosely categorized according to
their dimensionality. As shown in figure 1,
examples of nanoscaled magnetic materials
include 0-dimensional nanodots, 1-dimensional
nanowires, 2-dimensional ultrathin films or
multilayers, and more complex structures that
could have a combination of these characteristics.
Nanoscaled magnetic materials often exhibit new
and enhanced properties over their bulk
counterparts, so they not only offer ideal material
systems for exploring fundamental questions in
nanomagnetism, but also hold promise for
applications in information storage and magnetic
sensing.
Figure 1. Examples of nanoscaled magnetic
materials: (a) 0-dimensional nanodots, (b) 1dimensional nanowires, (c) 2-dimensional
ultrathin films or multilayers, and more complex
structures, such as (d) multilayered nano-rings,
(e) nanocomposite films with embedded
nanopillars (courtesy of R Ramesh) [19], and (f)
artificial spin ice system (courtesy of D Lacour
and S El Moussaoui)
The magnetic configuration of a nanoscaled
magnetic material is the result of energy
minimization in such a material system. In
general, the free energy of a nanomagnet is given
by
Etotal = EH + Eexch + Edemag + Eanis, (1)
where EH is the Zeeman energy, Eexch is the
exchange energy, Edemag is the demagnetizing
energy, and Eanis is the anisotropy energy. The
Zeeman energy, EH, is proportional to the external
magnetic field [27]. The exchange energy is most
commonly described by the Heisenberg model
with
(2)
E
  J S  S ,
exch
ij
i
j
i, j
where Jij is the exchange integral determined by
the overlap of the electronic wavefunctions of the
two atoms with angular momenta Si and Sj [28]. In
ferromagnets (FM), Jij is predominantly positive,
and thus favors a parallel alignment of magnetic
moments, resulting in ferromagnetic ordering. In
antiferromagnets (AFM), Jij is negative and favors
an antiparallel alignment of magnetic moments.
3
The demagnetizing energy, Edemag, is associated
with the long-range dipolar interaction, favoring
the formation of magnetic flux closure domains.
The anisotropy energy, Eanis, is due to magnetic
anisotropies
including
magnetocrystalline
anisotropy and surface anisotropy [29], favoring
magnetization along certain crystallographic or
surface directions.
When the size of the structure is comparable
to the important length scales of these energies,
the finite size will begin to have an influence on
the spin configuration. The interplay between size
confinement and proximity effect can result in
unique novel magnetic configurations, such as
magnetic domains and magnetic vortices, and
interesting magnetic phenomena, such as the GMR
effect, exchange bias and spin reorientation.
Although some of the novel nanomagnetic effects
have already been widely used in commercially
available magnetic data storage devices, such as
the exchange bias effect, many fundamental
questions related to these phenomena have yet to
be answered. The development and application of
a remarkable number of direct, real space
magnetic imaging methods, especially the
synchrotron based x-ray photoemission electron
microscopy (X-PEEM), make it possible to
experimentally investigate these problems. Below
we list some of such questions that have been
extensively investigated by the X-PEEM
technique.
2.1. Vortex dynamics in quasi-0D nanodots
When the sizes of all the three dimensions are
shrinked to submicron scale, a qusi-0D nanodot is
obtained. This type of structure is also often
called a nanodisk when it is in a shape of flat disk.
The magnetic configuration of a soft magnetic
nanodisk is determined by the competition
between the exchange and demagnetizing
energies, because the magnetic anisotropy can be
neglected in soft magnetic materials, such as FeNi
alloys.
The ground state of a finite-sized magnet will
typically try to minimize the field external to the
magnet by dividing into domains, in order to
reduce the demagnetizing energy. This can be
accomplished if the local spins are oriented in
random directions, such that the net moment of the
structure is zero. Deviations from parallel
alignment of spins costs exchange energy,
however, thus the spins will only be randomized at
length scales significantly above that of the
exchange length. The result is that the magnetic
will form into domains of local ferromagnetic
order, with long-range randomization of the
magnetization directions. These domains will have
a characteristic size that is governed by both the
exchange interaction and the magnetostatic
coupling. When the size of the structure
approaches this characteristic length, the finite size
will begin to induce order. The consequence of
this at the nanoscale is that the magnetization at
the edges will try to align along the tangential
directions. This will tend to drive the system to
flux closure states. For a square structure, the
simplest flux closure state is a four-domain state,
with the magnetization directions at right angles to
each other. Increasing the symmetry to a circular
structure results in a continuous circulation of the
magnetic flux around the disk. In this
configuration, the spin directions are governed
over most of the disk by dipolar interactions,
except at the center, where the shorter-range
exchange interaction dominates. In this region,
approximately the size of the exchange length, the
antiparallel alignment of spins would cost too
much energy, so the system compromises by
canting out-of-plane. This vertical core and
circulating disk structure is a magnetic vortex
state, as illustrated in figure 2. The magnetic
vortex state is one of the most interesting and
important nanomagnetic states.
Figure 2. Magnetic vortex state (courtesy of
K Buchanan)
The vortex state is of interest for both
fundamental and practical reasons. The unique
topology of the vortex state allows the LandauLifshitz-Gilbert equation to be solved analytically,
yielding great insights into magnetization
dynamics at the fundamental level. They also have
considerable importance for many device
applications. Vortex states arise frequently in
4
magnetic domain walls, and can have a strong
influence
on
their
motion.
Therefore,
understanding vortex dynamics is very important
for magnetization reversal, wall motion in
nanowires, and switching of nanomagnetic
devices. Also, because the magnetic vortex is a
uniquely stable flux-closure state that displays two
energy-equivalent polarities, there is interest in
using them in non-volatile logic applications such
as magnetic memories. Finally, the vortex state
exhibits a well-defined eigenmode structure that
offers the possibility of easily tuning through size,
shape, or possibly external fields. This makes
them very attractive for microwave applications.
The dynamics of the vortex state was
considered theoretically first by Thiele [30] for the
general case, and more recently in confined
geometries by others [ 31 , 32 , 33 , 34 , 35 ]. These
studies examined the excitation mode structure,
and described the radial, azimuthal, and the vortex
core translational modes. For a circular geometry,
the Landau-Lifshitz-Gilbert equation can be
written in the simplified form of a twodimensional single particle equation of motion in
the core position, X (Thiele’s equation),
  G  X

MX
 W X 
 0,
X
structures for memory and sensing elements. In
these more complicated geometries, finite size
effects have a significant impact on magnetization
reversal and the motion of magnetic domain walls.
One specialized situation of interest is the
magnetic nanowire, a quasi-1D geometry in which
domain walls are inhibited in one direction by the
small lateral size of the structure, but may form in
the orthogonal direction. In principle, this allows
more controlled manipulation of the domain wall,
and magnetic logic devices [37, 38] and memory
devices [39] have even been proposed based on
based on the field-driven or current-driven domain
wall motions, such as the domain-wall race track
memory, as illustrated in figure 3 [39].
(3)
This equation includes a cross term analogous to
the Magnus Force in the motion of fluid vortices,
such that the fundamental, or gyrotropic solution
to this equation describes a circular trajectory of
the core about the center of the disk, where the
sense of the rotation is determined solely by the
core polarity. The practical consequences in the
presence of damping for a vortex core perturbed
from the center are that it should follow a spiral
path towards the center of the disk.
Experimentally, the vortex core motion has been
studied in both the frequency domain[32,
33Error! Bookmark not defined.], and with realtime magnetic imaging studies[36].
2.2. Domain wall motion in quasi-1D nanowires
While vortices offer the potential for using
the core polarity as a non-volatile logic building
block, there are considerable practical questions
associated with addressing and sensing the state of
such devices. Therefore, there is still considerable
interest in using lower-symmetry magnetic
Figure 3. Magnetic nanowire-based domainwall race track memory (courtesy of S S P
Parkin) [39]
Magnetic field-driven domain wall motion
has been extensively studied [40, 41, 42, 43]. In a
nanowire geometry, Beach et al. [40] showed that
for field-driven wall motion, the domain wall
velocity is linearly dependent on the driving field,
up to a limit of ~3-4 Oe. Above this limit, there is
evidence for a more disordered motion.
Micromagnetic simulations suggest that in this
non-linear regime, the wall structure is modified
such that a vortex forms [44, 45]. In this case, the
motion of the vortex core is influenced by the
same cross term as in the simpler disk geometry.
This leads to a non-monotonic domain wall
position, and reversals of the core polarity when it
5
encounters the edges of the wire [46, 47]. The
result is a complex oscillatory motion of the wall
position.
Current-induced domain wall has also been of
great interests not only because of the promising
applications in novel spintronics devices, but also
because of the fundamental physics concerning the
mutual interaction of magnetization and electric
current [37, 48, 49, 50]. Although the prediction of
the effect of electric current on a domain wall
dated back to the 1970s [ 51 , 52 ], the first
experimental studies were reported much later [53]
because the high-current densities required for
driving domain wall motion was recently realized
in lithography patterned magnetic nanowires. It is
now generally accepted that the current-driven
domain wall dynamics can be described by the
Landau-Lifshitz-Gilbert equation with two
additional adiabatic and nonadiabatic spin-transfer
torque terms [54].
2.3. Magnetic coupling in quasi-2D multilayers
The magnetic multilayer is an interesting and
rich system of both scientific and technological
importance. In magnetic multilayers, different
kinds of ultrathin layers are stacked sequentially in
a periodic fashion. The advent of new film growth
techniques makes it possible to fabricate intricate
magnetic multilayer structures from various
materials with thicknesses down to one atomic
layer. The possibility of adjusting the constituent
materials, layer thickness, and periodicity in the
multilayers gives one the opportunity to fabricate
multilayers with unique magnetic and transport
properties. For example, magnetic multilayers
consisting of alternating ferromagnetic and
nonmagnetic metallic layers, such as Fe/Cr and
Co/Cu, exhibit the GMR effect [3, 4, 55, 56]. In
addition, the magnitude of the GMR oscillates as a
function of the thickness of the nonmagnetic
spacer layers, resulting from an oscillation in the
sign of the interlayer exchange coupling between
the ferromagnetic layers [ 57 , 58 ]. Below we
briefly review some interesting phenomena in
multilayers investigated by X-PEEM technique,
including exchange bias in ferromagnet /
antiferromagnet
hybrid
structures,
spin
reorientation transition in ferromagnet /
ferromagnet multilayers, and electric-field control
of ferromagnetism in ferromagnet / multiferroic
heterostructures.
2.3.1. Exchange bias in ferromagnet /
antiferromagnet hybrid structures
Exchange bias, discovered by Meiklejohn and
Bean in 1956 [59], is a very useful phenomena
occurring in bilayers or multilayers of ferromagnet
(FM) / antiferromagnet (AF) hybrid structures.
Exchange bias refers to a shift in the soft
magnetization hysteresis loop of the FM layer
caused by the hard magnetization behavior of the
adjacent AF layer, as illustrated in figure 4.
Figure 4. The exchange bias phenomenon: easyaxis magnetization hysteresis loop of a soft
ferromagnetic film (a) and an exchange-biased
bilayer consisting of a ferromagnet and an
antiferromagnet (b).
Determined by the competition of
demagnetizing
and
anisotropy
energies,
ferromagnetic films typically have an “easy”
magnetization axis, along which the spins prefer to
align. The two directions along this easy axis are
equivalent, and the same energy cost and the same
magnitude of the external magnetic field
(coercivity Hc) are required to switch the spins
from one direction to the other. Therefore, the
hysteresis loop of an FM is symmetric with respect
to the external field, as shown in figure 4 (a).
When an FM/AF system is grown in a magnetic
field or cooled down from above the AFM Néel
temperature in a magnetic field, the hysteresis loop
of the FM becomes asymmetric with respect to the
magnetic field and the center of the loop is shifted
from zero by a bias field, Hb, exhibiting exchange
bias, as shown in figure 4 (b). In the FM/AF
system, the originally equivalent two directions
along the easy axis in the FM layer is no longer
equivalent, and instead there is one preferred easy
magnetization direction for the FM, which is
opposite to the direction of Hb. The physics
6
underlying this exchange bias phenomenon is the
interfacial exchange interaction between the AF
and FM. The FM layer is pinned by the AFM
layer, and the magnetization reversal of the FM
requires an added energetic cost, corresponding to
the bias field Hb.
The exchange bias phenomenon has been
widely used in magnetic recording. The first
commercial device to employ the exchange bias
was IBM's anisotropic magnetoresistance (AMR)
disk drive recording head. In the mid-1990s, an
exchange bias layer started being used in the GMR
based spin valve head. However, the origin of
exchange bias phenomenon is still not well
understood because of the difficulty in spatially
resolving the magnetic domain structures in the
AF layer and at the FM/AF interface as well as the
correlation between the magnetic structure in the
FM layer and that in the AF layer. A directly
magnetic imaging technique with the sensitivity to
magnetic configurations in AF films and the
ability to resolve magnetic configurations at
different layers is needed to better understand the
origin of the exchange bias problem [60].
and interfaces causing a change of the spin-orbit
coupling are expected to change the magnetic
anisotropy. The magnetoelastic anisotropy
depends on the strain in the film and the Néel
surface anisotropy is related to the break of
symmetry at the surface or interface. Both of the
magnetoelastic anisotropy and the Néel surface
anisotropy depend on the magnetic layer
thickness. Therefore, magnetic anisotropy in
epitaxial thin films or magnetic multilayers
depends intricately on the film thickness. For
example, figure 5 illustrates the thickness
dependence of magnetic anisotropy in Co/Pd
multilayers [16, 68].
2.3.2. Magnetic anisotropy and spin reorientation
transition in ultrathin magnetic multilayers
In ultrathin magnetic films or multilayers
containing ultrathin magnetic layers, the complex
interplay among the short-rang exchange
interaction, the long-range dipolar interaction, and
the local magnetocrystalline anisotropy can cause
magnetic anisotropy and domain formation to
differ drastically from that in bulk magnetic
systems [61, 62, 63].
Magnetic anisotropy determines the easy
directions of magnetization in a sample and affects
the process of magnetization reversal. Magnetic
anisotropy of a magnetic sample is determined by
the competition of the demagnetizing energy and
the magnetic anisotropy energy, where the
magnetic anisotropy energy depends on the
magnetocrystalline
anisotropy
energy,
magnetoelastic anisotropy energy, and the Néel
surface anisotropy energy [16, 29]. The
magnetocrystalline anisotropy is a relativistic
manifestation of the coupling between the electron
spin and the orbital moment, as first calculated by
Bruno [64] and later experimentally verified by
XMCD studies [ 65 , 66 , 67 ]. Therefore,
modifications in the electronic structure at surfaces
Figure 5. The thickness dependence of magnetic
anisotropy in Co/Pd multilayers. Keff is the
effective magnetic anisotropy energy density,
and tCo is the individual Co layer thickness. The
vertical axis intercept equals twice the interface
anisotropy, KS, whereas the slope gives the
volume contribution, KV. (courtesy of M T
Johnson et al)[16]
The phenomenon, observed in many ultrathin
films or magnetic multilayers, that the
magnetization easy axis switches between out-ofplane and in-plane with magnetic film thickness is
called spin reorientation transition (SRT) [69, 70].
Both magnetic materials with magnetization easy
axis in the film plane and those with magnetization
perpendicular to the film plane have important
technological
applications.
Therefore,
investigation and control of the magnetic
anisotropy in magnetic multilayers are of both
fundamental and application importance.
There are two main challenges for the
experimental study of the SRT phenomenon. The
first is that the possibility for sample-to-sample
variations may overwhelm the effect to be studied.
7
With the advancement of thin film deposition
technology enables not only the film deposition
precisely controlled at atomic layer thickness, but
also the deposition of a wedged film, where the
thickness of one layer is varied continuously in
one single sample [69]. This wedge deposition
technique greatly increases the uniformity of
fabrication conditions, providing the solution to
the sample-to-sample variations problem. This
technique can also increase the effective
throughput in both the sample fabrication and
characterization steps.
The second challenge is the lack of
experimental information on magnetic anisotropy
and magnetic configuration of each individual
magnetic layer in a multilayer system. Nowadays,
the most widely used magnetic imaging method
that can provide sufficient spatial resolution for
the SRT study is magnetic force microscopy
(MFM). In MFM, an image of magnetic contrast
of the sample is obtained through the
magnetostatic interaction between a ferromagnetic
tip and the magnetic stray fields from the sample.
However, the MFM imaging can neither provide
the information on the origin of the magnetic
anisotropy nor distinguish magnetic configuration
of each individual layer in a multilayered sample
[71]. A directly magnetic imaging technique with
the sensitivity to magnetic anisotropy as well as
the ability to detect magnetic configuration in each
individual layer is needed.
2.3.3. Electric field control of ferromagnetism in
ferromagnet / multiferroic heterostructures
Recently, an epitaxial bilayer of ferromagnet /
multiferroic
heterostructure
has
been
experimentally studied by Chu et al,
demonstrating the control of local ferromagnetism
by electric field [72]. This work has added to the
richness of the field of multilayers involving 2D
magnetic layers and opened a new rout for
magnetism control in multilayered materials.
A ferroic is a material that possesses a
spontaneous, stable, and switchable internal
alignment [18]. For example, in ferromagnetic
materials a spontaneous magnetization is stable
and can be switched hysteretically by an applied
magnetic field; in ferroelectric materials a
spontaneous polarization is stable and can be
switched hysteretically by an applied electric field;
in
ferroelastic
materials
a
spontaneous
deformation is stable and can be switched
hysteretically by an applied stress. In general, a
multiferroic is a material that exhibits more than
one ferroic orderings simultaneously. However, in
current convention, the term “multiferroic”
primarily refers to a material that combines
ferroelectricity with ferromagnetism, or more
loosely, with any kind of magnetism [20].
Multiferroics have been of great interest because
of not only the fundamental science but also their
applications in magnetic data storage, spintronics
and high-frequency magnetic devices [18-20, 73].
Although the pioneer works on multiferroics
could be dated back to the 1960s and 1970s [74,
75, 76], the study of materials exhibiting coupled
magnetic and electrical ordering has been
revitalized in the last ten year or so because of
both the advances in crystal synthesis, thin film
deposition and various characterization techniques,
and the improvement of first-principles
computational techniques.
Figure 6. Magnetic and ferroelectric coupling in
ferromagnet / multiferroic heterostructures
Compared to bulk or single phase film
multiferroics, the ferromagnet / multiferroic
heterostructured bilayer is even more complex and
interesting, as illustrated in figure 6. In such a
system, a ferromagnetic layer is epitaxially grown
on a multiferroic layer, where the multiferroic
layer combines the ferroelectricity (FE) with
ferromagnetism (FM) or antiferromagnetism (AF).
The ferromagnetic layer can be magnetically
coupled with the multiferroic layer through the
magnetic exchange coupling (FM) or exchange
bias (AF). More importantly, the magnetization in
the ferromagnetic layer can be controlled by an
8
applied electric field through the coupling with
ferroelectricity in the multiferroic layer.
In contrast to the extensive experimental
studies reported on bulk or single phase thin film
of multiferroics, relatively fewer results have been
reported on the ferromagnet / multiferroic
heterostructured materials because of the
difficulties in not only sample fabrication but also
heterostructure characterization. The X-PEEM
technique provides a powerful tool for
characterization of the interface coupling in such a
heterostructure.
2.4. Magnetization reversal in complex
nanostructures
In addition to 0-dimensional nanodots, 1dimensional nanowires, 2-dimensional ultrathin
films or multilayers, nanostructures can be more
complex structures combining the above
characteristics.
One example is a multilayer stack of
alternating hard and soft ferromagnetic (FM) rings
separated by a nonmagnetic Cu ring [77, 78] (as
shown in figure 7), which is a very promising
design for realizing a MRAM device [79, 80]. This
pseudo-spin-valve design proposed by Zhu et al
[77] intend to take advantage of the vertical GMR
effect and the controlled chirality switching of the
magnetic vortex states in each FM ring. The key
point of this design is that the flux-closed vortex
state reduces interactions between neighboring
elements, enabling devices with high-density
elements and robust and repeatable switching
characteristics.
The second example is a nanocomposite thin
film in which nanopillars of the FM phase
embedded in a matrix of the multiferroic phase, as
illustrated in figure 1 (e) [19]. Recently, epitaxial
growth of ferromagnetic CoFe2O4 (CFO)
nanopillars embedded in a single crystalline
ferroelectric BaTiO3 or BiFeO3 (BFO) matrix has
been demonstrated and an electric-field-induced
magnetization switching has been observed in the
BFO-CFO nanostructures [81, 82, 83].
The third example is an artificial spin ice
system consisting of magnetic nanowires in a 2D
lattice, as shown in figure 1 (f) [84, 85, 86, 87,
88 ]. In such an artificial spin ice system, each
individual magnetic nanowire is a single-domain
magnetic dipole. The interactions among the
magnetic wires result in interesting phenomena,
such as magnetic frustration [84, 89] and magnetic
monopole effect [90, 91].
It is of great interest and significance to
investigate the magnetic property of each
individual
component
of
the
complex
nanostructures, such as the soft and hard magnetic
ring in the multilayered nanorings, and the
ferromagnetic and multiferroic phases in the
hybrid ferromagnetic-multiferroic nanocomposite
films. The unique element-specific X-PEEM
technique is an extremely powerful and
appropriate tool for characterization of these
complex nanostructures.
3. Photoemission electron microscopy (PEEM)
The basic principle of photoemission electron
microscopy (PEEM) is to create a full field image
of the electrons emitted from a sample surface in
response to photon absorption. The focusing of the
emitted electrons is usually done with a
combination of electrostatic and/or magnetic
lenses. Depending on the choice of excitation, this
provides a surface-sensitive spatial map of some
aspect of the samples electronic structure.
Photoemission electron microscopy was first used
with UV sources, and later came to exploit the
greater element selectivity available by excitation
with soft x-rays.
3.1. The instrumentation of PEEM
The structure of a basic electrostatic PEEM
instrument is shown figure 7. The first extraction
lens, along with the flat sample, forms an
immersion-type objective. At the focal point of
this objective, an aperture is usually placed to limit
the angular acceptance of the system, and with that
the spherical aberrations. This aperture is of
variable size, thus an optimization can be achieved
between total microscope transmission and spatial
resolution. Following the aperture is a projection
system usually consisting of two or three
projection lenses. These form an image at the back
focal plane, where an area detector is placed. The
detector is most often a multichannel plate and
scintillator screen, although solid-state detectors
have been used in some applications.[92] Since the
detection scheme relies on photoelectrons, most of
which are secondary electrons, the probing depth
will be limited to approximately the electron mean
free path in the sample. For soft x-ray excitation,
9
the electron energies are typically less than 10eV
therefore the escape depth is 2-5nm, according to
the universal curve.[93]
Figure 7. Schematic diagram of an electrostatic
PEEM instrument with x-ray excitation.
Photoelectrons are stimulated by the x-ray
beam, and are accelerated in to the column by
the first extraction lens, which forms an
immersion-type optic with the flat sample.
Following a contrast aperture, a projection
system forms an image at an area detector.
The performance, in terms of spatial
resolution, of a PEEM is governed by three
primary sources of focusing errors: spherical
aberrations, chromatic aberrations, and limitations
due to electron mean free path. Each of these may
limit the achievable spatial resolution under
certain operating conditions. For most electrostatic
PEEMs, spherical aberrations limit the spatial
resolution to ~20nm. However, with soft x-ray
excitation, the kinetic energy distribution of the
resulting photoelectrons is broadened, resulting in
greater chromatic aberrations. In this case, the
spatial resolution may be degraded to 50 to 100
nm. These errors may be minimized or corrected
using additional optical elements. For chromatic
aberrations, energy filtering of the photoelectrons
using a retarding grid or electron spectrometer can
improve the spatial resolution, at the cost of a
reduced image intensity. Spherical aberrations can
be corrected using electrostatic elements to
compensate them. In Section 5, we discuss
aberration-corrected microscopes in more detail.
3.2 The chemical sensitivity of PEEM
The spatial map of the photon absorption
coefficient provides information about the details
of the local electronic structure at the surface of
the sample. The nature of the information obtained
depends on the excitation energy used. With UV
radiation, the contrast obtained is mainly derived
from differences in work function across the
surface. This can give chemical sensitivity,
especially between metallic and oxidized surfaces.
With excitation in the soft x-ray range (200 – 2000
eV), the absorption coefficient of most solid
materials is dominated by strong core level
resonances. With higher photon energies and
tunable synchrotron sources, elemental contrast
can be obtained by choosing an appropriate core
level resonance. This can also provide sensitivity
to valence state for many materials, since the local
density of states in the final state will be sensitive
to chemical bonding.
3.3 The magnetic sensitivity of PEEM
By manipulating the polarization of the
incoming radiation, we can achieve sensitivity to
spin and orbital occupancies. With excitation in
the soft x-ray range, the absorption coefficient of
most solid materials is dominated by strong core
level resonances. Many of these resonances
involve transitions to the unfilled bands where the
magnetism of the material originates. These
resonances may be quite strong, especially for 3d
and 4f final states.
With circularly polarized radiation, there is an
additional dependence of the absorption
coefficient on the radiation polarization, resulting
in a strong x-ray magnetic circular dichroism
(XMCD) effect. This effect occurs for
ferromagnetic materials and arises from the spindependent density of states in the final state band.
The first experimental confirmation of the XMCD
effect was at the Fe K edge (1s-4p) [94], however
transitions that probe directly the states where the
ferromagnetic spins reside typically show much
greater effects [95]. For transition metals the L2,3
(2p-3d) transitions offer the largest effects, while
in rare earths, the M4,5 edges probe the f states
directly. In figure 8, we show schematically the
energy levels for an L (2p – 3d) resonance of a 3d
metal, along with example Mn L edge absorption
(I+ + I-) and dichroism (I+ - I-) spectra from a Mn
compound film. As the incident photon energy is
scanned through the core level binding energies,
10
electrons are excited to the conduction band,
creating a core hole. This core hole de-excites
quickly, resulting in either a fluorescent photon or
electron emission through Auger processes. The
absorption coefficient may be measure by
monitoring the yield of fluorescence photons,
electron or by measuring the transmitted beam
directly. For transition metal L edge absorption,
the majority of the de-excitation events are in the
form of electron emission, thus making this a
convenient way to collect absorption data.
Figure 8. The XMCD effect at a transition
metal L edge. Upper: Energy level diagram for
the x-ray absorption transition in a
ferromagnetic metal. Lower: Example Mn L
edge data for a Mn compound, in which the Mn
d electron are localized. The blue curve shows
the sum of the absorption (I++I-) for left and
right circularly polarized x-rays, while the gold
curve shows the difference (I+-I-).
Two primary resonances, L3 and L2, are
observed from the spin-orbit split 2p3/2 and 2p1/2
core levels, respectively. At each of these edges, a
significant difference in absorption coefficient can
be seen between left circularly polarized (LCP)
and right circularly polarized (RCP) radiation,
showing the XMCD effect. For 3d transition
metals and 4f rare earth elements, this XMCD
effect can be as high as ~30%, which translates
into very high sensitivity. At state of the art soft xray beamlines, detection of magnetic moments as
small as 0.01 B/atom is possible [96], and even
very small induced moments in magneticallydoped materials or at interfaces can often be
detected [ 97 ]. X-ray absorption and magnetic
circular dichroism is also very sensitive to the
band structure in the final states, making it a very
effective probe of the local electronic structure.
Finally, quantitative information can be extracted
about the spin and orbital moments through the
application of sum rules derived by Thole and
coworkers [98,99,100]
The sensitivity to spin has been used
extensively for imaging of magnetic materials and
structures. Using circularly polarized radiation, the
XMCD effect that occurs at certain resonances can
be exploited to create an image of the projection of
the local magnetization along the beam
propagation direction. In the ferromagnetic
transition metals, magnetic moments originate
from the exchange-split d-bands, thus the
transitions of interest are the L2,3 (2p-3d)
absorption edges, which mostly occur between
500 and 1000 eV. Since in this energy range the
absorption cross section is still quite high, and the
XMCD effect is very large at the L edges, very
high sensitivity magnetic images can be obtained.
Similarly, for the rare earth elements, the
magnetically important M4,5 (3d-4f) edges fall in
the 900 to 1500 eV energy range, making XPEEM a very attractive method for magnetic
imaging. Dichroic effects exist at other atomic
transitions where magnetic moments may be
present, such as the oxygen K, transition metal K,
and L edges of semiconductors. In principle, these
could be used for magnetic microscopy as well.
For PEEM imaging, however, the flux intensive
nature of the PEEM technique makes it difficult to
obtain sufficient signal to noise for these
transitions. Typically, a magnetic X-PEEM image
is formed by collecting left- and right-circularly
polarized images at the appropriate resonance and
taking the difference divided by the sum, as is the
practice for XMCD spectroscopy. It is also
possible to obtain magnetic images using the
contrast between L3 and L2 instead. In the case of
transition metals, this is accomplished by dividing
an L3 image by an L2 image. This has the
advantage of not requiring a polarization change,
11
which can be time-consuming or difficult with
some types of sources.
In addition to dichroism between circular
polarizations, it is possible to observe spindependent effects with linearly polarized light. Xray magnetic linear dichroism (XMLD) arises
when the spin-orbit coupling creates a
directionally dependent sub-orbital population.
Then, orbitals oriented parallel and perpendicular
to the electric field axis of the incident photon will
exhibit differing absorption coefficients. This
effect is more prominent in materials that exhibit
strong multiplet splitting, and is usually
manifested as a difference in peak height between
two multiplets. Because the XMLD effect is only
sensitive to the angle between the atomic moments
and the average electric field vector, it exhibits a
cos2 dependence. Therefore, while the spin
direction cannot be determined, XMLD is
sensitive to the spin axis and the magnitude of the
atomic moments. Consequently, XMLD is useful
for studying antiferromagnetic order. Because
other, non-magnetic, effects can contribute to
linear dichroism, care must be taken to account for
them.
3.2 Time-resolved X-PEEM
Using X-PEEM at synchrotron sources
conveys advantages beyond high flux, chemical
and magnetic sensitivity. The pulsed nature of
synchrotron radiation provides a natural ability to
obtain temporal data with a time resolution related
to the x-ray pulse width. This pulse width varies
between facilities and operating modes, but it
typically between 50 and 200 ps, thus offering
high sensitivity magnetic imaging with very high
time resolution. Therefore, time-resolved X-PEEM
is uniquely suited to tackle many of the most
compelling problems of magnetization dynamics
in confined structures. The number of photons
within a single x-ray pulse is insufficient to form
an image with adequate statistics, even at 3rd
generation sources. Therefore, time-resolved XPEEM is accomplished in a pump-probe mode. In
such experiments, shown in figure 9, an excitation
pulse is synchronized with the incoming x-ray
pulses and applied to the sample with a variable
delay. Then, by varying the delay and integrating
over many repeats of this cycle, the average
response of the sample to the excitation pulses can
be mapped out in time. When imaging at the
highest resolutions, the exposure times are
typically on the order of minutes, even at the
brightest synchrotron sources. Thus, the resulting
images are the average of several tens of millions
of pulses.
Figure 9. Schematic diagram of a pump-probe
time-resolved magnetic imaging experiment.
Time varying magnetic fields are generated by
current pulses in a lithographic waveguide or
stripline, onto which magnetic structures may
be deposited directly. A phase relation is
maintained by synchronizing the excitation
with the x-ray pulses via a timing signal from
the storage ring. By varying the phase between
the x-ray pulses and the field, the time
evolution of the magnetic configuration of the
sample can be mapped. The excitation current
pulses may be generated electrically, or with
pulsed lasers.
For the magnetization dynamics experiments
we discuss here, the excitation pulse is usually a
fast magnetic field pulse. The first demonstration
of pump-probe PEEM magnetic imaging was at
the BESSY-II synchrotron in Berlin [101]. In this
experiment a microcoil was used to generate field
pulses on a NiFe/Cu/Co trilayer, and partial
reversal of some of the NiFe domains was
demonstrated at a time scale of ~1 ns. While this
experiment demonstrated the possibility of pumpprobe PEEM, the use of a microcoil has two
significant drawbacks to it. First, the inductance
of the coil limits the response time, thus very fast
12
field transitions cannot be coupled to the magnetic
structures of interest. In the experiment by Vogel
et al. [100], this limited the field rise and fall times
to ~10ns. Second, the magnetic field may
influence the electron optics, create shifts or
distortions in the resulting image, and limiting the
spatial resolution.
Typically, it is desired to study the influence
of fast field transitions that at least approach the
time resolution available (~100ps), and at the
spatial resolution available from PEEM (~100nm).
To generate such fast field pulses in a way that
does not influence the optics, one approach is to
place the magnetic structures of interest onto
conducting striplines, which are then electrically
pulsed. This results in an induced field above the
stripline that influences the magnetic structures,
but extends only a few microns above them, thus
minimizing the effect on the emitted electrons. For
ultrafast field transitions requiring high bandwidth,
the stripline can be made as part of a coplanar
transmission line, which can easily be impedancematched to the source of the excitation pulses.
Two approaches have been widely used to
generate the electrical pulses: the photoconductive
switch approach, and direct electrical excitation. In
the former, a pulsed laser is used with a DC-biased
photoconductive switch integrated into the
stripline, thus generating an electrical pulse. This
scheme works well where ultrafast (sub-100ps)
unipolar pulses are needed, but is not well suited if
an arbitrary waveform is desired. Direct
connection to an electronic pulser can in principle
be used with any excitation waveform and is easier
and less expensive to implement. However,
ultrafast pulses may be harder to achieve at the
needed repetition rates, and it is very difficult to
use in a microscope design where the sample is
not electrically grounded.
The synchrotron-based x-ray photoemission
electron microscopy (X-PEEM) combines the
strength of polarized x-ray absorption enabling
both chemical and magnetic sensitivities, electron
microscopy enabling high spatial resolution, and
the pulsed nature of synchrotron x-ray sources
enabling temporal resolution. The chemical
sensitivity of PEEM makes this imaging technique
uniquely suited to address questions where more
than one magnetic element is present. This is
particularly useful for investigation of layered
nanomagnetic systems of interest for many
magnetic device applications, as well as a variety
of new, upcoming materials, which have complex
chemical and spin structures with the possibility of
multiple magnetic elements. The magnetic
sensibility based on the XMLD effect enables the
direct imaging of spin configurations in
antiferromagnets [ 102 ], which can hardly be
achieved by any other magnetic imaging method.
Time-resolved X-PEEM with high spatial (~100
nm) and temporal (< 1 ns) resolutions is an
excellent tool for studies of spin dynamics in
nanomagnetic structures. In the following, we
review
several
recent
experiments
that
demonstrate the significant role of X-PEEM
imaging in providing crucial information for
fundamental understandings of several problems
in nanomagnetism.
4. Recent PEEM Experiments in
Nanomagnetism
4.1. Time-resolved studies of vortex dynamics in
magnetic disks
Time-resolved X-PEEM is one of the best
techniques for directly imaging magnetic vortex
dynamics, because time-resolved X-PEEM has the
advantages of both the high spatial resolution and
the high temporal resolution.
4.1.1. Demonstration of gyrotropic vortex motion
in Co squares
In one of the first time-resolved PEEM
experiments, Choe et al.[103] used the PEEM-2
microscope at the Advanced Light Source (ALS)
to demonstrate the coplanar-waveguide based
pump-probe imaging technique, and explored the
dynamics of the magnetic vortex state in micronsized Co structures. Because the PEEM-2 is
configured such that the sample must be at the
accelerating voltage for the experiment, the
authors used a photoconductive switch and a
pulsed laser to generate field pulses. This allows
relatively fast (sub-100ps rise times) unipolar
pulses.
Co structures were grown on top of the
coplanar
waveguides
using
lithographic
techniques, and their sizes and thickness chosen
such that the fundamental mode was accessible
within the timing constraints of the ALS timing
mode used. The structures were demagnetized
13
such that they began with a Landau, or fourdomain flux closure state. When subjected to the
laser-driven field pulses (300-400ps FWHM), the
core of this state is perturbed from the center, and
then relaxes towards the center of the square. By
varying the delay between the laser pulses and
incoming x-ray pulses, the authors mapped the
resulting time-dependent domain structure, as
shown in figure 10 (a). By analyzing the domain
patterns, the location of the vortex core could be
derived, and the core trajectories could be plotted,
as shown in figure 10 (b).
(a)
was left untested by the ALS experiment, because
of the resonant nature of the experiment.
Additionally, the use of square structures
complicates the application of the analytical LLG
theory, because of the introduction of domain
walls.
4.1.2. Observation of non-gyrotropic motion in
NiFe squares
(b)
Figure 10. Gyrotropic vortex motion in Co
squares of different dimensions. (a) TRPEEM images of domains in Co squares of
Patterns I (1 by 1 m2), II (1.5 by 1 m2)
and III (2 by 1m2), taken at the specified
delay times after the field pulse. (b)
Trajectories of the vortex core. (courtesy of
S-B Choe et al.)[103]
Choe et al. found that for the magnetic
vortices driven resonantly at their excitation
frequency, the cores followed a circular (elliptical)
path for square (rectangular) shapes, as predicted
by the Landau-Lifshitz-Gilbert (LLG) theory. This
result was significant because it was the first direct
observation of this fundamental gyrotropic motion
in the time domain. The authors also confirmed
that the sense of the circular core motion
(clockwise or anticlockwise about the square
center) is dependent only on the polarity of the zcomponent of the magnetization of the vortex
core, and not on the chirality of the vortex itself
(see figure 10 (c)-(e)), as predicted by the LLG
theory.
One additional prediction of the LLG theory
is the fundamental eigenmode frequency, and its
dependence on the geometry of the structure. This
Figure 11. Non-gyrotropic vortex motion in 6
m NiFe squares. (a) time-evolution of the y
component of the magnetization, My, averaged
over the boxes shown in the right inset PEEM
image; (b) same data as in the lower half of (a)
but after subtracting the running average
together with a fit to the data as described in
the ref. [104]; (c) vortex displacement parallel
and perpendicular to Hp. The orientation of the
exciting field pulse and the polarization P are
sketched in the left inset of (a). (courtesy of J
Raabe et al.).
Interestingly, even though it is predicted by a
very fundamental and widely applicable LLG
theory, the gryotropic behavior is not universally
observed. In a similar experiment at the Swiss
Light Source (SLS) on Ni80Fe20 alloy (permalloy)
squares, Raabe et al. did not observe a circular
vortex core trajectory[104], as shown in figure 11.
In this experiment, the authors imaged the core
motion in 6-µm-square permalloy structure in a
flux closure state after a 6mT, ~400ps field pulse
14
generated by a pulsed laser. They found that the
core initially moves towards the edge of the square
in the direction perpendicular to the field for 1-2
ns. After reaching its maximum excursion of 750
nm, the core relaxes directly back to the center of
the square, along essentially the same path. This
relaxation takes until the next field pulse, which
occurs at t=16 ns. The authors also observed
precessional motion of the spins within a single
domain of the Landau state, and oscillations of
domain walls.
The lack of agreement on the presence of
gyrotropic motion between the ALS experiment on
Co squares and the SLS experiment on permalloy
squares is intriguing, since this should be an
intrinsic property of a vortex state. Raabe et al.
suggest that the difference may be explained by
the resonant nature of the ALS experiment as
opposed to their experiment, and the higher value
of the field pulse amplitude. It is also worth
mentioning that the two experiments involved very
different materials and different size structures. In
addition, both involved squares, which promote
the formation of domain walls, for which an
analytical solution to the LLG equation does not
exist.
4.1.3. Investigation of the free vortex core motion
in circular FeNi disks
In circular disks, analytical solutions do exist
for the LLG equation, and they make specific
predictions for the core trajectories and oscillation
frequencies. Thus, circular disks are an ideal
platform for exploring the dynamics of the vortex
state. Guslienko et al. used time-resolved X-PEEM
to investigate the free motion of the vortex core
and measure the fundamental eigenfrequencies in
the time domain [105]. In this experiment at the
Advanced Photon Source (APS), a direct electrical
pump pulse was applied, as opposed to a laserinduced field pulse, allowing for an arbitrary time
dependent field to be applied. The structures were
engineered such that their fundamental frequencies
were well above the excitation frequency of 6.53
MHz, determined by the synchrotron bunch
structure.
Figure 12. Free vortex core motion in circular
NiFe disks. (a) Time dependence of the ratio of
the y displacement of the vortex core to the
radius of 5.3 and 4.3 m diameter dots of
thickness 30 nm. (b) Comparison of the XMCD
experimental data (symbols) and analytical
theory for the dependence of the
eigenfrequency of the vortex translational
mode on the dot aspect ratio L/R of the
cylindrical permalloy dots. [105]
In this experiment, a 4mT field was applied to
disks of varying aspect ratio, and then the vortex
core motion was followed after the field was
removed. The authors measured the core
oscillation frequencies directly by deriving core
positions from the magnetic images. They found,
as shown in figure 12, that the core positions
oscillate in the several tens of MHz range, and that
the oscillation frequency dependence on the aspect
ratio matched well with the earlier predictions of
Guslienko et al.[ 106 ] for the gryotropic mode.
Interestingly, they also noted a mainly linear
oscillation, perpendicular to the direction of the
applied field. This result is consistent with those of
Raabe et al.[104], but is at odds with the
15
predictions of the LLG equation of motion. The
authors suggest, in this case, that the difference
may be due to insufficient contrast of the images
along the field direction. Since the vortex image
inherently has greater contrast in the direction
perpendicular to the x-ray beam, a determination
of the core position along the beam direction may
be subject to greater uncertainties, and that
component of the motion may be missed.
4.1.4. Nonlinear effects in vortex dynamics
The observation of non-gyrotropic motion in
the free vortex motion of circular permalloy
structures rules out the influence of domains walls
and resonant driving field as potential explanations
for the disagreement with the predictions of
theory. Therefore, the possibility remains that the
experiments have been conducted outside the
range of validity for the theory.
Cheng et al. [ 107 ] performed additional
experiments at the APS to determine the
dependence of the core trajectories on the
excitation field amplitude. In this experiment, they
found that gyrotropic motion is observed for low
excitation fields, while above a critical field of
~2.5 mT, an elliptical trajectory is observed. Thus,
they observe the two behaviors noted in the
literature within the same experiment, as shown in
figure 13. They also observe a transient domain
state in the first 500 ps of core motion for high
excitation fields, which calls into question the
ability to apply linear theory at least to the initial
motion.
This critical behavior suggests that a linear
regime exists in which the LLG predictions are
valid, and above which additional contributions to
the dynamics must be considered. Using
micromagnetic
simulations,
the
authors
reproduced the critical behavior seen in the
experiments. The simulations also showed the
formation of a cross-tie domain wall
corresponding to the transient domain state in the
PEEM images. The cross-tie wall includes several
vortex-antivortex pairs, which combine with the
original vortex as it moves down the wall. This
results in many core polarity reversals, effectively
randomizing the core polarity. This leads the
authors to suggest a alternate explanation for the
linear or elliptical trajectories seen above the
critical field. They suggest that since the
randomization of the core polarity from pulse to
pulse in the pump-probe experiment effectively
randomizes the sense of the core gyrotropic
motion, the component of the circular motion
along the beam would be canceled out, and an
apparent linear trajectory observed. The same
authors later showed that the core polarity and the
sense of gyrotropic motion is indeed randomized
by large field pulses [108].
Figure 13. Vortex core positions parallel (X)
and perpendicular (Y) to the applied field vs
time, along with fits (red/solid lines) to damped
sinusoids after (a) 1, (b) 2, and (c) 4 mT field
pulses. (d) shows the time-dependent applied
field profile. The upper inset shows the
experimental geometry indicating the relativefield (B) and photonmomentum (k) directions.
The lower inset shows the X-deflection/Ydeflection amplitude ratio vs the pulse-field
amplitude. (e) Transient domain states observed
both in experiment and simulation shortly after
the falling edge of a 5 mT excitation pulse
(removed at time t=0 ns). The experimental
PEEM images (top row) agree well with the
micromagnetic simulations (second row). [106,
107]
This explanation depends upon the limited
resolution of the imaging experiments. That is, if
the imaging technique has sufficient resolution to
16
see both components of the motion, the
randomization should be evident as the appearance
of two vortex cores at the maximum along the
field displacement. The authors above noted a
broadening of the core at this point in the motion,
but the spatial resolution is insufficient to resolve
the two cores. Wiegand et al.[109] performed a
similar experiment using a scanning transmission
x-ray microscope (STXM) at the Canadian Light
Source (CLS). Currently, the STXM technique has
a spatial resolution of ~25 nm. Thus, in the CLS
experiment, these authors were able to resolve the
two cores, and show that for large excitation fields
the pump-probe image is a superposition of two
gyrotropic trajectories of opposite gyration senses.
4.2. Studies of domain wall dynamics in nanowires
In many previous experimental studies of
domain wall dynamics, the domain wall position
was determined by indirect means, such as
electrical transport measurements [46, 110, 111]
and magneto-optical Kerr effect (MOKE)
measurements [40, 42]. However, these indirect
probing methods cannot provide detailed magnetic
configuration of the domain wall (transverse type
or vortex type), which is crucial for understanding
the fundamental physics underlying the domain
wall motion. X-PEEM has been greatly used in
imaging magnetic vortex structure because of its
sensitivity to in-plane magnetization as well as
high spatial resolution. Therefore, it is also
suitable for directly imaging the motion of domain
wall, especially the vortex type domain wall [112,
113]. More importantly, X-PEEM combining with
the coplanar waveguide technique can be used in a
pump-probe arrangement for time-resolved study
of domain wall dynamics [114]. X-PEEM imaging
at various temperatures can also study temperature
dependence of the domain wall dynamics [113].
Thomas et al. [112] used X-PEEM to explore
the injection and pinning of magnetic domain
walls in a 250-nm-wide, 20-nm-thick Ni80Fe20
nanowire with a micron-sized elliptical nucleation
pad [ 115 , 116 ] at one end and the other end
tapered to a sharp tip [117, 118]. The nanowire
itself is straight apart from one or two triangularly
shaped notches formed at the edge of the wire to
trap domain walls [118, 119 ]. Direct PEEM
imaging revealed that a vortex-like structure is
readily nucleated in the nucleation pad at low
magnetic fields (<15 Oe), whereas injection of a
domain wall into the nanowire requires
significantly larger fields (~60 Oe). The authors
also found domain walls can be pinned in the
nanowire at the notches and the chiralities of the
vortex-like domain walls vary in successive
experiments. These results show the subtle
complexities of the domain wall nucleation,
propagation, and pinning processes.
Laufenberg et al. [113] performed X-PEEM
imaging to determine the spin structure of head-tohead domain walls in Ni80Fe20 rings of various
dimensions
and
further
extracted
the
corresponding room temperature phase diagram
which exhibits two phase boundaries between the
wall types. In temperature-dependent X-PEEM
imaging as shown in figure 14, the authors
observed a thermally activated switching from
transverse walls to vortex walls at elevated
temperatures at a transition temperature between
260 °C and 310 °C. This provides direct
experimental evidence for the fact that transverse
and vortex walls are separated by an energy barrier
which can be overcome thermally.
Figure 14. PEEM images of a 7 nm thick and
730 nm wide ring imaged during a heating
cycle at temperatures of (a), (d) T=20 °C
(before and after heating, respectively), (b)
T=260 °C, and (c) T=310 °C. The two
transverse walls (a) are not visibly influenced
by heating (b) up to the transition temperature
(c), at which a thermally activated transition to
a vortex type occurs in both walls. (d) The
vortex walls are retained after cooling down.
The gray scale shows the magnetization
direction. (courtesy of Laufenberg et al.) [113]
X-PEEM has also been used to investigate
current-driven domain wall motion. Kläui et al.
[120] reported the effect of electrical currents on
the propagation and spin structure of vortex walls
17
in NiFe wires. They found that in the 1 µm wide
and 28 nm thick NiFe wires, the movement of
unperturbed vortex walls started at around
8.3x1011 A/m2 and the average wall velocity
increases with increasing current density, and the
typical velocity is below 1.0 m/s. In addition to
propagation of unperturbed vortex walls, they also
observed a number of different wall
transformations. Vortices were found to be
nucleated (as shown in figure 15) and annihilated
due to the spin torque effect, in agreement with
theoretical predictions [ 121 ]. The velocity was
found to be directly correlated with these
transformations and decrease with increasing
number of vortices. Uhlîr et al. [122] demonstrated
very high velocities for current-induced domain
wall propagation in NiFe/Cu/Co trilayered
nanowires with maximum velocities exceeding
600 m/s. These velocities are 4 to 5 times larger
than the maximum value reported for other inplane anisotropy systems [123].
Figure 15. (a) PEEM image of a vortex wall
after remagnetization (top) and after injection of
a pulse with j=8.3×1011 A/m2 (bottom). An extra
vortex has been nucleated downstream. (b) A
high resolution image of the double vortex wall
and a simulation with arrows (c) visualizing the
spin structure. (d) Image of an extended vortex
wall with a cross-tie structure at the center and
arrows indicating the magnetization direction.
(courtesy of Kläui et al.) [120]
Recently Uhlîr et al. [114] have used timeresolved X-PEEM to investigate the magnetization
dynamics induced by nanosecond current pulses in
NiFe/Cu/Co nanowires. They found that The
Oersted magnetic field present during the current
pulses induces a large tilt of the NiFe
magnetization, transverse to the wires, as shown in
figure 16. They also observed spin-wave-like
oscillations of the NiFe magnetization and
attributed these oscillations to precessional motion
about the effective field. Their results clearly show
that the effects of the Oersted field should be taken
into consideration when interpreting currentinduced domain wall motion in multilayered
nanowires.
Figure 16. Time-resolved PEEM images of the
NiFe layer of a 400 nm wide nanowire at time
delays of (a) 0 ns, (b) 0.35 ns, (c) 0.45 ns, (d) 1.9
ns, (e) 2.2 ns, (f) 2.3 ns, (g) 2.4 ns, (h) 3.3 and (i)
3.6 ns with respect to the beginning of the
positive part of the bipolar current pulse. These
delays are indicated on the bipolar pulse plotted
in (j), together with the magnetization tilt angle.
(courtesy of Uhlîr et al.) [114]
4.3. Studies of exchange bias in ferromagnetantiferromagnet interfaces
Element-specific
X-PEEM
imaging
techniques based either the XMCD effect or
XMLD effect allow the observation of the
microscopic spin structure on both sides of an
ferromagnet-antiferromagnet interface, providing a
extremely powerful tool for exploring a hotly
debated question: the origin of the exchange bias
in the ferromagnet-antiferromagnet system. Here
we review the first direct imaging investigation of
interfacial magnetic coupling in an exchange bias
system X-PEEM, followed by a discussion of the
importance of considering the anisotropy of
18
XMLD signals and the review of X-PEEM studies
of interfacial uncompensated spins of the
antiferromagnet.
4.3.1. The first X-PEEM observation of interfacial
magnetic coupling in exchange biased layers
In one of the first experiments to exploit the
element selectivity of PEEM in a layered system,
Nolting et al [124, 125] explored the interfacial
magnetism in exchange-biased bilayers. In this
experiment, the authors used x-ray magnetic
dichroism
in
PEEM
to
image
the
antiferromagnetic and ferromagnetic domain
structures of the Co/LaFeO3 bilayer. The authors
compared the linear dichroic images observed in
the antiferromagnetic LaFeO3 layer at the Fe L3
resonance with the corresponding circular dichroic
images at the Co L3 resonance, as shown in figure
17.
Figure 17. PEEM images and local spectra from
the antiferromagnetic and ferromagnetic layers
for 1.2-nm Co on LaFeO3/SrTiO3(001). (a) Fe
L-edge XMLD image; (b) Co L-edge XMCD
image. The spectra shown underneath were
recorded in the indicated areas and illustrate the
origin of the intensity contrast in the PEEM
images. (courtesy of Nolting et al) [124]
The authors discovered a clear correlation of
the ferromagnetic Co domains and the
antiferromagnetic LaFeO3 domains, indicating a
strong exchange coupling at the interface between
the two layers. They demonstrated that the
exchange bias occurred on a domain-by-domain
basis, confirming its microscopic origins. This
domain-by-domain exchange coupling is critical to
the formation of the exchange bias effect. They
also attempted to image the uncompensated Fe
spins directly with XMCD microscopy, but did not
succeed. They thought it was probably because of
the small concentration or magnitude of the
uncompensated spins.
4.3.2. Importance of experimental geometry in
interpreting the XMLD signal
In a follow up experiment to Nolting, et al.
[124], Czekaj et al. [ 126 ] examined the
geometrical dependence of the linear dichroism
signal in PEEM images of LaFeO3. In this
experiment, they continuously varied the angle
between the incoming photon polarization and the
antiferromagnetic
domain
magnetization
directions. In doing so, they were able to map out
the full angle dependence of the XMLD signal,
and they found that the magnetization directions
were aligned ~20° away from the crystalline axes
and the film plane. More importantly, they showed
that the XMLD signal depends strongly on the
magnetocrystalline anisotropy, and that it can
change sign, depending on the experiment
geometry.
Taking into account the anisotropy of XMLD
signal is very important for a correct interpretation
of antiferromagnetic domain images [ 127, 128].
For example, Arenholz et al [129] and Ohldag et al
[ 130 ] recently reported that the magnetic spins
near the surface of a NiO(001) single crystal are
perpendicular to those in the Co thin film
deposited on top of this NiO layer. These results
are based on a comparison between the XMCD
image of the Co layer and the XMLD image of the
NiO layer, which carefully took into consideration
of the XMLD anisotropy on the intensity ratio of
the two peaks at the Ni L2 edge. This perpendicular
alignment
of
the
ferromagnetic
and
antiferromagnetic spin in the Co/NiO system is
contradictory to an earlier report of a perfectly
parallel alignment of the ferromagnetic and
antiferromagnetic spins in the same system [131],
which did not take the anisotropic XMLD into
account. This shows that the relative orientations
of the crystalline axes, film plane, and photon
polarization must be well understood for the
interpretation of antiferromagnetic domain images,
and for understanding exchange bias effects.
19
4.3.3. Studies of uncompensated spins in
exchange-biased and coupled bilayers
Extensive research results suggest that the
exchange bias effect originates in uncompensated
and the pinned magnetic moments of the
antiferromagnet layer [15, 132 , 133 ]. The high
sensitivity of XMCD enhanced by dividing images
taken with left and right circularly polarized light
allows probing such uncompensated spins even if
their effective thickness is below a monolayer.
Figure 18. Figure 18. Patterned area of the
CoFeB/MnIr/NiFe system taken with PEEM at
the (a) Co and (b) Mn L3 edges. Arrows in some
domains of (a) represent their magnetization
direction as deduced from the XMCD contrast.
(courtesy of Eimüller et al.) [134]
Eimüller et al reported the direct XMCD
imaging of uncompensated Mn spins and the at the
CoFeB/MnIr interface with an antiferromagnetic
coupling between the Mn and the Co magnetic
moments, as shown in figure 18 [134]. A square
region of the Co86Fe10B4/Mn77Ir23 / Ni80Fe20/Si3N4
exchange bias film where material was removed
by focused ion beam (FIB) etching, except for
quadratic dots with edges of 1, 1.5, and 2 µm
length were imaged by X-PEEM at Co and Mn L3
edges. The magnetization of most Co domains
points along the exchange bias direction with a
spread of orientations relative to this direction of
less than 45°. The same domain structure observed
for Co was also observed at the Fe L3 edge (not
shown), thus proving parallel coupling of Co and
Fe spins. For the disordered γ-phase fcc alloy
Mn77Ir23 a vanishing XMCD is expected since its
magnetic Mn moments are compensated in the
(111) film plane, forming a noncollinear, so-called
3Q spin density wave (SDW) structure [ 135 ].
However, at the CoFeB/IrMn interface, the same
domain structure found in CoFeB was also
observed when the photon energy was tuned to the
Mn L3 edge, but with a reversed contrast,
indicating an antiparallel alignment between the
uncompensated Mn and the ferromagnetic CoFeB
spins.
Baruth et al [136] also observed interfacial
uncompensated spins of an antiferromagnet in a
trilayer with a single NiO layer sandwiched by two
Co/Pt multilayers with perpendicular magnetic
anisotropy [137, 138]. Interestingly, an oscillatory
coupling was observed in this system by Liu et al
through the insulating antiferromagnetic NiO
spacer [139, 140]. Magnetic coupling in layered
structures has been studied intensely ever since the
discovery of antiferromagnetic oscillatory
coupling through nonmagnetic layers, and the
concurrent GMR effect [55]. Most of work has
been on all-metallic systems, where an RKKY
mechanism was invoked to explain the oscillatory
interlayer thickness dependence [ 141 ]. For
insulating spacers, an oscillatory dependence was
not predicted [142,143].
To investigate the origin of the oscillatory
coupling in this system [Co/Pt multilayer]/ NiO
[Co/Pt multilayer] system, Baruth et al. using soft
x-ray spectroscopy and X-PEEM imaging at the
Ni and Co L edges. By comparing the Co and Ni L
edge XMCD signals, Baruth et al found a non-zero
Ni moment that was correlated with the sign and
strength of the observed interlayer coupling. For
antiferromanetic Ni, no XMCD is expected since
the net spin is zero. Therefore the observed signal
suggests that uncompensated spins at the Co/NiO
interfaces cant in the perpendicular direction as a
result of the Co-Ni exchange coupling. This
canting may be transmitted through the NiO layer
to the opposite Co layer, thus creating an indirect
coupling. The authors showed this net Ni moment
first in XMCD spectroscopy, however a crucial
test of whether it truly originates from interfacial
spins was performed by X-PEEM imaging at the
Co and Ni L3 edges. The PEEM images revealed
that the Co and Ni spins follow each other in a
domain-by-domain
fashion,
proving
the
microscopic nature of the coupling.
Miguel et al.[144] recently examined both the
ferromagnetic and antiferromagnetic domain
structure in an Fe/CoO exchange biased bilayer
system. Using the XMCD and XMLD effects at
the Fe and Co L3 edges, they were able to image
the Fe ferromagnetic domains, and Co
uncompensated
spins,
and
the
Co
antiferromagnetic domains separately, as shown in
figure 19.
20
Figure 19. XMCD–PEEM magnetic domain
images of 5 monolayers Fe at room temperature
(a) and at low temperature (b). Magnetic domain
image originating from uncompensated Co
moments at the interface as seen by Co L3
XMCD–PEEM (c) and antiferromagnetic
domains of the 8 monolayers CoO layer by Co
L3 XMLD–PEEM (d), both at low temperature.
The red lines are guides to the eye to follow the
overlap of Fe and CoO domain walls. (courtesy
of Miguel et al.) [144]
Miguel et al. found that the Co
uncompensated
spins
are
coupled
ferromagnetically to the Fe spin by the strong
exchange coupling. Further, the Fe and Co
ferromagnetic spins are collinear with the Co
antiferromagnetic spins. The authors showed also
that the antiferromagnetic domain structure is
strongly influenced by the ferromagnetic ordering
of the Fe layer. For no Fe coverage, the Co XMLD
contrast is barely recognizable, suggesting that the
domains are smaller than the spatial resolution of
the microscope. For 0-2 monolayers of Fe
coverage, below the critical thickness for FM
ordering, there is also very little XMLD contrast.
Above this critical thickness, the Fe layer orders
very clearly, and antiferromagnetic domains
appear that are correlated with the FM domains.
This correlation provides important clues to the
mechanism that underlies the exchange coupling.
4.4. Studies of spin reorientation transitions in
coupled magnetic multilayers
The full-field and element specific natures of
the PEEM technique in conjunction with a wedge
growth technique are uniquely suitable for
investigating the spin reorientation transition
(SRT) in ultrathin films or magnetic multilayers
[70, 145, 146, 147, 148, 149 ,150]. This was first
demonstrated by Kuch et al [70] in a study of SRT
in fcc wedged Co/wedge Ni/Cu(001) epitaxial
ultrathin films as a function of Co and Ni film
thickness. The Co thickness was varied from 1.4 to
2.6 atomic monolayers (ML) and the Ni thickness
from 11 to 14 ML. Both Co and Ni layers were
prepared as crossed wedges with slopes rotated by
90° with respect to each other. The authors
acquired a series of PEEM images at different
photon energies which cover the Ni 2p3/2 and 2p1/2
photon absorption regions. Therefore in addition
to the qualitative information about magnetic
domains obtained at one fixed peak photon energy,
full quantitative information can be extracted from
XMCD analysis of such a spectral series of images
with the same spatial resolution.
Figure 20. Results of the pixel-by-pixel sum-rule
analysis for the Ni effective spin moment (a) and
orbital moment (b). Different geometrical
projections onto the direction of the incident light
(from bottom to top, 30° above image plane) are
represented by different grayscales, as defined in
the legend. Ni and Co thicknesses are given at the
top and right axes, respectively. The dotted white
line highlights the spin reorientation line which
separates regions with out-of-plane magnetization
(bottom, two different grayscales) and in-plane
magnetization (top, four different grayscales).
(courtesy of Kuch et al) [70]
Kuch et al showed that the spin reorientation
occurs as a function of both Co and Ni thicknesses.
A switching of the magnetic easy axis from [001]
out-of-plane to <110> in-plane directions when
increasing the Co thickness or decreasing the Ni
thickness. Pixel-by-pixel sum-rule analysis of the
21
series of PEEM images results in images
displaying the Ni spin and orbital magnetic
moments, projected onto the propagation direction
of the illuminating x-rays, as shown in figure 20.
A constant effective Ni spin moment similar to the
bulk magnetic moment was observed. However,
the Ni orbital moment is distinctly higher for outof-plane magnetization than for in-plane
magnetization. The domain density of the
perpendicular magnetization increases towards the
spin reorientation transition line. The authors
explained the formation of perpendicularly
magnetized domains with decreasing size upon
approaching the spin reorientation transition by
magnetostatic stray field energy minimization for
decreasing domain wall energy. This experiment
help to elucidate the detailed mechanism of the
spin reorientation transition in the Co/Ni/Cu (001)
system.
Figure 21. (a) SRT position in the Fe-Ni
thickness plane. Magnetic bubble domain
images of (b) dFe = 2.9 ML, dNi =5.5 ML; (c)
dFe = 3.1 ML, dNi = 8.0 ML; (d) dFe = 3.6 ML,
dNi = 10.8 ML; (e) dFe = 4.1 ML, dNi = 13.6
ML.(courtesy of Choi et al.) [149]
Choi et al. [149] used PEEM imaging in
conjunction with a wedge growth technique to
examine thickness-dependent changes in magnetic
domain structure as a thin film system is driven
through a spin reorientation transition. In their
experiment, the influence of the anisotropy on the
formation of bubble domain phases in thin films
with perpendicular magnetic anisotropy was
examined in a crossed wedge Fe/Ni bilayer
structure deposited on a Cu(001) surface. By
varying the Fe and Ni thicknesses in orthogonal
directions, they created a 2D continuous library of
every thickness combination. This then allowed
them to map out the response of the system to the
thickness-driven changes in anisotropy by imaging
the wedge structure with X-PEEM, as shown in
figure 21. They found that a new bubble domain
phase forms near the thickness at which the
magnetic anisotropy switches from out-of-plane to
in-plane. This domain phase turns out to be a
metastable phase, appearing only with a small outof-plane field, and disappearing if the sample is
heated above 370 K.
4.5. Study of magnetization switching by electric
field in ferromagnet-multiferroic structures
Element-specific and layer-resolved X-PEEM
technique has recently been applied to study
ferromagnet-multiferroic bilayers. Chu et al [72]
investigate a bilayer consisting of a ferromagnetic
Co0.9Fe0.1 in intimate contact with a multiferroic
BiFeO3 layer. The magnetic state of the CoFe
layer was imaged by X-PEEM based on the
XMCD effect at the Co L-edge. The authors
determined that the CoFe forms domains that are
aligned 90o with respect to each other and that the
local magnetic domain structure exactly follows
the ferroelectric domain structure of BiFO3 imaged
by piezoresponse force microscopy (PFM), as
shown in figures 22 (a) and (b). These images
provide the first direct microscopic evidence of
exchange coupling between a ferromagnet and a
multiferroic at this length scale (~200 nm). The
authors attributed the observed one-to-one
mapping of the ferroelectric and ferromagnetic
domains to the collinear coupling between the
magnetization in the ferromagnet and the
projection of the antiferromagnetic order in the
multiferroic.
The authors further used X-PEEM to directly
image the change in magnetic state of the CoFe
layer in response to the electric switch of the
multiferroic BiFO3 layer, as shown in figure 22
(c)-(e). Their observations indicate that the
average
magnetization
direction
in
the
o
ferromagnet rotates by 90 on the application of an
electric field to the underlying BiFeO3 layer. On
electrical switching the BiFeO3 layer once again,
the average magnetization direction changes back
to the original state. Their results demonstrated the
possibility to locally control ferromagnetism with
an electric field.
22
Figure 22. (a)In-plane PFM image showing the
ferroelectric domain structure of a BFO film
with a large (10 μm, red square) and small (5
μm, green square) electrically switched region.
(b) Corresponding XMCD–PEEM image taken
at the Co L-edge for a CoFe film grown on the
written pattern. Direct matching of domain
structures is evident. XMCD–PEEM images
taken at the Co L-edge revealing the
ferromagnetic domain structure of the CoFe
features in such a coplanar electrode device
structure in the as-grown state (c), after the first
electrical switch (d) and after the second
electrical switch (e). Application of an electric
field is found to rotate the next magnetization of
the structures by 90o. (courtesy of Chu et al.)
[72]
4.6. Magnetization
nanostructures
reversal
in
nanomaterials. Zhao et al [ 151 ] studied the
magnetic structure as well as its response to an
external electric field in ferrimagnetic CoFe2O4
nanopillars embedded in an epitaxial ferroelectric
BiFeO3 film using XMCD-based X-PEEM
imaging. The film was first magnetized in an outof-plane magnetic field, resulting in a predominant
downward magnetization. Then, an electric
voltage of −12 V was applied in order to switch
the ferroelectric polarization in the BiFeO3 matrix
by scanning the PFM tip over a selected area.
Subsequent PEEM imaging was performed in an
area containing both poled and unpoled regions.
Magnetic switching was observed in both Co and
Fe magnetic sublattices of the ferromagnetic
CoFe2O4 nanopillars after application of an
electric field. The XMCD-based PEEM image
showed only black dots, indicating a uniform
downward magnetization distribution, in the
unpoled region (outlined by a box with red solid
lines in figure 23 (a)), while both black and white
dots can be seen in the poled area (outlined by a
box with green dashed lines in figure 23 (a)),
indicating switching of some of the nanopillars.
From XMCD spectroscopy experiments, about
50% of the CoFe2O4 nanopillars were measured to
switch their magnetization with the electric field,
implying an elastic-mediated electric-fieldinduced magnetic anisotropy change.
complex
4.6.1. Studies of ferromagnet - multiferroic
nanocomposit films
In addition to heterostructures consisting of
ferromagnetic
and
multiferroic
layers,
ferromagnets and multiferroics can also be
integrated in the format of nanocomposit films
containing magnetic nanopillars embedded in a
multiferroic film. X-PEEM has demonstrated its
unique strength of element specific magnetic
imaging in this type of complicated, multi-phased
Figure 23. Co-edge PEEM images at the
interface beween poled and unpoled regions.
(a) Magnetic information. (b) Chemical
information. The scale bars are 1 m.
(courtesy of Zhao et al.) [151]
To ascertain that the dichroism contras seen
in the PEEM image originates from the CoFe2O4
nanopillars, a Co element-contrast PEEM image
23
was obtained by taking the sum of two images
acquired on the left shoulder of the Co L3 peak
with right and left circular polarizations, as shown
in figure 23 (b). By comparing figures 23 (a) and
(b), the authors showed that the areas showing
magnetic contrast correspond to areas containing
Co. The combination of the magnetic information
and chemical information, obtained by X-PEEM,
of the unpoled and poled areas in the
nanocomposite CoFe2O4-BiFeO3 film convincely
demonstrated the magnetization switching of
CoFe2O4 nanopillars by the electric field.
4.6.2. Studies of multilayered nanodisks
The ability of PEEM to provide elementspecific magnetic images has proved very useful
for understanding of the behavior of ferromagnetic
spin valve structures. There is interest in using
metallic trilayers, such as NiFe/Cu/Co, patterned
into ring-shaped nanostructures, for magnetic
memory elements. The ring-shaped geometry
promotes the formation of vortex states, which are
useful for minimizing the stray fields from the
memory element, thus allowing greater density.
However, there is evidence that the layered
geometry may influence the formation of the
vortex states via the dipolar magnetic interactions
between the layers,[ 152 ] and micromagnetic
simulations suggest a complicated phase
diagram.[153]
Figure 24. PEEM image of the NiFe layer of
the multilayer rings exhibits diverse multidomain structures at remanence. This suggests
that imperfections in the rings strongly control
the magnetic reversal. [78]
observed images with layer-specific hysteresis
loops obtain by resonant x-ray magnetic
reflectivity. They found that although the
hysteresis loops showed zero remanent moment,
suggesting the formation of vortex states, the
PEEM images of the top NiFe layer showed a
multidomain structure, as shown in figure 24.
While the detailed domain structure displayed
some randomness, a four-domain pattern appeared
with greater frequency than would accounted for
by chance. Comparison of these multidomain
patterns with micromagnetic simulations suggests
that they are actually antivortex states. This
experiment therefore shows the importance of
imaging the real-space domain structure, even in
cases where layer-specific hysteresis loops on an
ensemble of nanostructures are available.
4.6.3. Studies of an artificial spin ice system
The kagome spin ice arrangement has been of
particularly interest because it is highly frustrated
and the three interactions at a vertex are
equivalent. The full-field nature of the PEEM
technique makes it uniquely suitable for
investigating a large array of magnetic
nanostructures, such as an artificial spin ice system
[86, 154]. Mengotti et al. [86] used X-PEEM to
examine the magnetic configurations in three basic
building blocks of an artificial kagome spin ice
consisting of one, two, and three rings. The
XMCD-based PEEM images exhibit a high
magnetic contrast with four distinct gray levels
associated with the kagome structure, allowing
one to determine unambiguously the direction of
the moments in all islands with a single image, as
shown in figure 25. Following a demagnetization
process involving rotation of the sample in a
magnetic field, the authors determined the
frequency of states as a function of the dipolar
coupling strength between the islands, modified by
varying the lattice parameter. They found the
important result that, as the number of rings
increases, there was a dramatic reduction in the
ability to achieve the low-energy states via
demagnetization.
Rose et. al. [78] used PEEM to image the
magnetic domain states of the permalloy layer in
NiFe/Cu/Co trilayer rings, and compared the
24
number of rings increases there is a drastic
decrease in the ability to achieve the low-energy
states. This PEEM imaging study of magnetic
configurations in three basic building blocks of an
artificial kagome spin ice provides crucial
information for understanding magnetic frustration
in an artificial spin ice system.
5. Future Prospects
In the preceding, we have outlined the recent
progress in static and time-resolved magnetic
imaging using PEEM over the last 10-12 years.
All of this work has been based on the standard
electrostatic PEEM design in conjunction with a
helical undulator at a 2nd or 3rd generation
synchrotron source. However, new sources and
electron optical designs are currently coming
online or are under development that promise to
provide orders of magnitude better performance in
sensitivity and spatial and temporal resolution.
Principle among these developments are the use of
aberration-corrected electron optics, which would
allow spatial resolution down to 2-5 nm, and
bunch slicing or compressing schemes to improve
the time resolution to sub 1 ps. In the longer term,
4th generation sources (x-ray free electron lasers)
are now beginning to become available, and may
produce opportunities for entirely new types of
PEEM experiments in the future.
Figure 25. Experimental frequency of lowenergy states (band 1) for strongly coupled
islands with increasing number of rings. (a)
Geometries for demagnetization with in-plane
and out - of- plane rotation axes and for in situ
switching in the PEEM, (b)XMCD images of
nine of the one-, two-, and three-ring
configurations following demagnetization about
an in-plane rotation axis, and (c) the frequency
of observed states plotted against the number of
rings. (courtesy of Mengotti et al.) [86]
They
also
directly
examined
the
magnetization reversal in an applied field in situ in
the PEEM. Following saturation of the
magnetization, they recorded XMCD snapshots of
the switching process on applying a reverse field,
increasing its value from zero in steps of
approximately 6 Oe, and recording images at
remanence after reducing the field between each
step back to zero. Their in situ magnetization
reversal imaging results also showed that as the
5.1. Aberration-corrected optics for improved
spatial resolution
In the most typical PEEM optical setup, the
spatial resolution is limited by aberrations in the
electron optics. The two most significant
contributions to these are chromatic aberration due
to the finite energy spread of photoelectron
emitted from the surface, and spherical aberration.
For excitation with soft x-rays, chromatic
aberrations effectively limit the spatial resolution
to ~100 nm. In the absence of chromatic
aberrations, spherical aberrations would limit the
resolution to ~20 nm. Indeed, this spatial
resolution has been achieved in PEEM systems
with UV excitation, in which the electron energy
spread is limited by the lower energy of the
incoming photons[ 155 ]. In the absence of any
aberrations in the optics, the spatial resolution is
limited by the effective area from which electrons
may be emitted. Since most of the electrons
emitted are secondary electrons, this area will be
25
defined by the mean free path in the material being
imaged. In most cases, the effective limit for the
resolution is 2-5 nm.
There are currently two efforts to build
improved resolution PEEM instruments at x-ray
facilities through various levels of aberration
correction. The first one, at BESSY-II, is called
SMART[ 156 ], and the second is the PEEM-3
instrument at ALS [ 157 ]. To reduce chromatic
aberrations, an energy filter is typically used to
restrict the energy distribution of the electron
beam. To correct for spherical aberrations, the
beam is deflected into an electron mirror which
introduces equal and opposite aberrations. If Fig.
xx, we show a schematic diagram of this approach
for the PEEM-3 instrument at ALS.
5.2. Short pulse sources at storage rings for
improved time resolution
In standard storage ring-based pump probe
experiments, the ultimately achievable time
resolution is governed primarily by the length of
the electron bunches in the ring. Other effects may
degrade the resolution from there, such as timing
jitter and reduced bunch purity, but the bunch
length clearly places a lower limit on the
resolution. While it is physically possible to
generate electron bunches in the 100 fs regime, the
use of storage rings limits the steady state bunch
length through Coulomb broadening. Therefore,
the bunch length is dependent on the storage ring
design, the bunch pattern used, and the charge per
bunch. At most 3rd generation sources, the
available bunch lengths range from ~50 – 100 ps
(FWHM). For much field-driven magnetization
dynamics research, 100 ps resolution is sufficient.
However, faster phenomena occur, in particular
optically-driven effects, at time scales from 100 fs
to 10 ps. One such example is thermal
demagnetization, which can take place in sub-ps
time scales, and is of interest for thermallyassisted magnetic recording technology [ 158 ].
Direct coupling of the magnetization to a laser
field can also produce magnetization dynamics in
the sub-ps to 10’s of ps scales [159]. There are
several potential methods to improve the pulse
length available at x-ray sources. These involve
either modifying the electron bunches to allow a
shorter “slice” of the x-ray pulse to be picked out,
or using next generation sources.
Figure 26. Diagram of the laser slicer for
generating femtosecond x-ray pulses at the
Advanced Light Source. From Schoenlein, et al.
(courtesy of .) [159]
Of the former, the most widely used approach
is the femtosecond laser slicer. In the laser slicing
technique, ultrafast x-ray pulses are created using
the interaction of a pulsed laser and the electron
bunches in the storage ring [160]. Laser pulses are
launched into the storage ring tube, where their
strong electric field modulates the energy of a
small slice of the electron bunch. This creates a
dispersion in the emitted photons from that bunch,
such that the resulting short x-ray pulse may be
separated from the rest of the pulse using a slit.
The laser slicing method has the advantage that it
is capable of generating very fast x-ray pulses
(~100 fs). However, the cost in available flux is
very high. First, by slicing 100 fs out of a 100ps
bunch, a factor of 103 is thrown away. Also, the
pulsing laser usually can only be operated at
~1kHz, such that all other x-ray bunches that are
not subject to a laser pulse must be gated out. As
storage ring repeat rates are usually above 1MHz,
this results in a loss of an additional factor of 103.
Therefore, the flux obtained from femtosecond
slicing beamlines is usually only about 104
ph/s/0.1% bandwidth, compared with 1012 for a
beamline designed for XMCD spectroscopy and
PEEM that can count every bunch. The capability
to do time resolved imaging with such low flux
numbers is extremely limited, but is possible in
principle. It was recently demonstrated by Boeglin
et al. that high quality XMCD spectroscopy,
sufficient for quantitative analysis of the spin and
orbital moment dynamics, is possible with the
laser slicer at BESSY-II [ 161 ]. With sufficient
stability and access to beamtime, a PEEM
experiment that involves fixed-energy imaging
could be possible, although to date none have been
reported.
Besides laser slicing, other approaches to
manipulating the electron bunches within the
26
storage can produce high flux and significant
improvements in the time resolution to the 1 – 10
ps range. At the BESSY-II light source, a special
bunch compression technique known as low-
mode provides <4 ps soft x-ray photon pulses,
with a bunch spacing of ~2 ns, and about 10x the
charge per bunch as in laser slicing [162]. Since
this bunch compression technique can be used
with every bunch, it yields significant
improvements in average flux over laser slicing.
Schönhense et al., used this special mode in a
field-driven pump-probe PEEM experiment on
micro-scale NiFe square and rectangular
structures[ 163 ], obtaining a time resolution in
imaging of 15 ps [ 164 ]. These experiments
revealed several transient features of the
magnetization process, including blocked domains
and transient domain walls and vortices. As in the
vortex studies of Guslienko et al. [105] and Cheng
et al. [107, 108], these states persist for <1 ns, and
do not exist in quasistatic magnetization processes.
The low- mode requires very low charge per
bunch, therefore in order to obtain the high
average flux needed for the PEEM experiments
and other user experiments, the repeat rate is kept
high, and the bunch spacing short. This limits the
total time available to follow magnetization
processes before the next pump pulse comes. Thus
the low- mode offers improved time resolution,
but has a limited dynamic time range. Another
proposed scheme for bunch manipulation may
offer very high time resolution, high average flux,
and repeat rates that approach normal synchrotron
operating modes. In this idea, a special rf cavity is
used to modify the momentum profile of the bunch
prior to their entry into an undulator, such that the
emitted photons from the upstream and
downstream ends of the bunch diverge [165]. A
horizontal slit is then used to take a time slice out
of each bunch, and a second cavity downstream of
the undulator restores the particle beam
characteristics, as shown in figure 27. This
technique can, in principle, be used at high repeat
rates with cryogenic cavities, while the storage
ring may be operated in a standard configuration.
It therefore offers potentially high average flux
with no impact to other users at the facility. This
approach has not been implemented at any facility
as of this writing, but is proposed for the
Advanced Photon Source in an upgrade plan
intended for completion in 2017. Estimates for the
proposed APS short pulse x-ray source (SPX)
suggest that pulse lengths of ~1 ps can be obtained
for hard x-rays, and ~4 ps for soft x-rays, with
~108 ph/s/0.1% bandwidth [ 166 ]. This type of
source would be very useful for studying sub-10
ps magnetization phenomena, as occur in
semiconductor spin dynamics and laser-driven
pump-probe experiments.
Figure 27. Bunch-deflection scheme of Zholents
[165] for generation of short x-ray pulses under
consideration at the Advanced Photon Source.
5.3. Fourth generation x-ray sources
For improvements in time resolution beyond
1 ps, a move beyond standard storage-ring based
x-ray sources is necessary. The most widely
implemented approach to this so far is the freeelectron laser (FEL), which produces ultrafast
pulses with very high peak flux, although usually
with low repeat rates. Two such facilities currently
exist, the Free Electron Laser in Hamburg
(FLASH) soft x-ray FEL at Hasylab, Hamburg,
Germany167, and the Linac Coherent Light Source
(LCLS) hard x-ray source at the Stanford Linear
Collider (SLAC) in California, USA 168 . Both of
these facilities provide 1012 – 1013 photons/pulse
(equivalent to ~ 10 s, or 107 pulses, of operation at
a typical storage ring) with pulse durations as low
as 10-50 fs. The photon energies available are
from the UV to 180 eV at FLASH, and 800 eV to
8.3 keV at LCLS as of this writing. In future,
LCLS may be able to provide energies down to
500 eV.
27
The available energy range of LCLS covers,
or will cover, most of the important edges in
elements important for magnetism. Therefore,
although the available polarization is currently
limited to linear, it is worthwhile to briefly
consider how a hypothetical circularly polarized
source at this facility would operate and contribute
to magnetism research. The average flux at LCLS
at 120 Hz is ~1015photons/s/1% bandwidth, an
improvement of only 10x over 3rd generation
storage rings. However, the pulse duration and the
peak flux allow this source to access new regimes
and new physics that are unattainable at storage
ring sources.
The most obvious advancement comes form
the ultrashort pulse length, as low as 10 fs
(FWHM). Absorption spectroscopies at this time
scale would enable, for the first time,
magnetization dynamics of sub-ps processes. This
would then allow probes of how atomic and
molecular transitions couple to the magnetization.
The coupling of ultrafast laser fields has been
shown to induce dynamics in the sub-ps time scale
in certain oxides159 which are not fully understood.
The ability to image at less than 100 fs would
undoubtedly produce outstanding results using the
same pump probe approaches as needed at
synchrotrons, and lead to new magnetization
physics at new time scales. However, the most
significant advancement from using such a source
will likely be the ability to go beyond stroboscopic
techniques. The flux per pulse of the LCLS is
equivalent to ~10 – 50 s of beamtime at a state-ofthe-art, 3rd generation soft x-ray synchrotron
beamline. A typical PEEM exposure time may be
10 s to 5 min, therefore the FEL is the first source
available that is able to form a useable PEEM
image from a single pulse. This removes a major
restriction of pump-probe imaging: its insensitivity
to any stochastic domain behavior.
Because stroboscopic imaging averages
several tens of millions of pulses, any behavior
that does not happen the same way with extremely
high repeatability will be missed. For most device
applications, non-reproducible behavior is of
course undesired, so pump-probe techniques
should work well. However, it is important to
understand the non-repeatable behaviors, as they
may appear unexpectedly in certain geometries, or
they may be an unavoidable consequence of the
geometry or the driving excitation. For example,
in the motion of a domain wall along a narrow
nanowire above the Walker limit, the complex
vortex-oscillation driven motion may have a
strong stochastic component. In general, the
motion of a magnetic vortex above its critical
velocity is characterized by a high degree of core
distortion, the formation of vortex-antivortex
pairs, and random core polarity reversals.
While a PEEM experiment at an FEL facility
would offer the ability to probe these stochastic
dynamical phenomena in a new way, it would be
subject to the same constraints that other FEL
experiments have come up against. The first is that
the peak power in each pulse is extremely high,
and may result in extensive damage to the sample,
especially for insulating samples. Therefore, rather
than an ensemble of experiments on a single
sample, a typical FEL experiment may involve a
series of single shot experiments on an ensemble
of samples. This brings in new considerations for
creating reproducibility across samples, to allow
for systematic comparisons to be made.
6. Summary
In the field of nanomagnetism, the important
length scales can range from the exchange length
(of order 10 nm) to the characteristic domain size
(several microns or more). Dynamical behavior
occurs at very fast timescales, ranging from sub-ps
to microseconds. Therefore, progress in this area
depends strongly on characterization techniques,
especially direct magnetic imaging, that offer high
sensitivity, high spatial resolution, and high time
resolution.
Using
photoemission
electron
microscopy with excitation by circularly polarized
soft x-ray sources, images with 100 nm spatial
resolution and very high magnetic sensitivity can
be obtained by tuning to an appropriate absorption
resonance. At the same time, the timing structure
of synchrotron sources is often ideal for
magnetization dynamics, enabling experiments in
a pump-probe mode. Consequently, PEEM
experiments have made many important
contributions to the field of nanomagnetism over
the last 20 years.
Earlier PEEM magnetic imaging experiments
concentrated on understanding the relative
magnetic order in several layered systems, in
which multiple chemically distinct magnetic
components are present. Here, the element
28
specificity inherent with soft x-ray excitation
provides definitive information about the spin
configuration, and therefore robust models of the
interfacial spin coupling can be developed and
tested. In addition to sensitivity to ferromagnetic
layers, X-PEEM provides sensitivity to
antiferromagnetic order in oxides and other
localized electron systems via directional coupling
of incident photons to multiplet states. Such
experiments have been very instructive in
examining the domain relation between
antiferromagnetic and ferromagnetic components
in exchange biased systems. These may also be
correlated with uncompensated interfacial spins
for a more complete understanding of the
interfacial spin structure. The orbital sensitivity
obtained with X-PEEM also provides directional
information, making it extremely useful for
exploring magnetic anisotropies.
due to higher applied field requirements.
Improvements in the spatial resolution of PEEM
through aberration correction optics have proved
to be challenging, and will likely remain a very
specialized technique. However, many prospects
exist for improvements to the achievable time
resolution through various bunch compression or
slicing schemes. While laser-slicing techniques are
likely too flux-limited to use with a PEEM, several
storage rings are operating or commissioning lowalpha modes, in which the pulse length is
shortened to several picoseconds, at the expense of
some flux. At the Advanced Photon Source, the
bunch deflection scheme of Zholents is under
consideration, which would achieve a pulse
duration of 1-2 ps. Finally, free electron lasers are
becoming available in the soft x-ray regime (e.g.,
LCLS and FLASH), which offer the potential for
sub-100 fs pulses, and single-shot imaging.
Another very useful aspect of PEEM with
synchrotron excitation is the inherent time
resolution that may be obtained from the pulsed
nature of the x-ray source. At most 3rd generation
storage rings, the timing structure of the electron
bunches results in x-ray pulse durations of 50 –
100 ps, with ~2 – 150 ns in between bunches.
Given that important magnetization dynamics
processes occur in frequency ranges from MHz to
THz, this timing structure is uniquely suited for
time resolved imaging of magnetic nanostructures,
using a pump-probe approach. Time-resolved XPEEM has been applied to the dynamics of
magnetic vortices in small (1 μm-sized and larger)
lithographic structures. These experiments have
yielded important understanding of the vortex
fundamental gyrotropic mode frequency, its
relation to shape, and the influence of non-linear
effects on the vortex core trajectories. Time
resolved PEEM has also been applied to the
questions of domain wall motion in nanostructures
and nanowires, and magnetization reversal.
To date, most PEEM experiments on
magnetic structures have involved objects of
~500nm size and larger, owing to the typical
resolution of 100nm. In future, many magnetic
device applications will involve smaller
dimensions and faster dynamics that go beyond
the present limits of pump-probe PEEM. In
addition, the use of newer, more anisotropic
materials in such devices may pose a challenge
29
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