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Detailed summary of responses and changes:
Article
ID:
Title:
ROP/136737/REV
Studies of nanomagnetism using synchrotron-based x-ray photoemission electron microscopy
(X-PEEM)
First referee's report
Studies of nanomagnetism using synchrotron-based x-ray photoemission electron microscopy (X-PEEM)
By Cheng and Keavney
This review article gives an excellent overview of the strengths of XMCD-PEEM. A broad range of
experiments are discussed comprising a range of 0D, 1D, 2D and hybrid magnetic systems. The article is
well written and of benefit to both the novice and more expert reader.
Recommendations:
- The term nanomagnetism is generally used in a broader sense than for the purpose of this article. It may
include for example molecular magnets and various forms of magnetic particles. I suggest therefore
specifying the area of nanomagnetism in the Introduction.
Because X-PEEM is most suitable for imaging solid materials in a planar geometry, we naturally focus on
work involving patterned thin films. We do not mean to exclude other forms of nanomagnetism, but due to
the nature of the technique we’re reviewing, this is a necessary choice. We have added “especially to the
research of artificially nanoscaled magnetic materials of various dimensions,” in the beginning of the last
paragraph of the “introduction” section, to better specify the area we are covering.
- The classification of quasi-0D, 1D and 2D in section 2 is very appealing and worth considering to repeat
in the layout of section 4.
The layout of section 4 has been changed to be parallel with that of section 2, using the categories of quasi0D, 1D and 2D.
- The introduction to magnetism is rather brief for the non-specialist reader, and I’d suggest to expand the
introduction to include concepts as domain walls, soft versus hard magnetism, exchange length.
The following is added to page 4 about domain walls and the exchange length:
“A magnetic domain wall is the boundary between two adjacent domains. It is the transition region in
which spins gradually reorient from the direction in one domain to that in the next. The typical domain wall
width, the thickness of the transition range, in an unpatterned magnetic film is called the exchange length.
The term exchange length is also used for the typical size of a magnetization vortex core [Ultrathin
Magnetic Structures IV: Applications of Nanomagnetism, Volume 4 By B. Heinrich, J. Anthony C. Bland].”
The following is added to page 4 about soft and hard magnetism:
“Along the preferred magnetization direction, the field needed to magnetize the magnetic material to
saturation is smaller than any other directions, so this preferred direction is called the easy axis. Soft
magnetic materials are those in which the magnetization processes (domain wall motion and domain
magnetization rotation) occur in weak fields, while hard magnetic materials refer to the materials that
require high magnetic field for magnetization reversal [30].”
- Explain symbols in eq. 3.
“where M is the vortex mass tensor, G is the gyrovector, and W(X) is the energy of the vortex deflected
from the disk center.” is added
- At times arguments are repeated in too much detail throughout the paper, like the benefit of element
selectivity and spatial resolution.
The authors feel it is worthwhile to emphasize some of the important features of PEEM by repeating from
various perspectives.
- The end of section 3.2 is a general argument rather than specifically about time resolved studies, and is
much suited for introduction.
The section number should be 3.4 and it was corrected. The end paragraph is not for section 3.4, but for
the entire section 3(PEEM). It serves as a transition between sections 3 and 4. So we prefer to keep it at the
end of section 3.
Typos and minor suggestions:
- Add square dot in Fig. 2
A square dot picture has been added to Fig. 2
- Page 4: qusi-0D --> quasi-0D
We have fixed this error.
- Page 5: Explain race track memory
We have added the following text to the description of Figure 3: In this proposal, information is stored in
the form of magnetic domains in a permalloy of 100 – 200 nm width. The domains are moved through the
wire using the spin-torque effect by passing current pulses through the wire. This may be realized in a
planar (Fig. 3(a)) or vertical (Fig. 3 (b)) geometry. As the domains pass over a magnetoresistive detector
(most likely a magnetic tunnel junction), a domain-orientation-dependent signal is generated, allowing
readout of the information (Fig 3 (c)). Writing is accomplished by pulsing a current line under the bit to be
written, switching its orientation (Fig 3(d,e)). The result is a non-volatile shift register, with potentially
high storage density.
- Page 7: explain spin valve
We have edited this discussion as follows: In the mid-1990s, an exchange bias layer was used in GMRbased spin valve read head. These heads relied on the magnetoresistance arising from two coupled
ferromagnetic layers through a nonmagnetic spacer. Exchange biased layer were used to pin one layer,
while the free layer flipped in response to the stray field from the disk medium.
- Page 7: short-rang --> short-range
We have fixed this error.
- Page 14: Fig 10 (c)-(e) --> Fig 10 b.
We have fixed this error.
- Page 16: explain cross-tie domain
A reference for cross-tie domain wall to [29 O’Handley R C Modern Magnetic Materials Principles and
Applications (Wiley-Interscience Publication, New York, 2000).] is added.
- Figure 16 and discussion is unclear
The following explanation added to page 19:
“Before the current pulse, the magnetization is aligned along the wire and no domain walls are present,
resulting in an almost homogeneous XMCD intensity (figure 16 (a)). The x-ray beam direction is parallel to
the magnetization direction before the current pulses in the bending parts of the nanowire. During the
pulses, the NiFe magnetization tilts away from the wire direction. The tilt is counter-clockwise (figures 16
(b)-(f)) for the positive part of the bipolar current pulse and clockwise (figures 16 (g)-(i)) for the negative
part, as can be inferred from the magnetic contrast in the differently oriented sections of the wire. The tilt
angle jt extracted from the time-dependent XMCD intensity in the bending parts of the nanowire is plotted
in figure 16 (j). This is expected because the Oersted field generated by the current pulse is in opposite
directions transverse to the wire for opposite current directions. The oscillations in jt at the beginning of
the positive and negative parts of the pulse indicate magnetization precession about the effective Oersted
field.”
- Page 28: repeat of FEL, Hamburg
These are not actually redundant, one is the name of facility (FLASH), one is the location (Hamburg). For
a little beter readability, we reworded this to: “Two such facilities currently exist, the soft x-ray Free
Electron Laser in Hamburg (FLASH) at Hasylab, Hamburg, Germany,…”
Article
ID:
Title:
ROP/136737/REV
Studies of nanomagnetism using synchrotron-based x-ray photoemission electron microscopy
(X-PEEM)
Second referee's report
Studies of nanomagnetism using synchrotron-based x-ray photoemission electron microscopy (X-PEEM)
by X M Cheng1 and D J Keavney2*
The manuscript represents a well though out summary of the work in the field of nanomagnetism as studied
by PEEM.
The only general comment is that I would prefer more extended discussion on some of the topics covered.
On number of places the statements are given without description of how they are obtained or what they are
based on. In number of topics one is forced to go to the original manuscript to fully understand the
conclusions. Clearly, many of the studies summarized are complex and require more explanation to be fully
comprehended.
Overall, the manuscript represent a comprehensive summary of the state of the field that will be useful
reference and should be published in the Reports on Progress in Physics after some minor issues listed
below are addressed.
page. 9
in the sentence below the “figure 7” should be Fig. 1 (d):
“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),”
We have fixed this error.
page11.
“may be measure” should read may be measured
We have fixed this error.
figure 8 needs to be more clear, what are the Auger electrons and what are the secondary electrons
mentioned in the text, needs better description
We have added the following to the discussion of Fig 8: These Auger electrons typically scatter
before escaping from the surface of the sample, resulting in secondary electrons.
the difficulty with K-edge PEEM magnetic imaging is not due to low flux but rather to very small XMCD
effect
We do not make the claim that K edge PEEM is difficult due to low incident flux alone, rather that oxygen
K, transition metal K, and semiconductor L edges are difficult due to low XMCD signal. At TM K edges in
particular, the absorption cross sections are smaller, resulting in a smaller interaction volume, and the
electron yield relative to fluorescence yield is smaller. However, in principle these low signals could be
overcome with greater flux, so there is some validity to the statement that PEEM at these edges is flux
limited. We have reworded this sentence slightly to reflect this point: “For PEEM imaging, however, the
small XMCD signals inherent at these transitions makes it difficult to obtain sufficient signal to noise with
the photon flux density presently available at typical sources.”
page 12
obtaining magnetic PEEM images by dividing L3 with L2 image contains other complexities that need to
be mentioned
We have added the following to the discussion of image generation: “However, obtaining magnetic PEEM
images by dividing L3 with L2 image doers not completely remove the charge contrast, therefore adding
complexity to the normalization of the images.”
need better description of XMLD, what is “average electric field vector” ?
“XMLD effect is only
sensitive to the angle between the atomic moments
and the average electric field vector”
We have edited this discussion and changed “average electric field vector” to “photon electric field
vector”, and added θ to the definition of the angle between the magnetization and the electric field vector.
This should make more clear the quantities being discussed.
page 13
explain why ~100ps and ~100nm or less are of interest:
“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).”
We have reworded this to read: “To make the best use of the available time resolution, it is
desirable to match the speed of the pump to that of the probe (~100ps).”
“because time-resolved X-PEEM has the
advantages of both the high spatial resolution and
the high temporal resolution.”
What are the disadvantages of X-PEEM, are there other time-and-space resoving imaging techniques and
how do they compare to X-PEEM ?
It is unclear where the referee is taking this quote from, however have added a short discussion of PEEM
vs Ker microscopyr and Transmission X-ray Microscopy to the previous section (3.1):
An additional characteristic of X-PEEM is the high surface sensitivity that is inherent with the detection of
secondary electrons. In most materials, the low kinetic energy of the secondaries (typically <10 eV) limits
the electron escape depth to 2 – 5 nm. This can be an advantage in some cases, however it can also limit
the range of samples that can be studied and introduce uncertainties due to incomplete knowledge of the
surface condition. If thick capping layers are present, it may be impossible to obtain a usable signal from
the later of interest. Conversely, if the sample is not capped but a thick oxidation layer has formed, this can
reduce the magnetic contrast dramatically.
Other magnetic imaging techniques can get around some of these problems, even in time-resolved imaging,
although with some drawbacks. Kerr microscopy offers very high sensitivity and probing depths of ~50 nm
or greater. However, it lacks element selectivity and the spatial resolution is limited by the diffraction in
the visible light optics, so imaging in the sub-micron regime is very difficult. Transmission x-ray
microscopy can be accomplished in the soft x-ray regime, offering high sensitivity to the entire thickness of
the sample and element selectivity. In addition, the spatial resolution is superior to non-aberrationcorrected PEEM, with a current state of the art of 15 nm as of this writing [add ref to H. Jung et al, APL
97, 222502 (2010).]. The principle disadvantage here is that the entire sample must be thin enough to
allow the soft x-ray beam through, or typically in the range of only 100 nm. This requires that the samples
be grown on special membranes, which are fragile and may preclude samples that require high deposition
temperatures or other processing.
Page 19
“demonstrated that the
exchange bias occurred on a domain-by-domain
basis, confirming its microscopic origins”
Explain how did they conclude this, explain what is domain-by-domain exchange bias ?
We have replaced the above sentence by “As shown in figure 17, the magnetic contrast in figure 17 (a)
arises from antiferromagnetic domains in LaFeO3 with an in-plane projection of the antiferromagnetic
axis oriented horizontally (light) and vertically (dark). The three distinct grey scales in figure 17 (b)
corresponds to ferromagnetic domains in Co aligned vertically up (black) and down (white), and
horizontally left or right (grey). Comparison of the in-plane projections of the antiferromagnetic axis and
the ferromagnetic spin direction in each individual domain shows that the ferromagnetic Co spins are
aligned parallel or anti-parallel to the in-plane projection of the antiferromagnetic axis.”
page. 20
the XMLD signal is anisotropic by definition, it would be better to say “the sign of the XMLD signal”
instead of “the anisotropy of XMLD signal”. Preferably, it requires more detailed statement on what is
actually meant here as indicated at the end of the paragraph but not related to term “the anisotropy of
XMLD”
We have reworded “the anisotropy of XMLD signal” to “the dependence of the XMLD signal on the
relative orientation of polarization, magnetic moments, and crystallographic axes”, “the angular
dependence of the XMLD signal”, or “the dependence of the XMLD signal on the experiment geometry”
remove “the” in :
“uncompensated Mn spins and the at the
CoFeB/MnIr interface”
We have fixed this error.
Page. 22
Section on “Studies of spin reorientation transitions in coupled magnetic multilayers” is confusing,
especially the Figure 20, the text sates that the Ni moment does not change in magnitude but figures shows
different shades of Ni spin moment ?
The contrast of the PEEM image reveals the component of the magnetization projected to the x-ray
propagation direction, which is determine by the magnetic moment and the angle between the
magnetization in each domain and the x-ray direction. To make it clearer,we have reworded the Ni spin
moment part as follows: “Taking into account the different grey scales in figure 20 (a), representing
projections of the different in-plane and perpendicular spin moments in different domains, and the spin
orientations in these domains, the authors calculated a constant effective Ni spin moment of about 0.65 μB,
similar to the Ni bulk magnetic moment.”
Page. 23
What is the meaning of white and black horizontal/vertical arrows at the bottom of the Fig.22 ?
“The grey scales of the PEEM images indicate different magnetization directions: parallel (white), antiparallel (black), or perpendicular (grey) to the x-ray propagation direction.” is added to the caption of
figure 22.
Pg.25
It would be useful to state what is the “low-energy states” configuration in these structures
Also the label of “out-of-plane” and “in-plane” axis is confusing as H appears to be allied in plane in both
cases.
“The low energy state for the one- ring kagome structure is the vortex state; the low energy states for the
two-ring kagome structure include the double-vortex state and the external-flux-closure state; and the low
energy states for the three-ring kagome structure include the ground state with two vortices and the
external-flux-closure state [86].” is added to explain the “low-energy states”
The “out-of-plane” and “in-plane” axis refer to the sample rotation axis in the demagnetization process,
not the applied field direction, as specified in the figure caption for figure 25 “(a) Geometries for
demagnetization with in-plane and out - of- plane rotation axes”
Pg.26
When using brighter synchrotron sources (4th generation) are there any concerns of space charge
limitation, there have been some discussion on the topic in the recent literature.
Space charge effects should not be a concern for the storage ring based short pulse generating schemes
discussed in this section, as they do not involve higher peak flux than a standard 3 rd generation source. For
FELs, this may be a concern, so we discuss that in section 5.3, as indicated below.
The statement “In most cases, the effective limit for the resolution is 2-5 nm.” should be supported with a
reference or/and argument on which this is based.
It is based on the mean free path of electrons in the material. We have edited last sentence to
make more this clear: “In most cases, this limit for the resolution will therefore be 2-5 nm.”
It would be informative to a reader what is the state of SMART and PEEM-3 project and what their target
resolution.
Added the following text to the end of section 5.1: The performance goal of the SMART PEEM/LEEM
instrument is 2 nm spatial resolution, with 2.6 nm recently being demonstrated in LEEM mode [add ref to
Th. Schmidt, et al., Ultramicroscopy 110, 1358 (2010)]. The PEEM-3 design goal is ~5 nm, currently it
operates at 50 nm or better without aberration correction, with the addition of the correcting mirror
planned for the future.
Pg. 29
What about space charge limitation on PEEM experiments with FEL sources ?
Since there have been no PEEM experiments at FELs so far, there is no work to reference here. However,
there has been work using ultrafast UV laser pumped PEEM, which is relevant to potential FEL
experiments. We have added the following discussion on this:
“An additional complication that can potentially arise from the very high instantaneous flux densities
present at FEL sources is image degradation due to space charge effects. Coulomb repulsion within the
cloud of emitted photoelectrons will broaden the cloud in real space and in energy distribution. This leads
to greater chromatic aberrations and general image blurring. The degree of spatial resolution degradation
will depend on the peak flux, and naturally the threshold where these effects become important will depend
on the baseline spectral resolution. In aberration corrected PEEMs, these effects will be of much more
concern.
Studies of space charge effects in PEEM using 4th generation soft x-ray sources have not been conducted
as of this writing, however these phenomena have been explored using ultrafast UV lasers. Recently,
Buckanie, et al. [Buckanie N M, Göhre J, Zhou P, von der Linde D, Horn-von Hoegen M, Meyer zu
Heringdorf F-J 2009 Space charge effects in photoemission electron microscopy using amplified
femtosecond laser pulses J. Phys.: Condens. Matter 21 314003] used 200 and 800-nm femtosecond laser
pulses to examine both the threshold of image degradation of a Ag island and the location in the electron
microscope column where space charge effects are most critical. They found that for 200 nm photons,
which are above the work function of the Ag island, significant energy broadening begins to occur at ~3μW
average laser fluence, corresponding to ~3 × 107 photons/pulse. Therefore, given the 1012 – 1013
photons/pulse available at an FEL, it is likely that space charge would present an even greater problem for
PEEM experiments than sample damage.