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Electric field tunability of microwave soft magnetic properties of Co2FeAl Heusler alloy
film
Shandong Li, Jie Xu, Qian Xue, Honglei Du, Qiang Li, Caiyun Chen, Ru Yang, Shiming Xie, Ming Liu, Tianxiang
Nan, Nian X. Sun, and Weiquan Shao
Citation: Journal of Applied Physics 117, 17B722 (2015); doi: 10.1063/1.4916112
View online: http://dx.doi.org/10.1063/1.4916112
View Table of Contents: http://scitation.aip.org/content/aip/journal/jap/117/17?ver=pdfcov
Published by the AIP Publishing
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JOURNAL OF APPLIED PHYSICS 117, 17B722 (2015)
Electric field tunability of microwave soft magnetic properties of Co2FeAl
Heusler alloy film
Shandong Li,1,a) Jie Xu,1 Qian Xue,1 Honglei Du,1 Qiang Li,1 Caiyun Chen,1 Ru Yang,1
Shiming Xie,1 Ming Liu,2 Tianxiang Nan,3 Nian X. Sun,3 and Weiquan Shao1
1
College of Physics Science, Key Laboratory of Photonics Materials and Technology in Universities of
Shandong, and Laboratory of Fiber Materials and Modern Textile, The Growing Base for State Key
Laboratory, Qingdao University, Qingdao 266071, China
2
Electronic Materials Research Laboratory, Key Laboratory of the Ministry of Education & International
Center for Dielectric Research, Xi’an Jiaotong University, Xi’an 710049, China
3
Electrical and Computer Engineering Department, Northeastern University, Boston,
Massachusetts 02115, USA
(Presented 4 November 2014; received 6 September 2014; accepted 15 November 2014; published
online 24 March 2015)
Co2FeAl Heusler alloy film with 100 nm in thickness was sputtered on (011)-cut lead zinc
niobate-lead titanate (PZN-PT) single crystal slabs. It was revealed that this multiferroic laminate
shows very large electric field (E-field) tunability of microwave soft magnetic properties. With the
increase of electric field from 0 to 8 kV/cm on PZN-PT, the anisotropy field, HK, of the Co2FeAl
film along [100] direction of PZN-PT is dramatically enhanced from 65 to 570 Oe due to the strong
magnetoelectric (ME) coupling between ferromagnetic Co2FeAl film and ferroelectric substrate. At
the same time, the damping constant a of Co2FeAl film dramatically decreases from 0.20 to 0.029.
As a result, a significantly shift of self-biased ferromagnetic resonance frequency, fFMR, from
1.86 to 6.68 GHz with increment of 3.6 times was obtained. These features demonstrate that
Co2FeAl/PZN-PT multiferroic laminate is promising in fabrication of E-field tunable microwave
C 2015 AIP Publishing LLC. [http://dx.doi.org/10.1063/1.4916112]
components. V
I. INTRODUCTION
Recently, multiferroic composite materials have drawn
increasing attention due to that the magnetoelectric (ME)
coupling structures offer an E-field manipulation of magnetic
properties (converse ME effect) or vice versa (direct ME
effect).1–4 The ME coupling gives rise to many novel multiferroic materials and devices. Of them, one important branch
is electrostatically tunable ferromagnetic/ferroelectric
composites, which led to many tunable devices such as a
picotesla sensitivity magnetometer filters,5 resonators, phase
shifters, etc.6–9 Comparing with the conventional magnetic
field tuned microwave magnetic devices, the electrostatically
tunable microwave multiferroic devices exhibit some advantages, such as more energy efficient, compact, lightweight,
and less noisy, etc.10
The ME coupling strength in multiferroic composites is
determined by many factors, such as piezoelectric/magnetoelastic parameters of the ferroic/magnetic phases, the interface interactions, the ME coupling mode, and the orientation
of the magnetic and electric fields. The strain/stress-mediated
multiferroic composites have effective energy transfer
between electric and magnetic fields; however, at microwave
frequencies, the strong ME coupling is difficult to be
achieved due to the large loss tangents of ferroic/magnetic
phases, leading to a very limited tunability in electrostatically tunable microwave multiferroic devices. The typical
a)
Author to whom correspondence should be addressed. Electronic
addresses: [email protected] and [email protected].
0021-8979/2015/117(17)/17B722/4/$30.00
tunable frequency range is less than 150 MHz, and the tunable magnetic field less than 50 Oe.8,11,12 Layered multiferroic
heterostructures with magnetic thin films give rise to great
opportunities for obtaining strong ME coupling at microwave frequencies due to improved interfaces, minimized
charge leakage of ferroelectric materials, and low loss
tangents of magnetic thin films.10,13 Heusler alloys, such as
NiMnSb, Co2FeAl, Co2MnSi, Co2FeSi, exhibit low coercivity HC, low damping constant a, and high anisotropy field,
HK, due to their itinerant-electron characteristics, showing a
potential high-frequency ferromagnetic properties.14–16
(011)-cut PZN-PT single-crystal slabs with 6% lead titanate
have high anisotropic piezoelectric coefficients.17 Therefore,
in this study, Co2FeAl Heusler alloy films as ferromagnetic
films were deposited on (011)-cut single-crystal PZN-PT
substrates to explore the ME effect in the layered ferromagnetic/ferroelectric multiferroic heterostructure.
II. EXPERIMENTAL PROCEDURE
The 100-nm Co2FeAl Heusler alloy films were deposited on PZN-PT substrates at room temperature under
2.8 mTorr Ar atmosphere with a floating rate of 20 sccm,
along with a RF power of 80 W for Co2FeAl target. The
(011)-cut single crystal PZN-PT substrates with a dimension
of 5 mm[100] 5 mm[01-1] 0.5 mm[011] have been
pasted on the sample turntable with their [100] direction
along the radial (R) direction. The magnetic properties were
measured by a vibrating sample magnetometer (VSM). The
microwave frequency performances of the multiferroic
117, 17B722-1
C 2015 AIP Publishing LLC
V
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17B722-2
Li et al.
J. Appl. Phys. 117, 17B722 (2015)
FIG. 1. The E-field dependence of hysteresis loops for the Co2FeAl/PZN-PT
multiferroic laminates along [100] and
[01-1] directions, respectively. (a) The
as-deposited Co2FeAl film at 0 kV/cm,
showing isotropic loops along [100]
and [01-1] directions of PZN-PT substrate; (b) at 8 kV/cm, showing a welldefined magnetic anisotropy induced
by E-field; and (c) and (d) the E-field
dependent loops along [01-1] and
[100] directions, respectively.
composites were evaluated by use of a vector network analyzer with co-planar waveguide fixture.
III. RESULTS AND DISCUSSION
Figure 1 shows the E-field dependence of hysteresis
loops for the Co2FeAl/PZN-PT multiferroic laminates along
[100] and [01-1] directions, respectively. As illustrated in
Fig. 1(a), the as-deposited Co2FeAl film does not exhibit
detectable magnetic anisotropy; however, when an external
E-field is applied on the [011] direction of PZN-PT substrate,
a well-fined uniaxial magnetic anisotropy was formed due to
the ME coupling [see Fig. 1(b)]. The E-field not only enhances the squareness ratio of film along the easy axis direction
of [01-1] [see Fig. 1(c)] but also dramatically increases the
magnetic anisotropy field, HK, along hard axis of [100]. As
illustrated in Fig. 1(d), the E-field dependent magnetic
anisotropy field, HK, dramatically increases from 65 Oe at
0 kV/cm to 570 Oe at 8 kV/cm along the HA [100] direction.
A large ME coupling coefficient of 63.1 Oe cm/kV is
obtained in Co2FeAl/PZN-PT multiferroic laminates. The
ferromagnetic resonance frequency, fFMR, of ferromagnetic
films can be expressed by Kittle equation,
fFMR ¼
c pffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi
HK ðHK þ 4pMS Þ;
2p
Figure 2 shows the E-field dependence of permeability
for the Co2FeAl/PZN-PT multiferroic laminates. As
expected, the ferromagnetic resonance frequency was dramatically shifted from 1.86 to 6.68 GHz, when the E-field
increased from 0 to 8 kV/cm. The fFMR increases by 3.6
times due to the ME coupling via the interface between
Co2FeAl film PZN-PT substrate. A large fFMR enhancement
ratio of 602.5 MHz cm/kV was achieved.
This E-field-induced fFMR shift can be explained by the
strain/stress-mediated electrostatic-field-induced in-plane
magnetic anisotropy field Heff. For the Co2FeAl/PZN-PT
multiferroic laminates, the Kittel equation [Eq. (1)] can be
rewritten as
fFMR ¼
c pffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi
ðHK þ Heff Þ ðHK þ Heff þ 4pMS Þ;
2p
(2)
(1)
c
2.8 MHz/Oe), HK is
where c is the gyromagnetic ratio (2p
the anisotropic field in plane, and 4pMS is the saturation
magnetization of ferromagnetic films. From Eq. (1), it can be
concluded that large enhancement of HK manipulated by
E-field will give rise to a large upward shift of fFMR. In other
words, the ferromagnetic resonance of Co2FeAl films will be
driven to a high frequency by E-field.
FIG. 2. The E-field dependence of permeability for the Co2FeAl/PZN-PT
multiferroic laminates.
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Li et al.
J. Appl. Phys. 117, 17B722 (2015)
where Heff is the orthogonal in-plane compressive and tensile
stress corporately induced internal effective magnetic field,
which could be positive or negative, and in our case it can be
expressed as
Heff ¼
3kY
ðd31 d32 ÞE;
M S ð1 þ Þ
(3)
where Y is the Young’s Modulus, is the Poisson’s ratio, k
is the magnetostriction constant, d31 ¼ 3000 pC/N along
[100] and d32 ¼ 1100 pC/N along [01-1] are linear anisotropic piezoelectric coefficients of PZN-PT, and E is the
applied external E-field strength. Heff is the effective magnetic anisotropy field under various E-fields.18 From Eqs. (3)
and (2), it can be concluded that the E-field will give rise to
an increase of Heff via ME coupling, and, therefore, drive the
ferromagnetic resonance upwards shifting to a high
frequency.
The free-energy density, Etotal, in the studied system
includes the Zeeman energy, Ezeeman , the demagnetization
energy; Eshape , the stress energy, Estress , and the uniaxial anisotropy energy, Euni . It can be written as
Etotal ¼ Ezeeman þ Eshape þ Estress þ Euni :
(4)
For the studied system, the main interaction between the
magnetic film and PZN-PT substrate occurs at the interface
(i.e., in-plane), so the contribution of demagnetization
energy can be ignored. In the case of a certain magnetic field,
the variation of total free-energy density is dominated by the
competition and transformation between Estress and Euni. The
uniaxial magnetic anisotropy of magnetic film is induced by
E-field tunable stress, and the uniaxial magnetic energy can
be manipulated by E-field via the magnetoelectric coupling.
Therefore, the anisotropy field exhibits an E-field tunability,
e.g., HK increases with the increase of E-field.
Figure 3 shows the relationship between magnetic anisotropy field, HK, and ferromagnetic resonance frequency,
fFMR, as well as the damping constant a, at various E-fields.
The Gilbert damping constant, a, represents magnetic loss of
the Co2FeAl magnetic film, which is deduced by fitting the
permeability spectra, as shown in Fig. 2, using LandauLifshitz-Gilbert equation. As illustrated in Fig. 3(a), the fFMR
and HK increase almost linearly with the E-field, and the Efield dependence of HK and fFMR exhibits a same trend at
various E-fields. These facts demonstrated that the E-field
induced HK dominates the ferromagnetic resonance frequency, as described in Eqs. (1)–(3).
On the other hand, the damping constant, a, dramatically
reduces from 0.20 to ca. 0.029 with the increase of E-field
from 0 to 8 kV/cm, indicating that the magnetic loss of the
multiferroic composites effectively decreases by the E-field.
It is excited to note that the ME coupling in the studied multiferroic laminates not only enhances the ferromagnetic resonance frequency but also reduces the magnetic loss at
microwave frequencies. This merit of the multiferroic composites gives great opportunity in practical application.
The intrinsic stress, in general, is randomly distributed
in the magnetic films, which give rise to a high damping
FIG. 3. The E-field dependence of (a) magnetic anisotropy field, HK, and ferromagnetic resonance frequency, fFMR, and (b) Gilbert damping constant a.
constant and decreasing resonant frequency. The investigated Co2FeAl film at zero E-field exhibits bad ferromagnetic resonance characteristics due to the magnetic isotropy
of as-deposited Co2FeAl film. As illustrated in Figs. 1, 2 and
3(b), the Co2FeAl film shows a higher coercivity of 65 Oe,
lower fFMR of 1.86 GHz with broad resonance peak, and very
large damping constant, a, of 0.20. The large and random
distributed intrinsic stress is responsible for the bad microwave performance of the Co2FeAl film at E ¼ 0 kV/cm.
However, if the E-field was applied on the PZN-PT substrate, a biaxial anisotropy stress, compressive stress along
[100] direction and tensile one along [01-1] direction of the
PZN-PT substrate will be induced by E-field.19 According to
magnetoelastic energy equation, EK ¼ 32 kS r cos 2 h, for a
positive kS (as the case in this study for the Co2FeAl films),
a compressive stress will drive the magnetic moments to be
aligned parallel to [01-1] direction of PZN-PT substrate.
Similarly, the tensile stress also leads the magnetic moments
along [01-1] direction. As a result, a uniaxial magnetic anisotropy with the magnetically easy axis along [01-1] direction is formed due to the ME coupling effect. As reported in
our previous work,20,21 if the biaxial stress induced uniaxial
magnetic anisotropy is formed in the ferromagnetic films,
the enhanced ferromagnetic resonance and improved microwave magnetic properties will be obtained due to the alignment of magnetic moments. As illustrated in Figs. 1–3, with
the increase of E-field, the coercivity and damping constant
reduce, and the HK and fFMR dramatically increase.
IV. CONCLUSIONS
Strong magnetoelectric coupling effect, corresponding
to a large magnetoelectric coefficient of 63.1 Oe cm/kV, was
observed in Heusler alloy Co2FeAl/PZN-PT multiferroic
heterostructure, leading to continuously E-field tunable
microwave frequency characteristics with fFMR upwards
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17B722-4
Li et al.
shifting from 1.86 to 6.68 GHz, i.e., an increment of
DfFMR ¼ 4.82 GHz or increment ratio of DfFMR/fFMR ¼ 256%
in E-field range of 0–8 kV/cm. The magnetoelectric coupling
between Co2FeAl film and PZN-PT substrate not only
enhances the ferromagnetic resonance frequency but also
reduces the magnetic loss at microwave frequencies, which
gives great opportunity in practical application of the multiferroic composites.
ACKNOWLEDGMENTS
This work was financially supported by National Nature
Science Foundation of China with Grant No. 11074040 and
Key Project of Nature Science Foundation of Shandong
Province with Grant No. ZR2012FZ006.
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