J. Magn. Soc. Jpn., 41, 46-51 (2017)<Review>

Control of Magnetization Dynamics in Ordered Alloy Systems
T. Seki*,**,***, W. Zhou*, T. Yamamoto*, and K. Takanashi*,****

Institute for Materials Research, Tohoku University, Sendai 980-8577, Japan*****

PRESTO, Japan Science and Technology Agency, Saitama 322-0012, Japan

Center for Spintronics Research Network, Tohoku University, Sendai 980-8577, Japan

L10-type alloys and L21-type Heusler alloys are key materials for future spintronic and magnetic storage devices.
L10-FePt with high uniaxial magnetic anisotropy is a promising material for ultrahigh density recording because of
its high thermal stability of magnetization at a nanometer scale. Co-based Heusler alloys showing high spinpolarization of conduction electrons enable us to enhance the magnetoresistance effect. In addition, utilizing themagnetization dynamics in these ordered alloys provides us with new paths in the development of spintronic andmagnetic storage devices. In this review, we introduce the control of magnetization switching field for L10-FePt
exchange-coupled with Ni81Fe19 by utilizing the spin waves in the bilayers, which will be useful for information
writing. A nanometer-scaled rf oscillator is also introduced, in which the magnetization dynamics is excited by spin
angular momentum transfer. We can improve both the rf output power and the oscillation quality simultaneously byusing Co2(Fe0.4Mn0.6)Si Heusler alloy.
Key words: L10-type ordered alloy, Heusler alloy, magnetization dynamics,
1. Introduction

We introduce our study on “Control of Magnetization
Dynamics in Ordered Alloy Systems” in this review
paper, which was a research subject of MSJ
Outstanding Research Award 2016.
Rapid progress in electronic devices enables us toaccess various information everywhere, and theaccessibility to a large amount of information makes ourlife comfortable and convenient. Among many kinds ofelectronic devices, magnetic and spintronic devices playsignificant roles in data storage technologies. Those
devices exploit the direction of magnetization in a small
magnet as an information bit, leading to thenon-volatile data storage without external power supply.
Since ultrahigh density data storage technologies are
essential to manage “Big Data”, one has highly desired
the further development in spintronic and magnetic
storage devices. In particular, it is important tosimultaneously achieve ultrahigh density storage andultralow power consumption for the device operation.
In addition to the existing data storage devices
mentioned above, various spin-related phenomena haveprovided new concepts for the development of spintronic
devices. For example, magnetization dynamics induced

Fig. 1 Schematic illustrations of crystal structuresfor (a) L10-ordered and (b) L21-ordered alloys.

46

by the torque due to spin angular momentum transfer,which is called spin torque, have recently been foundand demonstrated in nano-scaled devices 1). That has
opened a way to give novel functionalities to spintronic
devices. From the viewpoint of practical applications,however, the significant improvement of device
performance is still required.
This review paper is devoted to two research subjects:
one is the simultaneous achievement of ultrahighdensity storage and ultralow power consumption fordevice operation. The other is the improvement of
performance for spin torque devices, especially for aspin torque oscillator (STO) 2). Key materials for theabove research subjects are L10-type alloys and L21-type
Heusler alloys (Fig. 1), both of which are ordered alloys.
In addition to the ordered alloys, the magnetization
dynamics is crucial to solve the issues for the magnetic
storage and spintronic devices.
2. Spin-wave-assisted magnetization switching in
exchange-coupled systems having L10-FePt

Spintronic and magnetic storage devices face a serious
challenge in trying to simultaneously achieve ultrahighdensity storage and ultralow power consumption for
operation. In order to increase the storage density, thereduction of magnetic bit size is inevitable. A small
magnetic volume (V) gives rise to the instability ofmagnetic information due to the thermal energy (kBT,
kB is Boltzmann constant and T is temperature). Usinga material with high magnetic anisotropy (K) is one ofthe possible solutions for this problem. However, thehigh K material leads to another problem, which is thesignificant increase in the external energy required forthe information writing, i.e. magnetization switching.
This is a dilemma that all the magnetic storage devices
possess. A straightforward way to solve this dilemma is

Journal of the Magnetics Society of Japan Vol.41, No.3, 2017

that the switching field of a high K material is reduced
only when one needs to switch its magnetization.
L10-ordered FePt (L10-FePt) alloy 3) is a representativeof high K material, which exhibits the uniaxial
magnetic anisotropy (Ku) of 7 × 106 J/m3 along the c-axis
of face-centered tetragonal. Thanks to this high Ku,
L10-FePt is a candidate material for future ultrahighdensity magnetic recording media 4). As discussed above,
however, its high Ku also gives rise to the large
magnetization
switchingfield(Hsw).
Ifthemagnetization switching occurs through the coherent
rotation of magnetization 5), Hsw becomes equal toanisotropy field (Hani). Hani is given by 2 Ku / Ms, where
Ms is the saturation magnetization. Hsw is roughly
estimated to be larger than 100 kOe for fully
L10-ordered FePt. Thus, a method to efficiently reduce
Hsw for L10-FePt only during the information writing isneeded.
A promising method for the efficient Hsw reduction ismagnetization switching assisted by “external energy”
such as microwave energy or thermal energy that are
called microwave-assisted switching (MAS) 6,7) andheat-assisted switching 8). In the case of MAS,microwave is applied to a magnet in order to excite its
uniform magnetization precession mode, resulting in
the amplification of precession angle followed by themagnetization switching at a low magnetic field.
Although this MAS effectively reduces Hsw, theexcitation frequency for MAS will become close to the
sub-THz in case of high K materials such as L10-FePt
because the uniform magnetization precession
frequency is proportional to the effective field (Heff)
including Hani. From the practical point of view, thishigh frequency required for MAS is an obstacle todesign device structures. Instead of uniform
magnetization precession mode, we utilize spin wave
modes for an exchange-coupled bilayer system. In thefollowing,
we
introduce“spin-wave-assisted
magnetization switching method” 9-12), which enables us
to effectively reduce Hsw for L10-ordered FePt by
exciting the spin wave modes in a soft magnetic Ni81Fe19(Permalloy, Py).
Figures 2(a) and 2(b) schematically illustrate themagnetic structures in the exchange-coupled bilayer
consisting of the hard magnetic L10-FePt layer and thesoft magnetic Py layer. The L10-FePt and Py layers areexchange-coupled at the interface, resulting in themagnetic structure where all the magnetic moments arealigned in the same direction. We call this saturated
state. When small external magnetic field (H) wasapplied in the direction opposite to the magnetization
vectors, the magnetic moments in Py easily start torotate because of its low Hsw. However, the magnetic
moments of Py around the interface are pinned by thoseof L10-FePt. As a result, a spatially twisted magnetic
structure is formed in the exchange-coupled bilayer,
which we call twisted state. In order to experimentallyform this twisted magnetic structure, we prepared a

Fig. 2 Schematic illustrations of magnetic structuresat (a) saturated state and (b) twisted state for theexchange-coupled bilayer consisting of the hardmagnetic L10-FePt layer and the soft magnetic Pylayer. (c) Magnetization curve for the L10-FePt | Py
bilayer under the application of in-plane H [Ref. 11]
together with magnetic structures.

thin film on an MgO (110) single crystal substrate using
sputtering technique. The stacking of the thin film was
MgO Subs. || Fe (2) | Au (80) | FePt (10) | Py (100) |Au (3) (in nanometer). The FePt layer was grown on theAu buffer at 400 ºC. The substrate heating at 400 ºC
promoted the formation of L10 ordered structure for the
FePt layer, leading to the in-plane uniaxial magnetic
anisotropy along the in-plane [001] direction. The
magnetization curve for the L10-FePt | Py bilayer isshown in Fig. 2(c) 11), where the magnetization (M) was
normalized by the saturated value of M (Ms) and H wasapplied along the in-plane [001] direction of L10-FePt.
The gradual decrease in M was observed at the low H
region. In this H region, the magnetization reversal
process was reversible and the spring-back behavior
appeared when the minor magnetization curve wasmeasured. This reversible change indicates that the
L10-FePt and the Py are exchange-coupled at theinterface and the spatially twisted magnetic structure isformed in the bilayer by applying H.
The ferromagnetic resonance (FMR) spectra for thecontinuous film of the L10-FePt | Py bilayer are
displayed in Fig. 3(a) 11). The continuous film was put
facedown onto the coplanar waveguide (CPW), and thevector network analyzer FMR technique was employed,
where the frequency dependence of S11 (defined as theratio of the reflected voltage to the input voltage) was
measured. Three resonance peaks are observed both at
H = 200 Oe and H = -200 Oe corresponding to thesaturated state and the twisted state, respectively.
These peak frequencies (f0) are plotted as a function of
H in Fig. 3(b) 11). The open circles denote theexperimental data whereas the solid curves are theJournal of the Magnetics Society of Japan Vol.41, No.3, 201747

Fig. 4 Hsw as a function of excitation frequency for thedevice with the in-plane magnetized L10-FePt | Py
bilayer. Hrf = 140 Oe was applied to the device [Ref.
11].

Fig. 3 (a) FMR spectra for the continuous film of thein-plane magnetized L10-FePt | Py bilayer
measured at H = 200 Oe and -200 Oe, whichcorrespond to the saturated and twisted states,
respectively. (b) Peak frequencies as a function of H.
The open circles denote the experimental data
whereas the solid curves are the results of fitting
based the analytical model of PSSW mode [Ref. 11].

results of fitting based the analytical model of
perpendicular standing spin wave (PSSW) mode 13). The
analytical model of PSSW well explained the dispersion
relationship obtained experimentally. In addition to thisfact, the numerical simulation suggested that theseveral PSSW modes with the different number of nodes(n) were mainly excited in the Py layer 9). For example,
n = 0 was the lowest order of the PSSW mode, in which
the magnetic moments of Py were pinned at theinterface with the hard magnetic L10-FePt whereas themagnetic moments of Py at the other interface showed
the large-amplitude magnetization precession.
We examined the effect of the PSSW excitation on themagnitude of Hsw for the microfabricated L10-FePt | Py
bilayer element. The continuous L10-FePt | Py bilayer
was patterned into a rectangular element withdimensions of 2 µm × 50 µm by employing electron beamlithography and Ar ion milling. Figure 4 shows Hsw as afunction of excitation frequency, where the rf magnetic
field (Hrf) of 140 Oe was applied 11). When Hrf was notapplied, the device showed Hsw = 2350 Oe. Below f = 5.0
GHz, no remarkable change in Hsw occurred. However,
the application of Hrf = 6 GHz reduced Hsw to 1250 Oe.
The largest reduction was observed at f = 7 GHz, inwhich Hsw was obtained to be 350 Oe. This large Hsw
reduction is attributable to the excitation of PSSW mode.
As f was increased from 7 GHz to 20 GHz, Hsw increased
monotonically. The monotonic increase of Hsw with f is
the characteristic tendency for the spin wave-assisted

48

switching, which is determined by the dispersion
relationship of PSSW modes. In addition to the large
Hsw reduction, this spin-wave-assisted magnetization
switching showed the significant narrow Hsw
distribution 11). This narrowing in the Hsw distribution
suggests that the resonantly amplified spin wave
dynamics leads to the reduced Hsw. We also studied on
the magnetization switching conditions under the spinwave excitation and revealed the limited region ofswitching condition 14). This resonant switching
behavior suggests that the spin-wave-assisted switchinghas potential for a selective switching technique in
multilevel recording media.
In addition to the case of in-plane magnetized L10-FePt,
we have recently investigated the magnetization
dynamics of exchange-coupled bilayers with aperpendicularly magnetized L10-FePt, where the
L10-FePt layer was epitaxially grown on an MgO (100)single crystal substrate with a Au buffer 12). In order toexamine the effect of magnetization dynamics of theexchange-coupled bilayer on Hsw for the perpendicularly
magnetized L10-FePt, we exploited a nanodot consistingof the L10-FePt layers exchange-coupled with a soft
magnetic Py layer having a magnetic vortex. The
L10-FePt layer exhibited Hsw = 8.6 kOe without the Hrfapplication. When Hrf = 200 Oe with f = 11 GHz wasapplied, Hsw was reduced from 8.6 kOe to 2.8 kOe. By
comparing the experimental result with themicromagnetic simulation, we found that the vortex
dynamics of azimuthal spin waves in Py effectively
triggered the reversed-domain nucleation in L10-FePt at
a low H. This demonstrates that the excitation of spinwave leads to the efficient Hsw reduction even for theexchange-coupled system with the perpendicularly
magnetized L10-FePt.
Although the excitation of spin wave dynamics in theexchange-coupled systems is useful to reduce Hsw, thepresent bilayer structures having the soft magnetic
layer may lead to the degradation of thermal stability ofmagnetization compared with an L10-FePt single layer
film. The evaluation of thermal stability for the

Journal of the Magnetics Society of Japan Vol.41, No.3, 2017

exchange-coupled bilayer will be an issue to beaddressed in future.
3. Spin torque oscillator using Co-based Heusler alloy

Apart from the issue for ultrahigh density storage
discussed in the previous Section, we also need to open
up a way for new device applications for the further
progress in the spintronics research field. In otherwords, we should provide spintronic devices with novel
functionalities to realize a diversity of applications. An
emerging spintronic device is STO 2), which is a
nanometer-scaled rf oscillator, e.g. for on-chipcommunications or radar. The key physical phenomenon
to operate the STO is the spin torque-induced
magnetization dynamics. An STO consists of a layered
structure of a ferromagnetic (F1) layer | a nonmagnetic(N) metal (or an insulator (I)) | another ferromagnetic(F2) layer. In case that we inject the electric current
from F2 to F1, the conduction electrons are
spin-polarized by the F1 layer (a spin polarizer), which
exerts spin torque on the local magnetic moments of theF2 layer (a free layer). When the spin torque and the
intrinsic damping torque are balanced in the F2 layer,
the magnetization shows the self-sustained precession
motion around Heff. For current-perpendicular-to-plane(CPP) giant magnetoresistance (GMR) devices with F1 |N | F2 and magnetic tunnel junctions (MTJs) with F1 |I | F2, this steady magnetization precession produces achange in the device resistance via the GMR effect andthe tunnel magnetoresistance effect, respectively 15-17).
Since a dc electric current (Idc) is applied to the device,
the time-dependent device resistance (R(t)) is converted
into rf voltage (Vrf). Consequently, the device emits rf
output power (Pout). This is the central mechanism of
the STO, and its simple architecture composed of asingle nanometer-scaled element is one of the attractive
aspects of the STO.
However, the STOs have several crucial issues thatneed to be solved before the practical use: (i) the
enhancement of Pout, (ii) improvement of the rf
oscillation quality, and (iii) increasing the frequency
tunability by an electric current and/or a magnetic field.
MTJ-based STOs with an MgO tunnel barrier may solve
the first issue since the Pout is roughly proportional to
the square of the MR ratio. However, MTJ-based STOs
have other problems, which are a wide oscillation
linewidth observed generally 17), the risk of thedielectric breakdown of the tunnel barrier under large
bias voltages 18), and the non-negligible shot noise dueto the existence of tunnel barrier. On the other hand, a
CPP-GMR device generally exhibits a narrow oscillation
linewidth compared with the MTJ-based STOs 19).
Furthermore, a CPP-GMR stack is free from the risk ofdielectric breakdown of a tunnel barrier material, andhas the negligible contribution of shot noise. Thus, if the
CPP-GMR effect can be enhanced, it could be a
candidate of a high performance STO. One promising

Fig. 5 (a) Schematic illustration of CPP-GMR device
having CFMS | Ag | CFMS layers together with themeasurement setup for rf oscillation properties, and(b) its rf spectrum at perpendicular H = 3 kOe and Idc= 5.6 mA. The CPP nanopillar has an ellipsoidal
shape of 0.1 × 0.17 µm2. [Ref. 25]

way to enhance the MR ratio in a CPP-GMR device is touse half metallic ferromagnets showing the full
spin-polarization of conduction electrons, in which thedensity of states at the Fermi level exists only in one
spin channel.
Co2MnSi (CMS) is one of the Co-based Heusler alloys
theoretically predicted as a half metal 20) and showing
high GMR ratio experimentally 21), leading to theenhancement of Pout even for a CPP-GMR-based STO.
We reported that the large Pout = 1.1 nW was achieved
for the CPP-GMR device with the CMS | Ag | CMS
stack 22,23). Co2Fe0.4Mn0.6Si (CFMS) is another fullHeusler alloy exhibiting high spin polarization of
conduction electrons. In fact, the CPP-GMR devices
with CFMS | Ag | CFMS showed the MR ratios larger
than CMS | Ag | CMS 24). Thus, CFMS is also a
candidate material for developing high performance
STO 25,26).
Figure 5(a) depicts a structure of the CPP-GMR having
CFMS layers and the measurement setup to electricallydetect the spin torque-induced magnetization dynamics.
A stacking structure of Cr (20 nm) | Ag (40 nm) |CFMS (20 nm) | Ag (5 nm) | CFMS (3 nm) | Ag (2 nm)| Au (5 nm) was prepared on an MgO (100) single
crystal substrate using an ultrahigh-vacuum magnetron
sputtering system. The 20 nm-thick bottom CFMS andthe 3 nm-thick top CFMS layers were grown at room
temperature followed by in-situ annealing at 500ºC topromote the chemical ordering. The film was patterned
into a CPP nanopillar with an ellipsoidal shape (0.1 ×
0.17 µm2). The top and the bottom CFMS layers for the

Journal of the Magnetics Society of Japan Vol.41, No.3, 201749

present device behave as the free layer and the fixedlayer, respectively, against spin torque. The CPP-GMR
device was connected to the circuit with a two-terminal
rf probe and Idc was applied through a bias-Tee. Positive
Idc is defined as the direction, in which electrons flowfrom the upper to the lower CFMS layer. The output Vrf
was amplified by a preamplifier and was monitored infrequency-domain by a spectrum analyzer.
In order to understand the H direction dependence ofoscillation properties, we measured the spin torqueoscillation both under the in-plane and theperpendicular H. When in-plane H and Idc were applied
to the device along the long-axis of ellipsoidal pillar, thepeak appeared in the power spectral density. However,the small Pout (0.03 nW) was obtained and a wide
oscillation linewidth (Df) of 76 MHz was observed, whichwere attributable to the inhomogeneous effective
magnetic field (Heff). In contrast to the spin torqueoscillation under the in-plane H, we considerably
improved the oscillation properties by applying theperpendicular H. Figure 5(b) displays an rf spectrum at

Fig. 6 (a) L-MOKE loop and (b) magnetic domain
image
at
the
remanence
observedusing
PEEM-XMCD for the CFMS disks with the diameterof 2 µm and the thickness of 50 nm. [Ref. 28] (c) rf
spectrum for the CPP-GMR device having themagnetic vortex in the CFMS layer.

50

H = 3 kOe and Idc = 5.6 mA 25). The Idc dependence ofoscillation frequency (f0) showed the blue shift against
Idc, suggesting that the perpendicular H led to an
out-of-plane mode of magnetization precession. The
maximum value of Pout was obtained to be 23.7 nW
together with the narrow Df = 10 MHz. This Pout wasmuch larger than those reported in the CPP-GMR
devices using conventional ferromagnetic layers, andwas one order of magnitude smaller than theperformance of the previous MTJ-based STO showing
Pout = 0.14 µW 17). We also performed the micromagnetic
simulation to reveal the detail of magnetization
dynamics 25). Although we successfully achieved thelarge Pout by using the highly spin-polarized CFMS, thesimulated results suggested the formation of spatially
non-uniform magnetic structure during the spin torqueoscillation and the large incoherency in the precession
trajectory. This implies that the improvement ofcoherency in the spin torque oscillation enables us tofurther enhance Pout.
One of the possible ways to improve the coherency ofspin torque oscillation is to utilize the dynamics ofmagnetic vortex. A study on the vortex STO reported
the narrow Df, leading to the high Q factor (Df / f0) 27),
which is owing to the stable vortex oscillation. The
stability of vortex oscillation originates from the
topologically stable magnetic structure and goodconsistency of the magnetization distribution with thespatial distribution of the Oersted field formed by the dc
current. Thus, we examined the formation of magnetic
vortex and the control of vortex dynamics in the CFMS
nanodisk, in which the CFMS layers were epitaxiallygrown on the MgO substrate. Figures 6(a) and 6(b) show
the longitudinal magneto-optical Kerr effect (L-MOKE)
loop and the magnetic domain image at the remanence
observed using a photoemission electron microscopewith X-ray magnetic circular dichroism (PEEM-XMCD),
respectively, for the circular CFMS disks with thediameter of 2 µm and the thickness of 50 nm 28). The
shape of L-MOKE loop with low remanent
magnetization was attributed to the magnetization
process undergoing the formation of magnetic vortex. In
addition, the curling in-plane magnetic moments wereclearly observed in the magnetic domain image.
Consequently, one sees that the magnetic vortex can bestably formed even in the epitaxially grown CFMS disks.
Then, we fabricated the STO with the CFMS disk with adiameter of 240 nm, in which the magnetic vortex wasformed in the CFMS disk 29). The STO was made of thestack of MgO (100) Subs. || Cr (20 nm) | Ag (20 nm) |CFMS (20 nm) | Ag (5 nm) | CFMS (30 nm) | Ag (2nm) | Au (3 nm), in which 30 nm-thick CFMS waspatterned into the disk shape to stabilize the magnetic
vortex. Figure 6(c) shows an rf spectrum for the
CPP-GMR device having the magnetic vortex in the
CFMS layer. The sub-GHz oscillation with a narrow
oscillation linewidth (Df = 0.4 MHz) was observed, which
originated from the vortex dynamics. From the

Journal of the Magnetics Society of Japan Vol.41, No.3, 2017

systematic study on the oscillation properties for vortex
STO with CFMS, we found that the high Pout > 10 nW
and the high Q factor > 5000 were achieved
simultaneously at an optimum condition 29). Our
experimental results indicate that the utilization ofvortex dynamics in CFMS is a promising way to provide
a significant improvement of the performance of STOs.
4. Summary

In this review, we introduced two research topics:
spin-wave-assisted magnetization switching in thebilayers with L10-FePt and Py, and high performance
STOs using the CFMS Heusler alloy.
For the in-plane and perpendicularly magnetized
configurations of L10-FePt layers, we successfully
reduced Hsw for L10-FePt utilizing the spin waves in thebilayers. We expect that this spin-wave-assisted
magnetization switching will be used as an information
writing technique of future magnetic storage devices.
The STOs having the CFMS layers were developed,
and both the Pout and the Q factor were improved by
exploiting the highly spin-polarization of CFMS Heusler
alloy and the dynamics of magnetic vortex. We believethat vortex STO with CFMS serves as a key for thefurther progress in spintronic devices.
Acknowledgements
These works are partly incollaboration with Dr. H. Imamura, Dr. H. Arai and Dr.
T. Yamaji (AIST), Prof. Y. Nozaki (Keio Univ.), Prof. M.
Mizuguchi, Prof. T. Kubota, Dr. H. Yako (Tohoku Univ.),
Dr. Y. Sakuraba (NIMS), and Prof. M. Kotsugi (Tokyo
Univ. of Sci.). We also thank Dr. T. Ohkochi (SPring-8)
and Mr. Y. Murakami and Mr. I. Narita (Tohoku Univ.)
for their technical assistance. These works werepartially supported by a Grant-in-Aid for Young
Scientists A (25709056), Grant-in-Aid for Scientific
Research S (23226001), Grant-in-Aid for Scientific
Research B (16H04487), Institute for Division For
Interdisciplinary Advanced Research and Education,
Tohoku Univ. and the Japan Society for the Promotion
Science Research Fellowship for Young Scientist. The
device fabrication and the structural characterization
were partly performed at Cooperative Research andDevelopment Center for Advanced Materials, IMR,
Tohoku University.
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Received Feb. 08, 2017; Accepted Mar. 08, 2017

Journal of the Magnetics Society of Japan Vol.41, No.3, 201751