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ISSN 2432-0250

Vol.41 No.3
JournalJournal of the Magnetics Society of Japan
Electronic Journal URL: https://www.jstage.jst.go.jp/browse/msjmag

2017

Review：MSJ Awards 2016
Control of Magnetization Dynamics in Ordered Alloy Systems
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T. Seki, W. Zhou, T. Yamamoto, and K. Takanashi…46

Magnetic Recording
Impact of Damping Constant on Bit Error Rate in Heat-Assisted Magnetic Recording
T. Kobayashi, Y. Nakatani, F. Inukai, K. Enomoto, and Y. Fujiwara…52

Thin Films, Fine Particles, Multilayers, Superlattices
Morphology and Magnetic Properties of α”-Fe16N2 Nanoparticles Synthesized from Iron Hydroxide and Iron Oxides
M. Tobise, T. Ogawa, and S. Saito…58

Power Magnetics

Temperature Characteristics of the Dynamic Magnetic Loss of Ferrite
H. Saotome, K. Azuma and Y. Hamamoto…63

Biomagnetism / Medical Applications
Eﬀect of Amorphous Region on Magnetic Orientation of Poly (lactic acid) Blend Films with Diﬀerent Molecular Weight
R. Nakayama, H. Ikake, K. Kurita, S. Shimizu, S. Kurumi, K. Suzuki, K. Takahashi and H. Nojiri…66

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41

HP: http://www.magnetics.jp/ e-mail: msj@bj.wakwak.com
Electronic Journal: http://www.jstage.jst.go.jp/browse/msjmag

3

2017

INDEX

Journal of the Magnetics Society of Japan
Vol. 41, No. 3
Electronic Journal URL: https://www.jstage.jst.go.jp/browse/msjmag

CONTENTS
Review: MSJ Awards 2016
Control of Magnetization Dynamics in Ordered Alloy Systems… ……………………………………………………… T. Seki, W. Zhou, T. Yamamoto, and K. Takanashi

46

Magnetic Recording
Impact of Damping Constant on Bit Error Rate in Heat-Assisted Magnetic Recording… ………………………………… T. Kobayashi, Y. Nakatani, F. Inukai, K. Enomoto, and Y. Fujiwara

52

Thin Films, Fine Particles, Multilayers, Superlattices
Morphology and Magnetic Properties of α”-Fe16N2 Nanoparticles
Synthesized from Iron Hydroxide and Iron Oxides… ……………… M. Tobise, T. Ogawa, and S. Saito

58

Power Magnetics
Temperature Characteristics of the Dynamic Magnetic Loss of Ferrite… ………………………………………………………………… H. Saotome, K. Azuma and Y. Hamamoto

63

Biomagnetism / Medical Applications
Effect of Amorphous Region on Magnetic Orientation of Poly (lactic acid) Blend Films withDifferent Molecular Weight……………………………………………… R. Nakayama, H. Ikake, K. Kurita,		
S. Shimizu, S. Kurumi, K. Suzuki, K. Takahashi and H. Nojiri

66

Board of Directors of The Magnetics Society of Japan
President:

H. Fukunaga

Vice President:

K. Takanashi

Director, General Affairs:

Y. Takano, Y. Miyamoto

Director, Treasurer:

S. Sugimoto, K. Aoshima

Director, Planning:

C. Mitsumata, Y. Saito

Director, Editorial:

H. Saotome, K. Kobayashi

Director, Public Relations:M. Igarashi, H. Awano

Director, International Affairs:

A. Kikitsu, Y. Takemura

Director:

Y. Honkura

Auditor:F. Kirino, Y. Suzuki

INDEX
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

INDEX

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

INDEX

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

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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

INDEX

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

INDEXsystematic 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, 2017

51

INDEX
J. Magn. Soc. Jpn., 41, 52-57 (2017)<Paper>                            
          
             

                                   
              
                                                                                  
                       
   
         
           
     
                                                                            
                              


                                         
                       
                                                             52                                                                                                             
                   
                           
          
         
                                     
                                        
       
                            Journal of the Magnetics Society of Japan Vol.41, No.3, 2017

INDEX                
           
                                                                                           
      
                                                                                                                                                                                          





                      
         
                                                                                                                                                                                
        
                    
  
        
               
   
                                                                             

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INDEX                                                                                                
         
        
       
               
                                                                                                   
        
          
       
                                                                                                                                  
               
                
                                   
                                      

 
                                                        
       
             
          54         
                       

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

INDEX           

                                
                                                          
     
                 
                                                                      
                          






      
        
                                                                                                                                    

                                
                      
                  

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INDEX                                                                                                                                                                                                                                                                 
                            

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
                   

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


                       

                  

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INDEX                                                      

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                                               
     
                              
                          
       
       
                                
         
           



                                                                                                                          
                                                                                                                                                        
                                                
            

Journal of the Magnetics Society of Japan Vol.41, No.3, 2017  

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J. Magn. Soc. Jpn., 41, 58-62 (2017)<Paper>Morphology and magnetic properties of α”-Fe16N2 nanoparticles
synthesized from iron hydroxide and iron oxides
M. Tobise, T. Ogawa, and S. Saito

Department of Electronic Engineering, Graduate School of Engineering, Tohoku University,

6605, AzaAoba, Aramaki, Aobaku, Sendai 9808579, Japan

Metastable α”-Fe16N2 have attracted much interest as a candidate for rare-earth-free hard magnetic materials. To
realize high coercivity, it is necessary to utilize not only the magnetocrystalline anisotropy but also the shapeanisotropy of α”-Fe16N2 nanoparticles assemblies. An increase in magnetostatic couplings and intergranularexchange couplings among particles typically reduces the coercivity. Therefore, it is very important to evaluate theanisotropy and magnetic interactions among α”-Fe16N2 nanoparticles. We have examined the changes in morphology,
structure and magnetic properties through the synthesis of α”-Fe16N2 nanoparticles from various materials such as
α-FeOOH, ɤ-Fe2O3, and Fe3O4. The magnetic interactions were also estimated based on experimental results
obtained by analysis of the rotational hysteresis loss of randomly oriented nanoparticles. Hc and Hkptc for theα”-Fe16N2 nanoparticle assemblies for different starting materials ranged from 2.2 to 1.1 kOe, and from 11 to 12 kOe
respectively. Experimental results of the normalized coercive force and normalized switching field suggests that theexistence of large magnetic interactions among α”-Fe16N2 nanoparticles.
Key words: nanomagnetics, hard magnetic materials, iron nitride, magnetic anisotropy, rare earth element-free
1. Introduction
The metastable α”-Fe16N2 iron nitride phase withbody-centered tetragonal (b.c.t.) structure has received
much attention as a candidate for rare-earth-free hardmagnetic materials. After a surprising first report
where the α”-Fe16N2 phase exhibited giant saturation
magnetization as a thin film1), many researches haveattempted to produce single phase α”-Fe16N22,3). In 1993,
a thin film method was established to achieve a
relatively high volume fraction of α”-Fe16N2 using
reactive sputtering, where a saturation magnetizationof Ms ≃ 240 emu/g and uniaxial magnetocrystalline
anisotropy constant of Ku ≃ 1×107 erg/cm3 were
determined4). These values are very attractive withrespect to the realization of a permanent magnet basedon this material. Since it is still very difficult to produce
metastable α”-Fe16N2 in thin films, the synthesis ofsingle phase α”-Fe16N2 in bulk form is considered to be
extremely challenging5,6,7). In 2013, a procedure for thesynthesis of single phase α”-Fe16N2 nanoparticles wasestablished, and α”-Fe16N2 was reported8) to exhibit Ms≃ 226 emu/g and Ku ≃ 9.6×106 erg/cm3 at 300 K.
To produce a hard magnetic material using
magnetic particle, it is necessary to control theanisotropy of particle assemblies and the magnetic
interaction among these particles. Magnetostatic
couplings and intergranular exchange couplings among
particles typically reduce the coercivity. Therefore it isvery important to evaluate these magnetic interactions
among α”-Fe16N2 nanoparticles. In this work, we
examine the changes in the morphology, structure andmagnetic properties of α”-Fe16N2 nanoparticles
synthesized using different starting materials, such as
α-FeOOH, ɤ-Fe2O3, and Fe3O4. We also discuss the58

magnetic anisotropy and magnetic interactions basedon the experimental results obtained by the rotational
hysteresis loss analysis9) of randomly oriented
nanoparticles.
2. Experimental Procedure
The synthesis of α”-Fe16N2 nanoparticles was
conducted using commercially available α-FeOOH(Wako Pure Chemical Industries, Ltd.), ɤ-Fe2O3 (CIK
NanoTek Corporation) and Fe3O4 (Mitsui Mining &Smelting Co., Ltd.). The diameters of the iron hydroxide
and iron oxide particles ranged from several tens toseveral hundreds of nanometers. The temperature anddurationfor
reductionandnitridingweresystematically examined to obtain almost single phaseα”-Fe16N2, according to the following procedure.
α-FeOOH, ɤ-Fe2O3, and Fe3O4 nanoparticles were first
reduced at 250－600 °C for 1－5 h in a H2 gas flow.
Nitriding was subsequently performed at 100－200 °C
for 2 － 72 h in an NH3 gas flow. Crystal structure
analysis was conducted using powder X-ray diffraction(XRD) with CoKα radiation. Morphological changes
were observed using transmission electron microscopy(TEM). For magnetic properties measurements, α-Feand α”-Fe16N2 nanoparticles were packed into a disk
shape (6 mm diameter, 1 mm thick) with resin. The
magnetization σ, and coercivity Hc, were measured
using vibrating sample magnetometer (VSM) with amaximum applied magnetic field of 14.5 kOe at 300 K.
3. Results and Discussions
3.1 Morphological andnanoparticles preparedmaterials

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Fig. 1 shows TEM images of α-FeOOH as a starting
material, α-Fe produced by the reduction of α-FeOOH,
and α”-Fe16N2 formed by the nitriding of α-Fe. The
shape of the Fe hydroxide particles is spindle-like.
Reduction was performed in an atmosphere of flowing
H2 at 380 °C for 4 h, and produced aggregates ofnanoparticles by sintering. Subsequent nitriding was
performed in an atmosphere of flowing NH3 at 160 °C
for 8 h. No morphological differences were observed
between the reduced α-Fe and nitrided α”-Fe16N2
nanoparticle assemblies.
Fig. 2 shows TEM images of the ɤ-Fe2O3 starting
material, α-Fe reduced from ɤ-Fe2O3 and α”-Fe16N2 as
the nitriding product from α-Fe. The shape of the
ɤ-Fe2O3 particles is spherical. Reduction was conductedin an atmosphere of flowing H2 at 300 °C for 4 h. The
reduction process caused the nanoparticles to form
aggregates by sintering, as with the α-FeOOH starting

material. Nitriding was performed in an atmosphere offlowing NH3 at 145 °C for 8 h. The nitriding process did
not result in any morphological changes, in the same
way that no changes were observed with the α-FeOOH
starting material.
Fig. 3 shows TEM images of the Fe3O4 starting
material, α-Fe reduced from Fe3O4, and α”-Fe16N2 by thenitriding of α-Fe. The shape of the Fe3O4 particles is
cubic. Reduction was performed in an atmosphere offlowing H2 at 340 °C for 4 h. The reduction andnitriding processes did not generate any obvious
macroscopic morphological changes, in contrast to thatobserved with the α-FeOOH and ɤ-Fe2O3 starting
materials.
Fig. 4 shows representative XRD patterns of theα”-Fe16N2 nanoparticles produced from various starting
materials. In the case of α”-Fe16N2 nanoparticles
produced from α-FeOOH, ɤ-Fe2O3, all X-ray diffracted

Fig.1

TEM images of α-Fe and α”-Fe16N2 produced from α-FeOOH as a starting material.

Fig. 2

TEM images of α-Fe and α”-Fe16N2 produced from ɤ-Fe2O3 as a starting material.

Fig. 3

TEM images of α-Fe and α”-Fe16N2 produced from Fe3O4 as a starting material.

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Fig. 5 Torque loss curves of α”-Fe16N2 randomly
oriented particle assemblies produced from α-FeOOH
changing applying field. CW indicates clockwise
rotation and CCW indicates counter-clockwise
rotation.

Fig. 4 XRD patterns of the α”-Fe16N2 nanoparticles
produced from various starting materials. Asterisks
show super-lattice diffraction of N ordered site.

Fig. 6 Rotational hysteresis loss curves of α-Fe andα”-Fe16N2 nanoparticles assemblies produced from
α-FeOOH.

line were identified as α”-Fe16N2. On the other hand,very small diffracted line besides diffractions ofα”-Fe16N2 observed at diffraction angles around 48 and99 degrees for α”-Fe16N2 nanoparticles produced fromFe3O4. It is supposed that these additional diffracted
line correspond Fe4N and α-Fe, respectively.

linearly extrapolating to zero hysteresis loss. Hp is themagnetic field where Wr reaches its maximum whichcorresponds to the switching field of assemblies ofnanoparticles. Hkptc and Hp were evaluated with
applying magnetic field up to 15 kOe. In the case of
assemblies of particles which were magneticallyisolated ideally, the value of magnetic field where Wr
vanishes is independent of particle alignment(crystallographic orientation) and corresponds to Hkptc.
Similar rotational hysteresis loss analysis had been
already applied to the thin film recording media10,11).
Table 1 shows the magnetic properties of α-Fe andα”-Fe16N2 nanoparticle assemblies produced from thedifferent starting materials, as measured with the VSM
and torque magnetometer. Ms for the α-Fe nanoparticle
assemblies ranged from 205 to 198 emu/g according to
the starting material. Compared to the literature valueof the magnetization 218 emu/g for α-Fe, these values
are lower from 6 to 9 % than the literature value. It is
assumed that oxidation of surface of the α-Fe
nanoparticles may cause this decrease in magnetization.
Hc for the α-Fe nanoparticle assemblies ranged from 0.8
to 0.3 kOe according to the starting material, and Hkptc

3.2 Shape anisotropy with respect to theagglomeration of nanoparticles
Fig. 5 shows the torque loss curves of α”-Fe16N2
randomly oriented particle assemblies produced from
α-FeOOH changing applying field from 3 to 10 kOe.
Rotational hysteresis loss Wr is measured as the regionofenclosed
area
by
these
clockwise
andcounterclockwise torque curves. In the case of applying
magnetic field H is larger than the half of anisotropy
field Hk and smaller than Hk for the randomly oriented
particles, rotational hysteresis loss Wr is observed and
Wr changes as a function of magnetic field9). Fig. 6
shows rotational hysteresis loss curves of α-Fe andα”-Fe16N2 nanoparticles assemblies produced from
α-FeOOH. Hkptc was defined by plotting Wr versus
inverse applied field and finding the intercept by

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Table 1. Magnetic properties of α-Fe and α”-Fe16N2 nanoparticle assemblies
produced from various starting materials.

for the α-Fe nanoparticle assemblies also ranged from
5.4 to 7.2 kOe. For α-Fe nanoparticle assemblies, theanisotropy field Hkptc is considered to be due to theshape anisotropy of the nanoparticle assemblies
because the magnetocrystalline anisotropy constant K1
for α-Fe with cubic symmetry is too small (ca. 1×105
erg/cm3). It is considered that the differences of Hkptc arederived from the difference of shape anisotropy in theassemblies of α-Fe nanoparticles that are formed during
the reduction process. Hc for the α-Fe nanoparticle
assemblies decreased from 0.8 to 0.3 kOe even thoughHkptc of them increased from 5.4 to 7.2 kOe. It is
assumed that Hc is not determined by only shapeanisotropy. Further detailed investigation is necessary
to reveal the factor which affects Hc of α-Fe.
Shape anisotropy is expressed as Kushape = N·Ms2/2(or Hkshape=N·Ms).N is the difference of thedemagnetization coefficients between the long and shortaxes of a particle. The experimentally determined Hkptc
was used to estimate ΔNptc as an equivalentdemagnetization coefficients as nanoparticle assemblies.
Fig. 7 shows an example of the calculated result12) for

Fig. 7 Shape anisotropy field as a function of the
aspect ratio for the ellipsoid form.

Hkshape as a function of the aspect ratio, assuming an
ellipsoid form and that the Ms of α-Fe is 218 emu/g. In
this way, the values of ΔNptc were determined to range
from 3.3 to 4.6 for the corresponding aspect ratios of
α-Fe nanoparticle assemblies estimated with thisscheme, from 2.2 to 3.5. As shown in Fig. 7, themaximum shape anisotropy is expected to be 8×106
erg/cm3 with a high aspect ratio of more than 50 whichis considered to be very difficult to realize.
On the other hand, Hc for the α”-Fe16N2 nanoparticle
assemblies ranged from 2.2 to 1.1 kOe for different
starting materials. The reason why that Hc of α”-Fe16N2
nanoparticles which were produced from Fe3O4 is lower
than those of α”-Fe16N2 nanoparticles which were
produced from α-FeOOH and ɤ-Fe2O3 is considered dueto the additional α-Fe which was observed in X-ray
diffraction(Fig. 4). Hkptc for the α”-Fe16N2 nanoparticle
assemblies were not significantly different, ranging
from 11 to 12 kOe. The magnetocrystalline anisotropy
field Hkint of α”-Fe16N2 determined for a pseudo single-crystalline film was 11.2 kOe 13). This Hkptc for theα”-Fe16N2 nanoparticle assemblies are almost equal to
Hkint. If we assume α”-Fe16N2 nanoparticle assemblies
have the same magnetocrystalline anisotropy field Hkintof a pseudo single-crystalline film, the shape anisotropy
is considered to be very small in α”-Fe16N2 nanoparticle
assemblies.
3.3 Evaluation of magnetic interaction amongα”-Fe16N2 nanoparticle assemblies
Fig. 8 shows plots of the normalized coercive force(Hc/Hkptc) versus normalized switching field (Hp/Hkptc)
for α”-Fe16N2 nanoparticle assemblies produced fromthe various starting materials. Hc/Hkptc varied from 0.10
to 0.26 and Hp/Hkptc varied from 0.26 to 0.35. In theideal magnetization reversal mode due to the coherent
rotation of the magnetic moment with nonmagnetic
interaction (Stoner-Wohlfarth type)14) , Hc/Hkptc is equal
to 0.48 and Hp/Hkptc is equal to 0.51. According to themicromagnetic simulation, and it was demonstrated

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Fig. 8 Plots of Hc/Hkptc and Hp/Hkptc for α”-Fe16N2
nanoparticles prepared from various starting materials.

the aspect ratio of α-Fe nanoparticle assemblies were
estimated to be in the range of 2.2－3.5.
2. Hc for the α”-Fe16N2 nanoparticle assemblies
ranged 2.2 to 1.1 kOe for different starting materials.
Hkptc for the α”-Fe16N2 nanoparticle assemblies were notsignificantly different, ranging from 11 to 12 kOe.
3. The normalized coercive force (Hc/Hkptc) andnormalized switching field (Hp/Hkptc) are 0.10－0.26 and0.26－0.35 respectively, which indicates large magnetic
interactions.
4. As the expected shape anisotropy is in the orderof 106 erg/cm3 for α”-Fe16N2 nanoparticles even though ahigh aspect ratio is realized, magnetocrystalline
anisotropy on the order of 107 erg/cm3 is still required,in addition to improvement of the magnetic isolation
among nanoparticles to realize a permanent magnet.

that both Hc and Hp decrease with increasing magnetic
interaction among grains in thin film media11).
Compared to the ideal values of Hc/Hkptc and Hp/Hkptc,
these values of α”-Fe16N2 nanoparticles prepared fromvarious starting materials are relatively small. To
increase the coercivity of α”-Fe16N2 nanoparticles, it isnecessary to decrease magnetic interaction.

Acknowledgements This work was partially supportedby the New Energy and Industrial Technology
Development Organization (NEDO), the Matching
Planner Program from the Japan Science andTechnology Agency (JST) and the Technology Research
Association of Magnetic Materials for High-Efficiency
Motors (MagHEM).

3.4 Toward an increase in the coercivity of α”-Fe16N2
nanoparticle assemblies
The empirical figure known as the magnetic
hardness parameter = (K1/μ0Ms2)1/2 is a usefulparameter to consider how to approach the constructionof modern permanent magnet15). The  value for
Nd2Fe14B as a typical modern magnet, is 1.54, whilethat for α”-Fe16N2 is 0.53. It is recommended that  begreater than 1, which corresponds to a value of K1≃
3×107 erg/cm3, assuming the same magnetization of
Nd2Fe14B, 165 emu/g. The expected shape anisotropy isin the order of 106 erg/cm3 for α”-Fe16N2 nanoparticles
even though a high aspect ratio is realized (Fig. 7);
therefor, magnetocrystalline anisotropy in the order of
107 erg/cm3 is still required, in addition to improvementof the magnetic isolation among nanoparticles, forapplication as a permanent magnets.

References

4. Conclusion
The structural and magnetic properties of α”-Fe16N2
nanoparticles synthesized from different starting
materials were examined, and the magnetic anisotropyof nanoparticle assemblies and magnetic interactions
among nanoparticles were evaluated by rotational
hysteresis loss analysis.
1. α-Fe nanoparticle assemblies produced by
reduction and nitriding of different starting materials
show exhibited different values of Hkptc ranging from 5.4
to 7.2 kOe. It is considered that the differences of Hkptc
are derived from the difference of shape anisotropy in
the assemblies of α-Fe nanoparticles that are formed
during the reduction process. From these Hkptc values,

621) T. K. Kim, M. Takahashi: Appl. Phys. Lett., 20, 492 (1972)
2) J. M. D. Coey, K. O’Donnell, Q. Qinian, E. Touchais, K. H.
Jack: J. Phys.: Condens. Matter, 6, L23 (1994).
3) M. Komuro, Y. Kozono, M. Hanazono, and Y. Sugita: J. Appl.
Phys, 67, 5126 (1990)
4) M. Takahashi, H. Shoji, H. Takahashi, T. Wakiyama: IEEE
Trans. Magn., 29, 3040 (1993).
5) M. Takahashi, H. Shoji: J. Magn. Magn. Mater. 208, 145(2000).
6) E. Kita, K. Shibata, H. Yanagihara, Y. Sasaki, M.
Kishimoto: J. Magn. Magn. Mater., 310, 2411 (2007).
7) S. Kikkawa, A. Yamada, Y. Masubuchi, T. Takeda: Mater.
Res. Bull., 43, 3352 (2008).
8) T. Ogawa, Y. Ogata, R Gallage, N. Kobayashi, N Hayashi, Y
Kusano, S. Yamamoto, K. Kohara, M. Doi, M. Takano, M
Takahashi: Appl. Phys. Express, 6, 073007 (2013).
9) I. S. Jacobs, F. E. Luborsky: J. Appl. Phys., 28, 467 (1957).
10) M. Takahashi, T. Shimatsu, M. Suekane, M. Miyamura, K.
Yamaguchi, H. Yamasaki: IEEE Trans. Magn., 28, 3285
(1992).
11) I. A. Beardsley and V.S. Speriosu: IEEE Trans. Magn., 26,2718 (1990).
12) R. C. O’Handley, Modern Magnetic Materials: Principle
and Applications, 1st ed. (John Wiley & Sons, Inc., New
York, 1999), pp.38-43.
13) M. Takahashi, Y. Takahashi, K. Sunaga, H. Shoji: J. Magn.
Magn. Mater., 239, 479 (2002).
14) E. C. Stoner, E. P. A. Wohlfarth: Phil. Trans. R. Soc. Lond.,
A 240, 599 (1948).
15) R. Skomski, J. M. D. Coey: Scripta Materialia, 112, 3(2016)
Received Sep. 28, 2016; Revised Dec. 19, 2016; Accepted Mar.
08, 2017

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

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Manuscript

Click here to download Manuscript manuscript.pdf

J. Magn. Soc. Jpn., 41, 63-65 (2017)

LetterTemperature Characteristics of the Dynamic Magnetic Loss of Ferrite
H. Saotome, K. Azuma and Y. Hamamoto

Faculty of Engineering, Chiba University, 1-33 Yayoi-cho, Inage-ku, Chiba 263-8522, JapanThe dynamic magnetic loss in ferrites is obtained by subtracting the hysteresis loss, which is independent of theexcitation frequency, from the iron loss. In the high frequency excitation region, the dynamic magnetic loss is thedominant component of the iron loss in ferrites. The iron loss in ferrite is temperature-dependent and thisdependence has been described in product catalogs, where the hysteresis and dynamic losses are not separated. The
catalog data are measured using sinusoidal wave voltage excitation, whereas ferrite cores are commonly used underrectangular wave voltage excitation in DC-DC converters. In this paper, the experimentally obtained temperature
characteristics of the hysteresis and dynamic magnetic losses for rectangular wave voltage excitation are shown
separately, and it is found that the two are different. This suggests that the physical mechanisms involved are
different as well. Thus, it is important to consider the temperature characteristics of the dynamic magnetic loss to
produce low-loss ferrites.
Key words: ferrite, Mn-Zn, iron loss, dynamic magnetic loss, B-H loop, temperature
1. Introduction
Ferrite cores are mainly used in DC-DC converters,
where their switching frequencies are typically around
100 kHz. The eddy current loss in ferrites is negligible,
even when they are excited at such high frequencies,
because of the very high resistivity of grain boundaries.
Thus, the power loss of ferrites is considered to be themagnetic loss. For a given magnetic induction
maximum, Bm, the power loss of ferrites depends on thetemperature. This dependence has been described in a
product catalog, where the excitation voltage waveform
used was sinusoidal 1).
The magnetic loss in ferrites consists of hysteresisand dynamic magnetic losses 2). The area of the B-H loop
varies with the exciting frequency, i.e., the time
derivative of the magnetic induction, dB/dt. The
hysteresis loss (J/m3) is defined as the minimum B-Hloop area with respect to the exciting frequency when Bm
is kept constant, as shown by the broken line in Fig. 1.
When the excitation frequency, or dB/dt, is increased,
the B-H loop widens, as depicted by the solid line in Fig.
1. The dynamic magnetic loss (J/m3) is defined by thegray area in the figure.
Although it is of great interest to investigate thegeneration mechanism of the magnetic losses, this is
not easy. To determine whether the hysteresis anddynamic magnetic losses are generated by the same
B

O

Fig. 1

B-H loop.

H

Fig. 2

Toroidal core used in experiments.

mechanism, the present authors measured thetemperature behavior of both quantities in a previous
study 3). We found no significant difference of the
dynamic magnetic loss between at 0 °C and at room
temperature. In that study 3), we used TDK PC40
Mn-Zn ferrite, whereas here, we used TDK PC47, whose
power loss temperature dependence is approximately
1.8 times stronger than that of PC40 1). The dimensionsof the core used in the experiments is shown in Fig. 2.
The power loss measurements were carried out at 0 °C
and 30 – 70 °C.
2. Instrumentation
In order to set the temperature of the ferrite core to
0 °C and 30 – 70 °C, the temperature control setups
shown in Figs. 3(a) and 3(b), respectively, wereprepared. The core was completely submerged in
distilled water in both cases. In Figs. 3(a) and 3(b), thedistilled water was surrounded by ice and a thermal
insulator, respectively. In Fig. 3(b), the watertemperature was controlled with a power source, heater,
and thermocouple. The duration of the excitation was
short enough to keep the rise in core temperature below
1 °C.
The product catalog for PC47 shows the temperature
characteristics of the core loss under sinusoidal
waveform voltage excitation at 100 kHz and Bm = 200(mT). The measurement results obtained in the range of
30 °C to 70 °C using the present setup coincide with the

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63

INDEX

Shunt
resistori1

Rectangular
waveform
voltage source

v2

Rs ls

Ferrite corevsDistilled water

Ice(a) At roughly 0 °C
Shunt
resistor

Rectangular
waveform
voltage source

v2

i1

Rs ls
vs

(a) 0 °C

PID-controlled
power source

Ferrite core

Distilled water

Thermocouple

Heater

Thermal insulator(b) Above 30 °C
Measurement setups.

3. Measurement results
Six B-H loops measured at 0 °C and 70 °C are shownin Figs. 4(a) and 4(b), respectively. They were obtained
under rectangular waveform voltage excitation withdB/dt = 1, 200, 400, 600, 800 and 1000 mT/s. The
smallest and largest B-H loops in the figures correspondto 1 and 1000 mT/s, respectively. The maximum
magnetic flux density, Bm, was kept at 200 mT for each B-Hloop. Between 0 °C and 70 °C, the power loss of theferrite decreases with increasing temperature. The
hysteresis loss corresponding to the area of the smallest
B-H loop in each figure also decreases. Moreover, the64(b) 70 °C
Fig. 4B-H loops (Bm = 200 mT).

12
10

Loss (J/m3)

catalog data under the same conditions.
In this study, the ferrite core was excited by arectangular waveform voltage, i.e., at constant dB/dt,
because we wanted to measure the power loss of theferrite with respect to the parameter dB/dt. The
rectangular waveform voltage sources shown in Figs.
3(a) and 3(b) consisted of a function generator and an
amplifier, or an FET inverter. The amplifier and FET
inverter were used to generate dB/dt smaller and larger
than 200 mT/s, respectively.
To measure the excitation current, i1, a shunt
resistor with a known stray inductance was used. The i1
waveform was calculated by using the shunt resistor
voltage, vs, and the compensation for the deviation due
to the stray inductance. B-H loops are obtained with i1,
the time integral of the search-coil-induced voltage, v2,
and the core dimensions. When the excitation current is
too large, v2 deviates from the rectangular waveform
due to vs. In such cases, the output waveform of thefunction generator is adjusted to compensate for the
distortion 3).

8

6
4
2
0

0

10

20

30
40
50
Temperature (℃)

60

70

30
40
50
Temperature (℃)6070(a) Bm = 100 mT
25
20
Loss (J/m3)

Fig. 3

15

10
5

0

0

10

20

(b) Bm = 200 mT

Fig. 5 Temperature characteristics of the dynamic
magnetic loss with dB/dt = 1000, 800, 600, 400 and 200
mT/s from the top down, respectively. The bottom
broken lines show the hysteresis losses.

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

INDEX(a) Bm = 100 mT

the hysteresis losses. The dynamic magnetic loss
becomes larger than the hysteresis loss and dominateswith increasing dB/dt, regardless of the temperature. To
investigate whether the difference in temperature
characteristics between the hysteresis and dynamic
magnetic losses can be clarified, the normalized values
were re-plotted (see Fig. 6), with 100 % representing thevalues at 0 °C for each of the losses. Figures 6(a) and6(b) revealed that between 0 °C and 30 °C, thehysteresis loss decreased with temperature more
steeply than the dynamic magnetic loss; the normalized
hysteresis loss curves in Figs. 6(a) and 6(b) almost
coincide. Thus, the temperature characteristics of thehysteresis and dynamic magnetic losses are different,
which suggests that, microscopically, their generation
mechanisms are not precisely the same.
4. Conclusions(b) Bm = 200 mT
Fig. 6 Temperature characteristics of the dynamic
magnetic loss: normalized values with respect tothe values at 0 °C for dB/dt = 1000 (●) and 200 (○)
mT/s. The bottom broken lines show the hysteresis
losses normalized to the values at 0 °C.
dynamic magnetic loss decreases slightly. In Fig. 4, themagnetic field intensities fluctuate a little near ±Bm,i.e., the exciting current oscillates slightly when therectangular waveform voltage changes its polarity very
quickly. The fluctuation is caused by the stray
capacitance of the winding. However, it does not
influence the B-H loop areas, because the energy stored
in the stray capacitance does not affect the area in aperiod. The capacitance energy is restored to the voltage
source.
Similar to the experiments resulting in Fig. 4, B-H
loops were also measured at 30, 40, 50, 60 and 70 °C. The
temperature characteristics of the hysteresis anddynamic magnetic losses are shown in Figs. 5(a) and
5(b), where Bm is kept at 100 mT and 200 mT, respectively.
The measured dynamic magnetic losses are plotted withdB/dt = 1000, 800, 600, 400 and 200 mT/s from the top down,
respectively, and the bottommost broken lines represent

The hypothesis that the dynamic magnetic loss
corresponds to the eddy current loss in ferrite grains
has been disproved 4). Thus, the dynamic magnetic loss
must be caused by a magnetic phenomenon explainable
by electron spins, as in the case of the hysteresis loss,
whereas the eddy current can be explained by freeelectron motion. However, the experimental resultspresented here suggest that the generation mechanismof the dynamic magnetic loss cannot be exactly thesame as that of the hysteresis loss, given the differencein their temperature characteristics.
The hysteresis losses, normalized to the values at
0 °C, decrease with temperature to a greater extent
than the normalized dynamic losses. On the other hand,
the temperature dependence of the dynamic magnetic
loss in the raw data is stronger than that of thehysteresis loss. The dynamic magnetic loss is more
dominant under large dB/dt conditions. Thus, ferrite
manufacturers need to observe closely the temperature
characteristics of the dynamic magnetic loss, rather
than evaluating only the total magnetic loss.
References
1) http://product.tdk.com/ja/catalog/datasheets/ferrite_mn-zn_

material_characteristics_ja.pdf

2) H. Saotome and Y. Sakaki: IEEE Trans. Magn., 33, 728(1997)
3) H. Saotome and H. Ohta: J. Magn. Soc. Jpn. 39, 62(2015)4) H. Saotome: J. Magn. Soc. Jpn. 40, 19(2016)
Received Feb. 15, 2017; Accepted Mar. 27, 2017

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65

INDEX
J. Magn. Soc. Jpn., 41, 66-69 (2017)
LetterEffect of Amorphous Region on Magnetic Orientation ofPoly(lactic acid)
Blend Films with Different Molecular Weight
Rei Nakayama, Hiroki Ikake*, Kimio Kurita*, Shigeru Shimizu*, Satoshi Kurumi*, Kaoru Suzuki*, Kohki
Takahashi** and Hiroyuki Nojiri**
Graduate School of Science and Technology, Nihon Univ., 1-8-14 Kanda Surugadai, Chiyoda-ku, Tokyo 101-8308, Japan*

College of Science and Technology, Nihon Univ., 1-8-14 Kanda Surugadai, Chiyoda-ku, Tokyo 101-8308, Japan**

Institute for Materials Research, Tohoku Univ., 2-1-1 Katahira, Aobaku, Sendai, Miyagi, JapanThe magnetic-field-induced blend films, poly(L-lactide)(PLLA10) (Mn = 1.0 × 105) and poly(DL-lactide) (Mw = 1.0 ×
104 (PDLLA1) and 1.0 × 105 (PDLLA10)) were prepared in isothermal process under the magnetic-field 10 T. The
effects of amorphous region, i.e., molecular weight of PDLLA effect on crystallization and orientation of PLLA in thefilms were investigated using wide-angle X-ray diffraction and polarizing microscopy. The crystallinity of each films
showed 60% at crystallization growth time tc = 30 hrs irrespective of PDLLA molecular weight. The degree oforientation of PLLA10/PDLLA10 film increased with increasing tc. We concluded that the blending with amorphous
high-molecular-weigh PDLLA and crystallization in an applied high magnetic field is effective to achieve the PLLA
film that has higher crystallinity and orientation.
Key words: magnetic-induced-orientation film, poly(L-lactide), poly(DL-lactide), amorphous, blend film, highmagnetic field
1. Introduction
Recently, biomass plastics are interest for saving
petroleum resources and reduction of carbon dioxide. Itis especially expected that the biomass plastics
produced from plant-derived raw materials. For
example, poly(lactic acid)(PLA) is well-known biomass
plastic which are produced from renewable resources
and used in many field such as biomedical, packaging
and ecological fields.
There has been an extensive effort to investigate for
PLA such as crystal structure, crystal growth, melting
behavior, and applications.1) Many researchers havestudied stereocomplex consist of blend of PLLA andpoly(D-lactic acid)(PDLA) as a solution to improve thethermal and physical properties of PLLA.2) 3) However,
PDLA is the high cost as industrial raw material.
Another approach such as the orientation control is
proposed to improve the properties of PLLA.
Orientation control of PLLA is an important subject
because the physical properties of PLLA are strongly
affected by the orientation. By using mechanicalorientation, one obtains materials with ordered polymer
crystals. This method is often used in industry, however
there is a fear of deformation. PLLA is known to be
miscible with PDLLA and develop PLLA crystal in
PDLLA.4) 5) We previously reported that themagnetic-induced oriented films of PLLA/PDLLA,
which PDLLA has lower molecular weight than that ofPLLA.6)
Crystallization of PLLA has been developed in PDLLA
and shown the high degree of orientation.
In this work, the influence of molecular weight ofamorphous PDLLA in the PLLA/PDLLA blend films on66

the crystallization and orientation behaviors of PLLA
was investigated.
2. Experimental
2.1 Materials
PLLA of Mn = 1.0 × 105 (PLLA10), and PDLLA of Mw= 1.0 × 104 (PDLLA1) and Mw = 1.0 × 105 (PDLLA10)
were purchased from Sigma-Aldrich Co. The opticalpurity was determined by the specific optical rotation
measurement of PLA sample [α]sample using P-2300
polarimeter (JASCO Corporation, Japan) with awavelength of 589.3 nm (D-line of the sodium lamp) at
25 °C.

OP% =[α ] sample
× 100[α ] PLLA(1)where [α]PLLA is the specific rotation of PLLA with 100%of L stereoisomer and value of -156° was used.7) The€
percentage optical purity of PLLA10, PDLLA1 andPDLLA10 were 98.3, 0.4 and 0.5%, respectively. This
result shows both PDLLA10 and PDLLA1 areconsisting of racemic lactate units and are completely
noncrystallizable polymers.
2.2 Preparation of PLLA/PDLLA solution-cast
films
Two
kinds
of
solution-castblend
films,
PLLA10/PDLLA1 film and PLLA10/PDLLA10 film,
were prepared. Concentration of 10 wt% of PLLA10
chloroform solution and 10 wt% of PDLLA1 or
PDLLA10 chloroform solution were mixed and stirred
for 2 hrs, and the solution was put in petri dish and was

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

INDEX

stood for 24 hrs. PLLA/PDLLA films were dried at
70 °C for 2 hrs in vacuum oven. The thickness of eachfilms was 100 ± 20µm.
2.3 Preparation of PLLA/PDLLA oriented films
under high magnetic fieldPreparation of PLLA/PDLLA oriented films in amagnetic field was carried out at Institute for Materials
Research, Tohoku University. The temperature
program for the crystallization of PLLA in the blend
films was illustrated in Figure 1. The films were heatedfrom room temperature to the annealing temperature
185°C at a rate of 3°C/min and held constant for 10 min.
Then, films were cooled to 140°C (isothermal
crystallization
temperature)for
isothermal
crystallization time tc = 0, 2, 15 and 30 hrs. The
magnetic field of 10 T was applied throughout thewhole thermal process.

respectively. 8) 9) 10) The intensity of these diffraction
peaks
increasedgraduallywithincreasing
crystallization time. However, refraction peaks of
110/200 and 203 of the PLLA10/PDLLA10 film for tc = 2
hrs have been not detected.

Fig.2 WAXD patterns of PLLA10/PDLLA1 films.

Fig.1 Thermal process for PLLA10/PDLLA1
and PLLA10/PDLLA10 films. Throughout
the whole thermal process, the magnetic fieldof 10 T was applied.
2.4 Measurements
WAXD measurement was carried out using X'Pert
PRO MPD(PANalytical, Japan) operated at 45 kV and
40 mA to generate Ni-filtered CuKα X-ray beam.
Scanning speed was 0.01°/s and measurement range
was 3 – 60° at room temperature. Crystals growth in
the films was observed by polarization microscope(BX53-33P-OC-1, Olympus, Japan).
3. Results and discussion
3.1 Crystallization of oriented films
In Figures 2 and 3, WAXD pattern of
PLLA10/PDLLA1 and PLLA10/PDLLA10 films after
annealing at 140°C for different periods was shown.
PLLA shows the two dominant diffraction peaks at
16.7° and 19.1° of α-form crystal. These diffraction
peaks are assigned to the 110/200 and 203 reflections,Fig.3 WAXD patterns of PLLA10/PDLLA10 films.
The relationship between tc and the degree ofcrystallization Xc was shown in Figure 4. From the peakareas of WAXD patterns, Xc was calculated by thefollowing equation,

XC % =SPLLA
× 100
SPLLA + SPDLLA(2)

where SPLLA and SPDLLA are PLLA and PDLLA€
Journal of the Magnetics Society of Japan Vol.41, No.3, 2017

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INDEX

diffraction peaks areas.11) Xc of neat PLLA film which
prepared in the same condition also was shown forcomparison. From the figure, it is clear that theinduction period of PLLA crystallization of blend films
are longer than that of neat PLLA10 film. The
induction period of PLLA10/PDLLA1 is short incomparison with PLLA10/PDLLA10. This could beinterpreted that the mobility of PLLA10 chains arelarge
becausethe
meltviscosityof
low-molecular-weight PDLLA1 are lower than that of
PDLLA10. The crystallization rate of both blend films isfaster than that of neat PLLA after the induction period.
The value of Xc of all films, however, attained about
60% for tc = 30 hrs.

neat PLLA films are constant for all range of tc, i.e., thisshows that applied magnetic field has no effect on theorientation of these films.

Fig. 5 The degree of orientation fc by WAXD
measurementof
PLLA10/PDLLA1
and
PLLA10/PDLLA10 films.

Fig. 4 The degree of crystallinity Xc by WAXD
measurementof
PLLA10/PDLLA1
and
PLLA10/PDLLA10 films.

3.3 Isothermal crystalline behavior of oriented
films
Figure 6 shows a polarizing optical micrograph of (A)PLLA10/PDLLA1, (B) PLLA10/PDLLA10, and (C) neat
PLLA films. Optical micrographs were taken at
different crystal growth time ranging from 0 to 30 hrs.
These films show the formation of spherulite that wasgrown isothermally at 140°C, and its content increase
with increasing tc. As is seen in Figure 6(C), thespherulite radius of neat PLLA10 increased with

3.2 Magnetic orientation of oriented films
The c-axis of PLLA crystal in the PLLA/PDLLA film
was oriented in the parallel direction to the magnetic
field.6) The degree of orientation was defined as theazimuthal angle of reflected intensity of 110/200 planes.
The degree of the magnetic-field-induced orientation fc
was calculated by the following equations,2 2 
I( ) cos2 sin d
(3)
cos
= 0 
2
I( ) sin d0
3 cos2
fc =
2

1

(4)

where φ is the azimuthal angle and I(φ) is theazimuthal intensity. Figure 5 shows fc against tc of
blend films and neat PLLA film.
In the case of PLLA10/PDLLA10 film, fc increased withincreasing tc. In contrast, a fc of PLLA10/PDLLA1 and68

Fig.6 Polarized photomicrographs of films.
(A): PLLA10/PDLLA1 and (B): PLLA10/PDLLA10
at tc = 0, 2, 15 and 30 hrs. (C): neat PLLA10 at tc =0, 2, 12 and 30 hrs.

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

INDEX

increasing tc, and the behavior agrees with theliterature.12) On the other hand, the crystallization
behavior of blend films could be explained by following.
Increasing of spherulites in both blend films is due totrapping amorphous PDLLA in the spherulite of
PLLA.5) 12) For tc = 0 hr, the photomicrographsdemonstrate small crystallites grew spherically fromsingle nuclei. For tc = 2 hrs, numbers of small
crystallites increase in PLLA10/PDLLA1 film, but afaintly increase in PLLA10/PDLLA10 film. For tc > 15
hrs, it is clear that the spherulites contents in eachfilms increased with increasing crystal growth time. For
tc = 30 hrs, PLLA10/PDLLA1 film shows large opticaltextures in Figure 6(A).
Comparison
withspherulite
structureof
PLLA10/PDLLA1 and PLLA10/PDLLA10, the latter ismore disordered than the former. Formation of theselarge spherulites observed in PLLA10/PDLLA1 can be
explained by the homogeneous crystallization. In
contrast, PLLA10/PDLLA10 film shows small opticaltexture(Figure 6(B)). This is due to heterogeneous
nucleation, i.e., the locally orientated domain.
4. Conclusions
We have investigated the magnetic-field-induced
PLLA in amorphous PDLLA oriented films without
nucleating agents in a 10 T magnetic field. The
dependence of molecular weight of PDLLA on
crystallization of PLLA in the blend films was notobserved in the crystallization in the magnetic field.
However, orientation behavior is quite different
between PLLA10/PDLLA1 and PLLA10/PDLLA10 films.
That is, trapping of low-molecular-weight PDLLA in thespherulite of PLLA10 show disturb the orientation of
PLLA crystal. On the other hand, trapping
high-molecular-weight
PDLLA
promotehigherorientation film. Hence, the blending with amorphous
high-molecular-weigh PDLLA and crystallization in an
applied magnetic field is effective to achieve the PLLA
film that has higher crystallinity and orientation.
References

1) R. Auras, L.-T. Lim, S. E. M. Selke, and H. Tsuji, Eds.:
Poly(lactic acid) :Synthesis, Structures, Properties, Processing,and Applications, Wiley, New York, (2010).
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Macromolecules, 20, 906 (1987).
3) H. Tsuji, and Y. Ikada: Polymer, 40, 6699 (1999).
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5) P. Pan, Z. Liang, B. Zhu, T. Dong, and Y. Inoue:
Macromolecules, 42, 3374 (2009).
6) R. Nakayama, H. Ikake, K. Kurita, S. Shimizu, S. Kurumi,
K. Suzuki, K. Takahashi, and K. Watanabe: Kobunshi
Ronbunshu, 72, 661 (2015).
7) D. A. S. Marques, S. Jarmelo, C. M. S. G. Baptista, and M.H.
Gil: Macromol. Symp., 296, 63 (2010).
8) W. Hoogsteen, A. R. Postema, A. J. Pennings, G. tenBrink,
and P. Zugenmaier: Macromolecules, 23, 634 (1990).

9) J. Zhang, Y. Duan, H. Sato, H. Tsuji, I. Noda, Sh. Yan, and Y.
Ozaki: Macromolecules, 38, 8012 (2005).

10) P. Pan, W. Kai, B. Zhu, T. Dong, and Y. Inoue: Macromolecules, 40,
6898 (2007).

11) L. Bouapao, H. Tsuji, K. Tashiro, J. Zhang, M. Hanesaka: Polymer,
50, 4007 (2009).

12) H. Tsuji, and Y. Ikada: Polymer, 37, 595 (1996).

Received Dec. 08, 2016; Accepted Jan. 05, 2017

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Editorial Committee Members・Paper Committee Members
H.Saotome and K. Kobayashi (Chairperson), T. Kato, K. Koike and T. Taniyama (Secretary)
T. Daibo
Y. EndoA.Fujita
T. Hasegawa
H. Hashino
N. Hirota
T. Ichihara
S. Ikeda
S. Inui
K. Iramina
M. Kakikawa
S. Kasai
A. Kikitsu
K. Miura
H. Morise
H. Naganuma
T. Nichiuchi
T. Ohji
M. Ohtake
M. Oogane
T. SatoM. Sonehara
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Journal of the Magnetics Society of Japan
Vol. 41 No. 3 (通巻第 291 号) 2017 年 5 月 1 日発行

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