Magnetic Dynamics in Antiferromagnetically-Coupled Ferrimagnets: The Role of Angular Momentum

86 Pages·2020·3.445 MB·English

by  Takaya Okuno

Springer Theses
Recognizing Outstanding Ph.D. Research
Takaya Okuno
Magnetic Dynamics in
Antiferromagnetically-
Coupled Ferrimagnets
The Role of Angular Momentum
Springer Theses
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Takaya Okuno
Magnetic Dynamics in
Antiferromagnetically-
Coupled Ferrimagnets
The Role of Angular Momentum
Doctoral Thesis accepted by
Kyoto University, Kyoto, Japan
123
Author Supervisor
Dr. Takaya Okuno Teruo Ono
JapanPatent Office Institute for Chemical Research
Chiyoda, Tokyo,Japan KyotoUniversity
Kyoto, Japan
ISSN 2190-5053 ISSN 2190-5061 (electronic)
SpringerTheses
ISBN978-981-15-9175-4 ISBN978-981-15-9176-1 (eBook)
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’
Supervisor s Foreword
Conventional electronics utilize mostly the “charge” of electrons, whereas tradi-
tional magnetic devices use mainly the “spin” degree offreedom of electrons. The
field of spintronics, which aims at controlling both the charge and spin in a single
solid-state device, is developing rapidly and reshaping the information technology
field. The focus of spintronics has long been ferromagnets because they have
significant net magnetization, which enables an easy control of the magnetic state
using the magnetic field. Recently, antiferromagnets, a class of magnets where
neighboring moments are coupled to align antiparallel so that net magnetization is
zero, have been gaining increasing attention for device applications. Robustness
against perturbation caused by magnetic fields, absence of the production of stray
fields eliminating the unintentional magnetic crosstalk between neighboring devi-
ces, and ultrafast magnetic dynamics are advantageous characteristics of such
devices.However,thestudiesonantiferromagnetshavebeensignificantlylessthan
those on ferromagnets because of the difficulty in inducing antiferromagnetic
dynamics owing to their high stability against the magnetic field, which prevents
the manipulation of magnetic moments.
3d transition-metal and rare-earth alloys are types offerrimagnets, in which the
magnetic moments of the transition metal and rare earth, each with a different
magnitudeofmagnetization,coupleantiferromagnetically,resultinginnonzeronet
magnetization. Therefore, ferrimagnets are expected to have both
ferromagnetic-like (nonzero net magnetization) and antiferromagnetic-like (neigh-
boring moments are coupled antiparallel) characteristics. In 2017, Kab-Jin Kim
et al. reported that the field-driven magnetic domain wall (DW) velocity of a
ferrimagnet increased remarkably at the angular momentum compensation tem-
perature(T Þ,atwhichthenetangularmomentumoftheferrimagnetwascanceled
A
out. The stark increase in DW velocity is attributed to antiferromagnetic-like
dynamicsinferrimagnets.Thisstudyhasopenedupthepossibilitythattheultrafast
antiferromagnetic dynamics in ferrimagnets can be driven by magnetic field,
thereby overcoming the difficulty in inducing antiferromagnetic dynamics. It has
led to many investigations on ferrimagnets that seek to realize spintronic devices
based on ferrimagnets.
v
vi Supervisor’sForeword
In this thesis, Takaya Okuno has investigated the role of angular momentum in
ferrimagnetic dynamics in a gadolinium–ironcobalt (GdFeCo) alloy, in which
rare-earth (Gd) and transition-metal (FeCo) moments are coupled antiferromag-
netically. First, the current-induced torque on magnetic DW motion, spin-transfer
torque (STT), is examined in a wide range of temperatures across T . It is con-
A
cludedthatthenon-adiabaticSTTinantiferromagnetsactsasastaggeredmagnetic
field,i.e.,itexertseffectivemagneticfieldsofequalmagnitudesandoppositesigns
on the antiferromagnetically-coupled moments, which can be used for the efficient
control of antiferromagnetic DWs. Moreover, the non-adiabaticity parameter is
significantly larger than the Gilbert damping parameter, challenging the conven-
tional understanding of the non-adiabatic STT based on ferromagnets. Next, the
Gilbertdampingparameterofferrimagnets,i.e.,therateofenergydissipationfrom
ferrimagnetic dynamics, is investigated using two independent experiments: the
field-drivenDWvelocityandferrimagneticresonance.Incontrastwiththeprevious
reports,inwhichtheeffectiveGilbertdampingparameterofferrimagnetsincreases
remarkably as the temperature approaches T and diverges at T , the theoretical
A A
and experimental investigations in this thesis reveal that the Gilbert damping
parameter of ferrimagnets is insensitive to temperature even across T . These
A
studies uncover the nature offerrimagnetic dynamics in terms of angular momen-
tum and serve as a significant step forward to clarify the dynamics of antiferro-
magnetic moments. This thesis provides a scientific platform for further
investigation into the fundamental physics of ferrimagnetic and antiferromagnetic
dynamics as well as realizing the next-generation spintronic devices.
Kyoto, Japan Teruo Ono
August 2020
List of Published Articles
Parts of this thesis have been published in the following journal articles:
[1] TakayaOkuno,Duck-HoKim,Se-HyeokOh,SeKwonKim,YuushouHirata,
TomoeNishimura,WooSeungHam,YasuhiroFutakawa,HirokiYoshikawa,
ArataTsukamoto,YaroslavTserkovnyak,YoichiShiota,TakahiroMoriyama,
Kab-Jin Kim, Kyung-Jin Lee, and Teruo Ono, ‘Spin-transfer torques for
domain wall motion in antiferromagnetically coupled ferrimagnets’ Nature
Electronics 2, 389-393 (2019).
[2] Duck-Ho Kim, Takaya Okuno, Se Kwon Kim, Se-Hyeok Oh, Tomoe
Nishimura, Yuushou Hirata, Yasuhiro Futakawa, Hiroki Yoshikawa, Arata
Tsukamoto, Yaroslav Tserkovnyak, Yoichi Shiota, Takahiro Moriyama,
Kab-Jin Kim, Kyung-Jin Lee, and Teruo Ono, ‘Low Magnetic Damping of
FerrimagneticGdFeCoAlloys’,PhysicalReviewLetters122,127203(2019).
[3] Takaya Okuno, Se Kwon Kim, Takahiro Moriyama, Duck-Ho Kim, Hayato
Mizuno, Tetsuya Ikebuchi, Yuushou Hirata, Hiroki Yoshikawa, Arata
Tsukamoto, Kab-Jin Kim, Yoichi Shiota, Kyung-Jin Lee, and Teruo Ono,
‘Temperature dependence of magnetic resonance in ferrimagnetic GdFeCo
alloys’, Applied Physics Express 12, 093001 (2019).
vii
Acknowledgements
Many people have supported me in my research and personal life during my five
yearswiththeOnogroup,twoyearsasamaster’sstudentandthreeyearsasaPh.D.
scholar. First, I would like to express my sincere gratitude to my supervisor Prof.
TeruoOnoforhisvaluablesuggestionsandadvicenotonlyinmyresearchbutalso
inpersonallife.Heinspiredmetodevelopinterestinnewphysicalphenomenaand
developideas.IwouldalsoliketothankProf.TakahiroMoriyama,whohastaught
mehowtothinklogicallyandhowtodealwithexperimentaldata.ProfessorYoichi
Shiota helped me with the simulation program for macrospin simulation and
reviewedmyabstractsforconferences.IalsothankDr.Duck-HoKim,withwhomI
discussed many experiments. The idea of the spin-transfer torque experiment took
shape during my discussions with him.
I would also like to express my thanks to my collaborators overseas. Professor
Kab-jin Kim at the Korea Advanced Institute of Science and Technology was my
supervisor in the Ono group for one and a half years. While he performed his
researchontheferrimagneticDW(publishedinNatureMaterials),hehandeddown
some research on ferrimagnets to some of us at the Ono lab. He is not only a
distinguishedscientistbutalsoathoughtfulgentleman.Heisoneofmyrolemodels
as a scientist. The research in this thesis is owed mostly to him.
Furthermore, I would like to thank Prof. Se Kwon Kim at the University of
Missouri. I have learned a lot from my discussions with him, most of which took
placeovere-mails.Hisvaluable adviceandsuggestionshaveimproved thequality
ofmyresearch.Myinteractionswithhimhelpedmerealizetheimportanceofbeing
prompt and thoughtful in research and business. Next, I would like to thank Prof.
Kyung-Jin Lee at Korea University. His suggestions have been path-breaking.
Notwithstanding his preoccupations as a leading theoretician in spintronics, I have
benefitted from his views whenever I needed guidance in my research.
I would like to express my gratitude to the students in the Ono group: Mizuno
san, Nishimura san, Ishibashi san, Hirata san, Li san, Ikebuchi san, Hung san, and
the master’s course students. In particular, I would like to thank my friends Ando
san and Oda san, with whom I have spent five years. Moreover, I have received
indirect supportfrom manypeople. Iamgrateful toKusudasanandIchikawa san,
ix
x Acknowledgements
the managing staff at the Liquid Helium Experiment facility. I also express my
sincere gratitude to the secretaries, Yamaguchi san and Tanigawa san, of the Ono
group. I really appreciate all the kindness and support from each one of them.
Finally,Iwouldliketoshowmygreatestappreciationtomyfamily.Icouldnot
have accomplished my Ph.D. degree without their moral support and warm
encouragements.