Oral Session A1 (Tuesday, February 13)
Chair: Satoru Nakatsuji, University of Tokyo

Keynote Talk
10 : 15 - 10 : 55

Kyung-Jin Lee
Korea Advanced Institute of Science and Technology (KAIST), Korea

Orbital torque and orbital pumping
The orbital Hall eﬀect [1,2] describes the generation of the orbital current flowing in aperpendicular direction to an external electric field, analogous to the spin Hall eﬀect. As the orbital
current carries the angular momentum as the spin current does, injection of the orbital current
into a ferromagnet can result in torque on the magnetization [3], which provides a way to detect
the orbital Hall eﬀect. With this motivation, we examine the current-induced spin-orbit torques invarious ferromagnet/heavy metal bilayers by theory and experiment [4]. Analysis of the magnetic
torque reveals the presence of the contribution from the orbital Hall eﬀect in the heavy metal,
which competes with the contribution from the spin Hall eﬀect. In particular, we find that net
torque in Ni/Ta bilayers is opposite in sign to the spin Hall theory prediction but instead consistent
with the orbital Hall theory. This orbital torque can enhance net spin-orbit torque via an eﬃcientorbital-to-spin conversion [5]. We also present a theory of orbital pumping [6], which is theOnsager reciprocity of orbital torque.
References[1] H. Kontani et al., Giant orbital Hall eﬀect in transition metals: Origin of large spin and anomalous Halleﬀects. Phys. Rev. Lett. 102, 016601 (2009).
[2] D. Go and D. Jo, C. Kim, and H.-W. Lee, Intrinsic spin and orbital Hall eﬀects from orbital texture.
Phys. Rev. Lett. 121, 086602 (2018).
[3] D. Go and H.-W. Lee, Orbital torque: Torque generation by orbital current injection. Phys. Rev. Research
2, 013177 (2020).
[4] D. Lee et al., Orbital torque in magnetic bilayers. Nat. Commun. 12, 6710 (2021).
[5] S. Lee et al., Eﬃcient conversion of orbital Hall current to spin current for spin-orbit torque switching.
Commun. Phys. 4, 234 (2022).
[6] S. Han et al., Theory of orbital pumping. arXiv:2311.00362 (2023).

Oral Session A1 (Tuesday, February 13)
Chair: Satoru Nakatsuji, University of Tokyo

Contributed Oral
10 : 55 - 11 : 15

Observation of current-driven fast magnetic domain-wallmotion in noncollinear antiferromagnets
M. Wu1, 2, T. Chen1, 3, 4, T. Nomoto5, H. Isshiki1, 6, Y. Nakatani7, T. Higo1, 4, 6, T.
Tomita1,4,6, K. Kondou2, 6, R. Arita2, 5, 6, S. Nakatsuji1, 4, 6, 8, Y. Otani1, 2, 6, 8
1. The Institute for Solid State Physics, The University of Tokyo; Kashiwa, Chiba 277-8581, Japan.
2. Center for Emergent Matter Science, RIKEN; 2-1 Hirosawa, Wako 351-0198, Japan.
3. School of Physics, Southeast University; Nanjing 211189, China.
4. Department of Physics, University of Tokyo; Hongo, Bunkyo-ku, Tokyo 113-0033, Japan.
5. Research Center for Advanced Science and Technology, University of Tokyo; 4-6-1 Meguro-ku, Tokyo, 153-8904, Japan.
6. CREST, Japan Science and Technology Agency (JST); 4-1-8 Honcho Kawaguchi, Saitama 332-0012, Japan.
7. Department of Computer Science, University of Electro-Communications; 1-5-1 Chofugaoka, Chofu-Shi, Tokyo
182-8585, Japan.
8. Trans-Scale Quantum Science Institute, The University of Tokyo, Bunkyo-ku, Tokyo 113-0033, Japan

Antiferromagnets have the natural advantages of ultrafast magnetization dynamics and negligible stray fields
compared with ferromagnets, thus appealing for next-generation magnetic domain-wall applications. So far,however, the study of the magnetization dynamics in antiferromagnets has been challenging due to theirinsensitive magneto-electric responses. Recently, remarkable developments on noncollinear antiferromagnets
Mn3X (X = Sn, Ge) enabled us to detect and manipulate their antiferromagnetic domain states [1-7]. Here, we
demonstrate a current-driven magnetic domain-wall motion in a single crystal Mn3X wire by means of magnetooptical Kerr observation technique [8]. It reveals that Néel-like domain-walls can be accelerated up to 750 m/s
with a current density of only 7.6×1010 A/m2. It also shows extremely high mobility compared with ferromagnets
and ferrimagnets. Moreover, our theory reveals that the domain-wall motion in noncollinear antiferromagnets isdriven by the spin-transfer torque derived from the current-induced nonequilibrium spin accumulation. These
our findings open a new route to develop a mechanism responsible for antiferromagnetic domain-wall-based
applications.

References[1] S. Nakatsuji et al., Nature 527, 212–215 (2015).
[2] T. Higo, et al., Nat. Photon. 12, 73–78 (2018).
[3] H. Tsai, et al., Nature 580, 608–613 (2020).
[4] Y. Takeuchi et al., Nat. Mater. 20, 1364–1370 (2021).
[5] T. Higo, et al., Nature 607, 474–479 (2022).
[6] B. Pal et al., Sci. Adv. 8, 24 (2022).
[7] J-Y. Yoon et al., Nat. Mater. 22, 1106–1113 (2023).
[8] M. Wu submitted (2023).

Oral Session A1 (Tuesday, February 13)
Chair: Satoru Nakatsuji, University of Tokyo

Contributed Oral
11: 15 - 11 : 35

Stroboscopic magneto-optical imaging of current-induced
domain wall dynamics in ferrimagnet GdFeCo
K. Ogawa1, N. Yoshikawa1, M. Ishibashi1, A. Tsukamoto2, M. Hayashi1 & R. Shimano1,3
1. Dept. of Phys., Univ. of Tokyo
2. College of Science and Technology, Nihon Univ.
3. Cryogenic Research Center, Univ. of Tokyo
Current-induced domain wall motion (CIDWM) is expected to find applications in spintronic devices such as
memory and logic. Recently, it is known that CIDWM in ferrimagnetic materials can exceed 1 km/s at the angularmomentum compensation point temperature TA [1,2]. A method to visualize such ultrafast dynamics of CIDWM isnecessary to understand its physics and achieve accurate manipulation of domain walls (DWs).
Here, we developed a stroboscopic magneto-optical imaging system capable of detecting DW displacements ofsub-µm with a time resolution of approximately 160 ps [3]. In our experiment, DWs were generated in aferrimagnetic GdFeCo/Pt wire by all-optical magnetization reversal and ns current pulses were injected to observe
CIDWM. The CIDWM dynamics unveil a time varying DW velocity during the pulse, especially, an acceleration
noticeable approximately 1 ns after the start of current injection. These results can largely be attributed to the gradualtemperature change caused by Joule heating and an enhanced DW velocity at TA. Our method provides access to thespatial and stochastic magnetization dynamics in sophisticated structures driven by external stimuli and should beapplicable to a wide range of current-driven phenomena.
References[1] Kim, K. et al. Nat. Mater. 16, 1187–1192 (2017).
[2] Caretta, L. et al. Nat. Nanotechnol. 13, 1154–1160 (2018).
[3] Ogawa, K. et al. Phys. Rev. Res. 5, 033151 (2023).

Oral Session A1 (Tuesday, February 13)
Chair: Satoru Nakatsuji, University of Tokyo

Keynote Talk
11: 35 - 12 : 15

Cheng Song
Korea Advanced Institute of Science and Technology (KAIST), Korea

Electrical 180° switching of Néel vector inaltermagnets
Altermagnet is an emerging magnetic phase with alternating spins and spin splitting band
structure, thus combining the advantages of both antiferromagnets and ferromagnets [1-3].
However, as crucial components, the electrical detection and electrical 180o switching of the Néelvector as well as the corresponding spin-splitting, are very challenging. We demonstrated that inaltermagnets Mn5Si3 [4] and CrSb [5], the unique anomalous Hall eﬀect can be adopted forelectrical readout of opposite Néel vectors. We proposed a new mechanism for the electrical 180o
switching of the Néel vector via spin-orbit torques by designing asymmetric switching barriers andexperimentally achieved it. It is made possible by the fixed chirality between Néel vector and tiny
relativistic net moment due to the Dzyaloshinskii-Moriya interaction. Based on their novel readout
and manipulation methods, we fabricated prototype Hall devices that can accomplish robust write
and read cycles. By suitable design of crystal distortion via strain, field-free and fully electricalswitching of altermagnetic Néel vector is realized. Furthermore, controllable Néel vector enables
tunable spin-charge interconversion through altermagnetic and inverse altermagnetic spin splitting
eﬀect [2,3,6].
References[1] L. Šmejkal, et al., Phys. Rev. X 12, 040501 (2022)[2] H. Bai, C. Song, et al. Phys. Rev. Lett. 128, 197202 (2022)[3] H. Bai, C. Song, et al. Phys. Rev. Lett. 130, 216701 (2023)[4] L. Han, C. Song, et al. accepted by Sci. Adv.
[5] Z. Y. Zhou, C. Song, et al. In preparation.
[6] Y. Liu, C. Song, Q. Y. Jin, et al. Adv. Optical Mater. 2300177 (2023).