From Riken, Magnetic Octupoles Overcome Problems with Antiferromagnets

From RIKEN, Saitama, Japan

Physicists at RIKEN have demonstrated how ultrafast, low-power-consumption memory devices could be realized by replacing conventional magnetic materials with novel ones ⁽¹⁾.

Figure 1: Conceptual image showing a cluster magnetic octupole comprising eight sublattice moments (small arrows) arranged in an octahedron. The long green arrow indicates the net magnetic moment.
(© 2024 RIKEN Center for Emergent Matter Science)

In conventional HDDs, the magnetic disk has to be physically moved to read to and write from the disk. This has the disadvantages of being slow and susceptible to mechanical problems.

A better approach would be to use electrical currents to move the walls between magnetic domains – microscopic regions in a magnetic material in which the magnetic moments align in consistent fashion. So-called domain-wall devices are promising for realizing rapid, low-power memory devices.

Yoshichika Otani and co-workers have demonstrated rapid motion of magnetic octupole
domain walls using electrical currents for noncollinear antiferromagnets.
(© 2024 RIKEN)

“In the case of a hard disk drive, you have a small coil which has to be physically moved around,” explains Yoshichika Otani, RIKEN Center for Emergent Matter Science. “But for devices based on domain walls, you don’t need any mechanical movement. Rather, the domain wall moves, and you can read and write information electrically without any mechanical motion.”

Domain-wall devices have been investigated using ferromagnetic domains in which all the spins in a domain are parallel with each other. But these require high current densities to push the domain walls around, which results in high power consumptions. The domains also generate stray magnetic fields, which makes it challenging to cram a lot of them into a small space, making miniaturization difficult.

Antiferromagnetic domains in which the spins are arranged in alternating directions could overcome both these problems. But their low net magnetic fields are a double-edged sword – they are beneficial for miniaturization, but they make it difficult to manipulate and detect the domains.

Now, Otani and his co-workers have demonstrated a new approach for realizing domain-wall devices based on antiferromagnetic materials that overcomes this difficulty.

The secret to their approach was to use noncollinear antiferromagnets in which sublattice moments form cluster magnetic octupoles (Fig. 1). This contrasts with the much more commonly used magnetic dipoles, which have 2 poles and resemble tiny bar magnets.

Using this structure, the team was able to accelerate domain walls to speeds of 750 meters/second using about a hundredth of the current density needed to move ferromagnet domain walls.

The findings came as a nice surprise to the team.

“We weren’t confident that it would work with octupoles,” says Otani. “But it actually worked when we tried it, and so we were pleasantly surprised.”

Article: Current-driven fast magnetic octupole domain-wall motion in noncollinear antiferromagnets

Nature Communications has published an article written by Mingxing Wu, The Institute for Solid State Physics, The University of Tokyo, Kashiwa, Chiba, 277-8581, Japan, and Center for Emergent Matter Science, RIKEN, 2-1 Hirosawa, Wako, 351-0198, Japan, Taishi Chen, The Institute for Solid State Physics, The University of Tokyo, Kashiwa, Chiba, 277-8581, Japan, Key Laboratory of Quantum Materials and Devices of Ministry of Education, School of Physics, Southeast University, Nanjing, 211189, China, and Department of Physics, University of Tokyo, Hongo, Bunkyo-ku, Tokyo, 113-0033, Japan, Takuya Nomoto, Research Center for Advanced Science and Technology, University of Tokyo, 4-6-1 Meguro-ku, Tokyo, 153-8904, Japan, Yaroslav Tserkovnyak, Department of Physics and Astronomy and Bhaumik Institute for Theoretical Physics, University of California, Los Angeles, Los Angeles, CA, 90095, USA, Hironari Isshiki, The Institute for Solid State Physics, The University of Tokyo, Kashiwa, Chiba, 277-8581, Japan, and CREST, Japan Science and Technology Agency (JST), 4-1-8 Honcho, Kawaguchi, Saitama, 332-0012, Japan, Yoshinobu Nakatani, Department of Computer Science, University of Electro-Communications, 1-5-1 Chofugaoka, Chofu-Shi, Tokyo, 182-8585, Japan, Tomoya Higo, The Institute for Solid State Physics, The University of Tokyo, Kashiwa, Chiba, 277-8581, Japan, Department of Physics, University of Tokyo, Hongo, Bunkyo-ku, Tokyo, 113-0033, Japan, and CREST, Japan Science and Technology Agency (JST), 4-1-8 Honcho, Kawaguchi, Saitama, 332-0012, Japan, Takahiro Tomita, The Institute for Solid State Physics, The University of Tokyo, Kashiwa, Chiba, 277-8581, Japan, Department of Physics, University of Tokyo, Hongo, Bunkyo-ku, Tokyo, 113-0033, Japan, and CREST, Japan Science and Technology Agency (JST), 4-1-8 Honcho, Kawaguchi, Saitama, 332-0012, Japan, Kouta Kondou, Center for Emergent Matter Science, RIKEN, 2-1 Hirosawa, Wako, 351-0198, Japan, and CREST, Japan Science and Technology Agency (JST), 4-1-8 Honcho, Kawaguchi, Saitama, 332-0012, Japan, Ryotaro Arita, Center for Emergent Matter Science, RIKEN, 2-1 Hirosawa, Wako, 351-0198, Japan, Research Center for Advanced Science and Technology, University of Tokyo, 4-6-1 Meguro-ku, Tokyo, 153-8904, Japan, and CREST, Japan Science and Technology Agency (JST), 4-1-8 Honcho, Kawaguchi, Saitama, 332-0012, Japan, Satoru Nakatsuji, The Institute for Solid State Physics, The University of Tokyo, Kashiwa, Chiba, 277-8581, Japan, Department of Physics, University of Tokyo, Hongo, Bunkyo-ku, Tokyo, 113-0033, Japan, CREST, Japan Science and Technology Agency (JST), 4-1-8 Honcho, Kawaguchi, Saitama, 332-0012, Japan, and Trans-Scale Quantum Science Institute, University of Tokyo, Hongo, Bunkyo-ku, Tokyo, 113-0033, Japan, and Yoshichika Otani, The Institute for Solid State Physics, The University of Tokyo, Kashiwa, Chiba, 277-8581, Japan, Center for Emergent Matter Science, RIKEN, 2-1 Hirosawa, Wako, 351-0198, Japan, CREST, Japan Science and Technology Agency (JST), 4-1-8 Honcho, Kawaguchi, Saitama, 332-0012, Japan, and Trans-Scale Quantum Science Institute, University of Tokyo, Hongo, Bunkyo-ku, Tokyo, 113-0033, Japan.

Abstract: “Antiferromagnets (AFMs) have the natural advantages of terahertz spin dynamics and negligible stray fields, thus appealing for use in domain-wall applications. However, their insensitive magneto-electric responses make controlling them in domain-wall devices challenging. Recent research on noncollinear chiral AFMs Mn₃X (X = Sn, Ge) enabled us to detect and manipulate their magnetic octupole domain states. Here, we demonstrate a current-driven fast magnetic octupole domain-wall (MODW) motion in Mn₃X. The magneto-optical Kerr observation reveals the Néel-like MODW of Mn₃Ge can be accelerated up to 750 m s^-1 with a current density of only 7.56 × 10¹⁰ A m^-2 without external magnetic fields. The MODWs show extremely high mobility with a small critical current density. We theoretically extend the spin-torque phenomenology for domain-wall dynamics from collinear to noncollinear magnetic systems. Our study opens a new route for antiferromagnetic domain-wall-based applications.“

(1) Wu, M., Chen, T., Nomoto, T., Tserkovnyak, Y., Isshiki, H., Nakatani, Y., Higo, T., Tomita, T., Kondou, K., Arita, R. et al. Current-driven fast magnetic octupole domain-wall motion in noncollinear antiferromagnets. Nature Communications 15, 4305 (2024). doi: 10.1038/s41467-024-48440-9