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

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.

Strongly Coupled Spin Waves and Surface Acoustic Waves at Room Temperature.

Here, we report the observation of strong coupling between magnons and surface acoustic wave (SAW) phonons in a thin CoFeB film constructed in an on-chip SAW resonator by analyzing SAW phonon dispersion anticrossings. We employ a nanostructured SAW resonator design that, in contrast to conventional SAW resonators, allows us to enhance shear-horizontal strain. Crucially, this type of strain couples strongly to magnons. Our device design provides the tunability of the film thickness with a fixed phonon wavelength, which is a departure from the conventional approach in strong magnon-phonon coupling research. We detect a monotonic increase in the coupling strength by expanding the film thickness, which agrees with our theoretical model. Our work offers a significant way to advance fundamental research and the development of devices based on magnon-phonon hybrid quasiparticles.

Perpendicular full switching of chiral antiferromagnetic order by current.

Electrical control of a magnetic state of matter lays the foundation for information technologies and for understanding of spintronic phenomena. Spin-orbit torque provides an efficient mechanism for the electrical manipulation of magnetic orders1-11. In particular, spin-orbit torque switching of perpendicular magnetization in nanoscale ferromagnetic bits has enabled the development of stable, reliable and low-power memories and computation12-14. Likewise, for antiferromagnetic spintronics, electrical bidirectional switching of an antiferromagnetic order in a perpendicular geometry may have huge impacts, given its potential advantage for high-density integration and ultrafast operation15,16. Here we report the experimental realization of perpendicular and full spin-orbit torque switching of an antiferromagnetic binary state. We use the chiral antiferromagnet Mn3Sn (ref. 17), which exhibits the magnetization-free anomalous Hall effect owing to a ferroic order of a cluster magnetic octupole hosted in its chiral antiferromagnetic state18. We fabricate heavy-metal/Mn3Sn heterostructures by molecular beam epitaxy and introduce perpendicular magnetic anisotropy of the octupole using an epitaxial in-plane tensile strain. By using the anomalous Hall effect as the readout, we demonstrate 100 per cent switching of the perpendicular octupole polarization in a 30-nanometre-thick Mn3Sn film with a small critical current density of less than 15 megaamperes per square centimetre. Our theory reveals that the perpendicular geometry between the polarization directions of current-induced spin accumulation and of the octupole persistently maximizes the spin-orbit torque efficiency during the deterministic bidirectional switching process. Our work provides a significant basis for antiferromagnetic spintronics.

Chirality-Induced Magnetoresistance Due to Thermally Driven Spin Polarization.

Chirality-induced current-perpendicular-to-plane magnetoresistance (CPP-MR) originates from current-induced spin polarization in molecules. The current-induced spin polarization is widely recognized as a fundamental principle of chiral-induced spin selectivity (CISS). In this study, we investigate chirality-induced current-in-plane magnetoresistance (CIP-MR) in a chiral molecule/ferromagnetic metal bilayer at room temperature. In contrast to CPP-MR, CIP-MR observed in the present study requires no bias charge current through the molecule. The temperature dependence of CIP-MR suggests that thermally driven spontaneous spin polarization in chiral molecules is the key to the observed MR. The novel MR is consistent with recent CISS-related studies, that is, chiral molecules in contact with a metallic surface possess a finite spin polarization.

Giant field-like torque by the out-of-plane magnetic spin Hall effect in a topological antiferromagnet.

Spin-orbit torques (SOT) enable efficient electrical control of the magnetic state of ferromagnets, ferrimagnets and antiferromagnets. However, the conventional SOT has severe limitation that only in-plane spins accumulate near the surface, whether interpreted as a spin Hall effect (SHE) or as an Edelstein effect. Such a SOT is not suitable for controlling perpendicular magnetization, which would be more beneficial for realizing low-power-consumption memory devices. Here we report the observation of a giant magnetic-field-like SOT in a topological antiferromagnet Mn3Sn, whose direction and size can be tuned by changing the order parameter direction of the antiferromagnet. To understand the magnetic SHE (MSHE)- and the conventional SHE-induced SOTs on an equal footing, we formulate them as interface spin-electric-field responses and analyzed using a macroscopic symmetry analysis and a complementary microscopic quantum kinetic theory. In this framework, the large out-of-plane spin accumulation due to the MSHE has an inter-band origin and is likely to be caused by the large momentum-dependent spin splitting in Mn3Sn. Our work demonstrates the unique potential of antiferromagnetic Weyl semimetals in overcoming the limitations of conventional SOTs and in realizing low-power spintronics devices with new functionalities.

Electrical manipulation of a topological antiferromagnetic state.

Electrical manipulation of phenomena generated by nontrivial band topology is essential for the development of next-generation technology using topological protection. A Weyl semimetal is a three-dimensional gapless system that hosts Weyl fermions as low-energy quasiparticles1-4. It has various exotic properties, such as a large anomalous Hall effect (AHE) and chiral anomaly, which are robust owing to the topologically protected Weyl nodes1-16. To manipulate such phenomena, a magnetic version of Weyl semimetals would be useful for controlling the locations of Weyl nodes in the Brillouin zone. Moreover, electrical manipulation of antiferromagnetic Weyl metals would facilitate the use of antiferromagnetic spintronics to realize high-density devices with ultrafast operation17,18. However, electrical control of a Weyl metal has not yet been reported. Here we demonstrate the electrical switching of a topological antiferromagnetic state and its detection by the AHE at room temperature in a polycrystalline thin film19 of the antiferromagnetic Weyl metal Mn3Sn9,10,12,20, which exhibits zero-field AHE. Using bilayer devices composed of Mn3Sn and nonmagnetic metals, we find that an electrical current density of about 1010 to 1011 amperes per square metre induces magnetic switching in the nonmagnetic metals, with a large change in Hall voltage. In addition, the current polarity along the bias field and the sign of the spin Hall angle of the nonmagnetic metals-positive for Pt (ref. 21), close to 0 for Cu and negative for W (ref. 22)-determines the sign of the Hall voltage. Notably, the electrical switching in the antiferromagnet is achieved with the same protocol as that used for ferromagnetic metals23,24. Our results may lead to further scientific and technological advances in topological magnetism and antiferromagnetic spintronics.

Magnetic and magnetic inverse spin Hall effects in a non-collinear antiferromagnet.

The spin Hall effect (SHE)1-5 achieves coupling between charge currents and collective spin dynamics in magnetically ordered systems and is a key element of modern spintronics6-9. However, previous research has focused mainly on non-magnetic materials, so the magnetic contribution to the SHE is not well understood. Here we show that antiferromagnets have richer spin Hall properties than do non-magnetic materials. We find that in the non-collinear antiferromagnet10 Mn3Sn, the SHE has an anomalous sign change when its triangularly ordered moments switch orientation. We observe contributions to the SHE (which we call the magnetic SHE) and the inverse SHE (the magnetic inverse SHE) that are absent in non-magnetic materials and that can be dominant in some magnetic materials, including antiferromagnets. We attribute the dominance of this magnetic mechanism in Mn3Sn to the momentum-dependent spin splitting that is produced by non-collinear magnetic order. This discovery expands the horizons of antiferromagnet spintronics and spin-charge coupling mechanisms.

Publisher Correction: Magnetic and magnetic inverse spin Hall effects in a non-collinear antiferromagnet.

In this Letter, the formatting of some of the crystallographic axes was incorrect. This has been corrected online.

Clear variation of spin splitting by changing electron distribution at non-magnetic metal/Bi2O3 interfaces.

Large spin splitting at Rashba interface, giving rise to strong spin-momentum locking, is essential for efficient spin-to-charge conversion. Recently, a Cu/Bismuth oxide (Bi2O3) interface has been found to exhibit an efficient spin-to-charge conversion similar to a Ag/Bi interface with large Rashba spin splitting. However, the guiding principle of designing the metal/oxide interface for the efficient conversion has not been clarified yet. Here we report strong non-magnetic (NM) material dependence of spin splitting at NM/Bi2O3 interfaces. We employed spin pumping technique to inject spin current into the interface and evaluated the magnitude of interfacial spin-to-charge conversion. We observed large modulation and sign change in conversion coefficient which corresponds to the variation of spin splitting. Our experimental results together with first-principles calculations indicate that such large variation is caused by material dependent electron distribution near the interface. The results suggest that control of interfacial electron distribution by tuning the difference in work function across the interface may be an effective way to tune the magnitude and sign of spin-to-charge conversion and Rashba parameter at interface.

Experimental detection of domain wall propagation above the Walker field.

The domain wall (DW) velocity above the Walker field drops abruptly with increasing magnetic field, because of the so-called Walker breakdown, where the DW moves with a precessional mode. On applying the higher field, the DW velocity again starts to increase gradually. We report the DW propagation around this local minimum regime in detail, investigated through the time-resolved electrical detection technique, with a magnetic tunnel junction. Just above the Walker field, we succeeded in detecting the precessional motion of the DW in a real-time regime, while a different mode appeared around the local minimum of the DW velocity.