Session F24: Novel Cross Coupling Effects and Manipulation Methods of Chiral Antiferromagnets

Piezomagnetic switching of the anomalous Hall conductivity in an antiferromagnet at room temperature

Piezomagnetism couples strain linearly to magnetic order, implying that it can produce and control magnetization. However, unlike magnetostriction, which couples magnetization quadratically to strain, it enables bidirectional control of a net magnetic moment. If this effect becomes large at room temperature, it may be technologically relevant, similar to its electric analogue, piezoelectricity. However, current studies of the piezomagnetic effect have been primarily restricted to antiferromagnetic insulators at cryogenic temperatures. Here we report the observation of strong piezomagnetism in the antiferromagnetic Weyl semimetal Mn₃Sn at room temperature. This material is known for its nearly magnetization-free anomalous Hall effect. We find that a small uniaxial strain on the order of 0.1% can control both the sign and size of the anomalous Hall effect. Our experiment and theory based on a Landau free energy functional show that the piezomagnetism can control the anomalous Hall effect, which will be useful for spintronics applications.   

Unconventional Spin-Transfer Torque in Noncollinear Antiferromagnetic Junctions

Ferromagnetic spin valves and tunneling junctions are crucial for spintronics applications and are one of the most fundamental spintronics devices. Motivated by the potential unique advantages of antiferromagnets for spintronics, we theoretically study junctions built out of noncollinear antiferromagnets that exhibit a spin-polarized current. We demonstrate a large and robust magnetoresistance and spin-transfer torque. This is in contrast to simple antiferromagnets in which these effects can also exist but are very sensitive to disorder. We show that the spin-transfer torque is capable of ultrafast switching between parallel and antiparallel states of the junction, analogously to the ferromagnetic junctions. However, we also demonstrate a novel behavior stemming from the non-collinear magnetic order. In particular, we find that, unlike in the ferromagnetic junctions, the spin-transfer torque is present even in the parallel or anti-parallel states of the junction and that a new type of self-generated torque appears in the noncollinear junctions. In addition to the junctions, we discuss the self-generated torque in non-collinear bulk systems.   

Detection and 180° Electrical Switching of Néel Vector in Altermagnets

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 180^o switching of the Néel vector as well as the corresponding spin-splitting, are very challenging. We demonstrated that in altermagnets Mn₅Si₃ [4] and CrSb [5], the unique anomalous Hall effect can be adopted for electrical readout of opposite Néel vectors. A new mechanism was proposed for the electrical 180^o switching of the Néel vector via spin-orbit torques by designing asymmetric switching barriers and experimentally achieved it. Thus it is termed as barrier-asymmetric spin-orbit torque. It is made possible by the fixed chirality between Néel vector and tiny relativistic net moment due to the Dzyaloshinskii-Moriya interaction. Based on the novel readout and manipulation methods, we fabricated prototype memory devices that can accomplish robust write and read cycles. Furthermore, controllable Néel vector enables controllable spin-charge interconversion through altermagnetic and inverse altermagnetic spin splitting effect [2,3,6].   

Electrical manipulation and detection of non-collinear spin textures in the chiral antiferromagnet Mn3Sn

Antiferromagnetic (AF) materials have garnered significant attention for their favorable properties in device applications, including very small stray fields and ultrafast spin dynamics [1]. Among antiferromagnets (AFMs), those with macroscopic time-reversal symmetry breaking (TRSB) with non-trivial spin structure have been extensively studied for their potential to detect and control spontaneous responses akin to ferromagnets. The non-collinear AFM Mn₃Sn [2], a prominent example of such TRSB AFMs, is a magnetic Weyl semimetal with unique chiral AF ordering hosting cluster magnetic octupoles and topological band structures, which lead to large transverse responses induced by the momentum-space Berry curvature [2,3]. The research targets are shifting fundamental studies, which have yielded fruitful results discussed above, towards exploring spintronics properties, focusing on cross-coupling effects such as spin-electron and -lattice coupling.

This presentation focuses on our studies involving heterointerfaces based on Mn₃Sn. We have successfully manipulated the chiral AF order in bilayer films composed of polycrystalline Mn₃Sn and heavy metals, demonstrating the potential for spin-orbit torque (SOT) [4]. This research extends to bilayer films comprising epitaxial Mn₃Sn and heavy metals [5,6], where we have achieved SOT-induced perpendicular magnetic recording for the first time in AFMs [6], leveraging high-quality Mn₃Sn films fabricated by MBE methods. Additionally, we have observed magnetoresistance effects at room temperature in an AF tunnel junction composed of Mn₃Sn/MgO/Mn₃Sn multilayers [7]. These results offer promising avenues for the development of chiral AF spintronics.

[1] Jungwirth et al., Nat. Nano. 11, 231 (2016). [2] Nakatsuji, Kiyohara, & TH, Nature 527, 212 (2015); Nakatsuji and Arita, Annu. Rev. Condens. Matter. Phys. 13, 119 (2022). [3] Kuroda, Tomita et al., Nat. Mater. 16, 1090 (2017). [4] Tsai, TH et al., Nature 580, 608 (2020). [5] Takeuchi et al., Nat. Mater. 20, 1364 (2021). [6] TH, Kondou et al., Nature 607, 474 (2022). [7] Chen, TH, Tanaka et al., Nature 613, 490 (2023).   

Observation of spin swapping effect in non-collinear antiferromagnets

A pure spin current without a charge current carries spin angular momentum with minimal carriers in ferromagnetic (FM) metals and no carriers at all in FM insulators.  Pure spin current phenomena, including injection/detection of pure current and spin orbit torque switching, have been realized in collinear FM materials with a large magnetization (M), but not in collinear antiferromagnetic (AF) materials with M = 0 unless under a sizable external field.  Spin swapping effect, where the spin current direction and spin index interchanged as it traverses through a material, has been an intriguing spintronic effect since its prediction in 2009 [1], but not observed in collinear FM and AF materials.  Spin swapping effect has recently been observed in non-collinear AF spin structure of LuFeO₃ [2] and LuFeO₃ [3].  The pure spin current effects in non-collinear AFs increase, as opposed to those in collinear FM materials that decrease, with increasing temperatures indicating new mechanisms [4].  One may exploit spin swapping effect for lateral spintronic devices, and non-collinear AFs for low-field AF spintronics.