The spin splitting torque (SST) was theoretically proposed to combine advantages of conventional spin transfer torque (STT) and spin-orbit torque (SOT) as well as enable controllable spin current (1). This talk presents experimental evidences of SST and the inverse effect in collinear antiferromagnet RuO2 films. First, according to spin torque ferromagnetic resonance (ST-FMR) measurements of RuO2 films, we exhibit that spin current direction is correlated to the crystal orientation of RuO2 and spin polarization direction is dependent on (parallel to) the Néel vector, indicating the existence of SST in RuO2 (2). Second, based on spin Seebeck effect and THz emission measurements, we demonstrate that inverse SST effect can convert spin current polarized along Néel vector into charge current, which manifests as Néel vector-dependent spin Seebeck voltage signals and THz emission signal (3). These findings not only presents a new member for the spin torques besides traditional STT and SOT, but also proposes RuO2 for both promising spin source and spin sink for spintronics. 

**References:** 
(1) R. González-Hernández, et al., Phys. Rev. Lett. 126, 127701 (2021). 
(2) H. Bai, C. Song, et al. accpeted by Phys. Rev. Lett. 128, 197202 (2022). 
(3) H. Bai, C. Song, et al. In preparation. 

![](https://s3.eu-west-1.amazonaws.com/underline.prod/uploads/markdown_image/1/image/24291f53f50a837b79979b16c59135f7.png)

Observation of the spin splitting torque and its inverse effect in an antiferromagnet

The spin current with out-of-plane spin polarization (sigma-z) is favorable for the efficient switching of perpendicular magnetized devices, which is required for high-density memories [1,2]. Recently we observed sigma-z both in non-collinear antiferromagnets Mn3Pt [3] and Mn3SnN [4], as well as in collinear antiferromagnets Mn2Au [5,6] and RuO2 [7] thin films. Strong out-of-plane polarized spin current is generated when the charge current is applied along the magnetic mirror plane of non-collinear antiferromagnet, but tiny when perpendicular. With the aid of strong sigma-z, current induced spin-orbit torque (SOT) switching of adjacent perpendicular ferromagnet is realized without external magnetic field. In Mn2Au films, sigma-z is generated at two spin sublattices with broken spatial symmetry, and the antiparallel antiferromagnetic moments play an important role. The out-of-plane polarized spin current can be controlled by the current- and electric field-induced Néel vector switching. Thus this Néel vector-dependent spin Hal effect is termed as “antiferromagnetic spin Hall effect”, which adds another twist to the atomic-level control of spin currents via the antiferromagnetic spin tructure. RuO2 is a tetragonal itinerant collinear antiferromagnet with nonmagnetic atoms at noncentrosymmetric positions [8]. This particular crystal structure produces anisotropic band structure，enabling RuO2 a efficient spin source due to the spin splitting effect. Spin current generated by spin splitting effect do not meet conventional “cross product relationship”. The spin current flow direction is related to the crystal structure，while the spin polarization direction is related to the Néel vector. Spin splitting effect has more efficient spin-charge convention and more controllable spin polarization. The role of sigma-z for the field-free SOT switching is theoretically analyzed, and different roles of sigma-z and conventional in-plane spin polarization (sigma-y) are presented with different external magnetic field. Our findings would help to use out-of-plane polarization for efficient SOT-MRAM to combine the advantage of SOT and spin-transfer torque.

![](https://assets.underline.io/uploads/markdown_image/1/image/2cd5af1ecc73195bd6fc92b4627ec76c.jpg)

Fig. 1. Schematic of out-of-plane polarized spin current and field-free spin-orbit torque switching.

Out-of-plane spin polarization and antiferromagnetic spin Hall effect

The discovery of the spin Hall effect (SHE) enabled the efficient generation and manipulation of the spin current. The magnetic spin Hall effect provides a unique opportunity to control the spin current and relevant device performance with controllable magnetization. In this talk, we report the magnetic spin Hall effect both in non-collinear antiferromagnet Mn3Pt and a collinear antiferromagnet Mn2Au. We generate tiny out-of-plane polarized spin current when the charge current is applied along the axis perpendicular to the magnetic mirror plane of Mn3Pt/permalloy bilayers, but robust when the current is parallel to the magnetic mirror plane in all of the Mn3Pt films with different orientations. In Mn2Au, the spin currents are generated at two spin sublattices with broken spatial symmetry, and the antiparallel antiferromagnetic moments play an important role. Therefore, we term the Néel vector-dependent spin Hal effect the ‘antiferromagnetic spin Hall effect’. The out-of-plane spins from the antiferromagnetic spin Hall effect are favourable for the efficient switching of perpendicular magnetized devices, which is required for high-density applications. The antiferromagnetic spin Hall effect adds another twist to the atomic-level control of spin currents via the antiferromagnetic spin structure.

Antiferromagnetic spin Hall effect

EOA: Antiferromagnetic Spintronics I

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Presentations

Observation of the spin splitting torque and its inverse effect in an antiferromagnet

Out-of-plane spin polarization and antiferromagnetic spin Hall effect

Antiferromagnetic spin Hall effect

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