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

Abstract

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.  Fig. 1. Schematic of out-of-plane polarized spin current and field-free spin-orbit torque switching.