Spin-orbit torque in antiferromagnets

Abstract

Antiferromagnets with zero net magnetic moment, strong anti-interference and ultrafast switching speed have potential competitiveness in high-density information storage [1], [2]. Electrical switching of antiferromagnets is at the heart of device application [3]. We will present our recent progress in the current-driven magnetization switching through spin-orbit torque (SOT) in three antiferromagnetic systems, including Mn 2 Au, and [Co/Pd]/Ru/[Co/Pd] synthetic antiferromagnets (SAF). Body centered tetragonal antiferromagnet Mn 2 Au with opposite spin sub-lattices is a unique metallic material for Neelorder spin-orbit torque (SOT) switching. The SOT switching in quasi-epitaxial (103), (101) and (204) Mn 2 Au films prepared by a simple magnetron sputtering method will be discussed. We demonstrate current induced antiferromagnetic moment switching in all the prepared Mn 2 Au films by a short current pulse at room temperature, whereas different orientated films exhibit distinguished switching characters. A direction-independent reversible switching is attained in Mn 2 Au (103) films due to negligible magnetocrystalline anisotropy energy, while for Mn 2 Au (101) and (204) films, the switching is invertible with the current applied along the in-plane easy axis and its vertical axis, but becomes attenuated seriously during initially switching circles when the current is applied along hard axis, because of the existence of magnetocrystalline anisotropy energy [4]. SAF were proposed to replace ferromagnets in magnetic memory devices to reduce the stray field, increase the storage density and improve the thermal stability. We will discuss the SOT in a perpendicularly magnetized Pt/[Co/Pd]/Ru/[Co/ Pd] SAF structure, which exhibits completely compensated magnetization and a high exchange coupling field of 2200 Oe. The magnetizations of two Co/Pd layers can be switched by spin-orbit torque between two antiparallel states simultaneously. The magnetization switching can be read out due to much stronger spin-orbit coupling at bottom Pt/[Co/Pd] interface compared to its upper counterpart without Pt. Both experimental and theoretical analyses unravel that the torque efficiency of antiferromagnetic coupled stacks is significantly higher than the ferromagnetic counterpart, which conquers the exchange coupling field, leading to the critical switching current of SAF comparable to the ferromagnetic coupled one [5], [6]. Besides the fundamental significance, the efficient switching of antiferromagnets by current would advance magnetic memory devices with high density, high speed and low power consumption.