Spin-orbit torque switching of metallic antiferromagnets and ferrimagnets

Spin-orbit torque (SOT) control of magnetic order in antiferromagnetic (AFM) and ferrimagnetic (FIM) materials is of great current interest, motivated by their exchange-dominated high-frequency dynamics and small (or absent) macroscopic magnetization, making them excellent candidates for high-speed and robust memory devices.

Here we present our recent results in SOT control of AFM and FIM order. First, we discuss SOT control of AFM order in two metallic antiferromagnets (PtMn and non-collinear IrMn₃) [1, 2]. We show that pillars of both AFMs, grown on a heavy metal (HM) layer, can be reversibly switched between different magnetic states by electric currents. We also present an experimental protocol to unambiguously distinguish current-induced magnetic and nonmagnetic switching signals in AFM/HM structures. A six-terminal double-cross device is constructed, with an IrMn₃ pillar placed on one cross. The differential voltage is measured between the two crosses after each switching attempt. For a wide range of current densities, reversible switching is observed only when write currents pass through the cross with the IrMn₃ pillar. This eliminates the possibility of non-magnetic switching artifacts, which complicated the interpretation of most previous AFM/HM switching experiments.

Next, we present a strategy for deterministic field-free switching of perpendicular ferrimagnetic films by using chiral symmetry-breaking to eliminate the need for an external magnetic field [3]. Bias-field-free SOT switching is demonstrated in a perpendicular CoTb film with an engineered vertical composition gradient. The vertical structural inversion asymmetry induces strong intrinsic SOTs and a gradient-driven Dzyaloshinskii–Moriya interaction (g-DMI), which dynamically breaks the in-plane symmetry during the switching process. This approach is scalable to large wafer size.

[1] Nature Elec. 3, 92, (2020)

[2] Nature Comm. 12, 3828, (2021)

[3] Nature Comm. 12, 4555, (2021)