Predicted multiple Walker breakdowns for current-driven domain-wall motion in antiferromagnets

Spintronics based on antiferromagnets (AFMs) has attracted significant attention due to their advantages over ferromagnets including absence of stray fields and high-speed operation. It is well-known that a ferromagnetic domain wall (DW) suffers from Walker breakdown (WB) under a high current, causing the DW to lose rigidity and decrease velocity. The DW in AFMs has been proposed to be immune to WB with the speed limited by maximal magnon velocity. We challenge this belief by theoretically discovering reentrant WBs for current-driven DWs in AFMs, in fundamental contrast to the unique WB in ferromagnets. Using micromagnetic simulation, we first find the efficiency of current-induced staggered spin-orbit torque (SOT) in layered AFMs (e.g., Mn2Au, CuMnAs) over spin-transfer torque (STT) to drive DW motion. Intriguingly, the DW velocity driven simultaneously by STT and SOT is not a simple addition of those driven by each torque separately, and it is nonlinear in current. We resolve these mysteries by considering the Lorentz contraction of DW width in AFMs to find an analytical DW velocity that agrees well with simulation. The width contraction induces a novel nonlinear dependence of the DW hard-axis tilt angle on current, which induces emergence of multiple WBs that stems from competition between STT plus SOT and torques exerted by antiferromagnetic exchange interaction and anisotropy energy. Our work will provide important information for the development of spintronics based on antiferromagnetic DWs.