Current-driven nucleation and propagation of antiferromagnetic skyrmionium

We present a theoretical study on nucleation and propagation of antiferromagnetic skyrmionium induced by spin current injection. A skyrmionium, also known as 2 pi skyrmion, is a vortexlike magnetic structure characterized by a topological charge, the so-called skyrmion number, being zero. We find that an antiferromagnetic skyrmionium can be generated via a local injection of spin current with toroidal distribution. We examine the threshold current density for successful skyrmionium nucleation and the robustness of the proposed mechanism against thermal fluctuation. A spatially uniform spin current is then demonstrated to induce propagation of the skyrmionium. We derive an expression for the skyrmionium velocity based on a collective-coordinate model, which agrees well with our numerical results.