Spin-Transfer-Driven Dynamics of Magnetic Vortices and Antivortices in Dots With Crystalline Cubic Anisotropy

We study the magnetization dynamics of a vortex- or antivortex-containing nanodot driven by an out-of-plane polarized electric current within micromagnetic simulations. The dot is an ultra-thin structure created using a material with strong crystalline cubic anisotropy. It is in-plane ordered with an effective fourfold anisotropy. In the case of the antivortex, we consider astroid-shaped dots, while in the case of the vortex, the circular dots and the astroid-shaped ones. Unlike in the soft-magnetic dots, the vortex (antivortex) textures in thin layers with sufficiently strong fourfold anisotropy consist of four closure domains independent of the dot shape. The magnetization in the domain walls (DWs) deviates from the dot plane under the action of the out-of-plane polarized electric current normal to the dot, which drives the DW propagation-a rotation of the texture around the vortex (antivortex) center. The DW velocity is dependent on the distance from the vortex (antivortex) core; thus, the DWs deform under the current creating a fourfold spiral shape. For sufficiently hard cubic magnets, we find a regime of the oscillatory dynamics of the dot [a cyclic switching between the spiral state and the closure-domain vortex (antivortex) state]. For softer magnets, we consider a spin-transfer-driven fast magnetization reversal of the vortex state mediated by the creation of the spiral state.