Current-induced dynamics of topological spin structures in ferromagnetic and antiferromagnetic heterostructure systems

Abstract

Motion of a topological spin structure is one of the long-standing issues in magnetism, because it plays an essential role in spintronic devices that use dynamics of spin structures, such as racetrack memory. Recently, magnetic skyrmions, which are topologically stable chiral spin texture, have been considered as promising candidates for future spintronic devices as information carriers because of their tiny size and high mobility. Control of skyrmion motion in a device geometry is essential, and thus, the understanding of skyrmion dynamics in a confined geometry is needed. Recent studies show that a magnetic skyrmion can exist in heavy metal (HM) / ferromagnet (F) / oxide heterostructure systems. In this geometry, when electric current flows in the HM layer, an effective torque which is called spin-orbit torque (SOT) is exerted on the F layer. This SOT can be used to manipulate the spin in the F layer, so that it can move a magnetic skyrmion. In this study, we investigated current-induced skyrmion dynamics in a nano-scale circular magnetic disk structure. We show that the magnetic skyrmion has three excitation modes: a gyrotropic mode, a breathing mode and a dual excitation mode of gyrotropic and breathing modes. This dual excitation mode is related to the inertial mass of skyrmions. Therefore, this study will give an insight to the understanding of the inertia of the skyrmion which is crucial to its motion. In addition, we also considered a HM / antiferromagnet (AF) / oxide heterostrucutre and explored SOT-induced dynamics of AF domain wall structure by means of analytical and numerical calculations. It has been known that dynamics of antiferromagnetic spin texture has a relativistic form whose “speed of light” is played by the maximal group velocity of spin waves. In this study, we show that relativistic dynamics can be realized in the current-induced motion of the AF-DW within realistic range of current density.