Observation of Spin-orbit Torque-induced Skyrmion Dynamics Revealed by Time-resolved X-ray Imaging

Abstract

Magnetic skyrmions are topologically protected spin textures and have received great interest in utilizing the information memory bit due to their unique topological characteristics. Recent studies have reported that chiral skyrmions are formed in heavy metal (HM)/ferromagnet (FM)/oxide metallic heterostructures that exhibit strong interfacial Dzyaloshinskii-Moriya interaction (DMI) such as Ta/CoFeB/TaOx [1,2], Pt/CoFeB/MgO [3], Pt/Co/Ir [4] or Pt/Co/MgO [3]. Unlike dipolar field-stabilized bubbles, chiral skyrmions show unique behavior such as spin-orbit torque-driven skyrmion displacement [1-3], lattice formation [5] and emergent electrodynamics [6]. However, for development of ultrafast spin devices, it is important to understand dynamic phases such as skyrmion breathing [7], gyration [8], or skyrmion Hall effect [9,10] during the applied current pulses while most studies take the static images before and after the current pulses. In this work, direct observation of nanosecond-dynamics of a 100 nm-size chiral skyrmion driven by currentinduced spin-orbit torque is revealed by using time-resolved soft X-ray microscopy [11]. We observed two distinctive dynamic excitation behaviours, breathing-like and translational behaviors, which can simply be manipulated by changing the magnitude of current-induced spin-orbit torques. Then, the physical origin of these dynamic phases is discussed by using micromagnetic simulations. Moreover, we demonstrate the skyrmion generation by applied bipolar current pulses in the absence of magnetic field. In sum, our observations offer the efficient way to manipulate the dynamic phases of magnetic skyrmions, which could realize for the development of ultrafast skyrmionic applications in the future.