Time-Resolved X-ray Microscopy for Direct Observation of Spin-Torque and Oersted-Field Driven Vortex Gyration

Due to their symmetry, magnetic vortices are ideal candidates for studying the influence of the spin-transfer torque on the local magnetization. The out-of-plane magnetization of the vortex, the vortex core, can be excited to gyrate around its equilibrium position by in-plane magnetic fields or spin-polarized currents. Here we present results from time-resolved X-ray microscopy on permalloy squares with a vortex in the center. Spin-polarized currents with densities of $4.7~-~12~\cdot $~10$^{10}$~A/m$^2$ are laterally driven through the permalloy sample, and the gyration is imaged for different phases of the ac-excitation. The results are compared to micromagnetic simulations, to good agreement. For vortices having opposite chirality a chirality-dependent phase shift of $40^{\circ}$ is observed that is attributed to Oersted fields from the spin-polarized current. An analytical model estimates corresponding field strengths of $40~\mu$T. This study confirms our assumption that Oersted fields from spin-polarized currents cannot be neglected in spin-torque experiments and shows the sensitivity of the measurement technique.