Direct Imaging of Non-Adiabatic Spin Torque Effects on Vortex Core Orbits

Recently high frequency, current induced vortex motion has received a great deal of interest from a spintronic perspective, as it suggests a possible low power, high speed writing process. However, understanding the processes that govern this motion, specifically the relative contributions of adiabatic and non-adiabatic spin torque effects, has been difficult due to experimental constraints. We developed a novel TEM sample stage in which we apply high frequency currents in-situ to excite resonant motion in Permalloy disc structures (2000x2000x50nm) with high spatial resolution ($<$5nm for dynamic measurements). We have imaged the time-averaged vortex trajectory through resonance. We find that the orbital amplitudes are drastically different for clockwise and counterclockwise chiralities, indicating the presence of both Oersted fields and non-adiabatic spin torque effects, and that the orbital size scales linearly with current density varied between (7.1-10.0)x10$^{10}$ A/m$^{2}$. These results allow us to extract a value for the non-adiabatic spin torque with unprecedented precision. Additionally, we report on off-resonance effects, such as tilting and variations in the ellipticity of the orbit as it is swept through resonance, with first of their kind experimental observations.