Time-resolved x-ray imaging of spin-torque-induced magnetic vortex oscillation

The spin transfer phenomenon provides a new method to manipulate magnetization without applying an external magnetic field and a new playground to study the spin degree of freedom of electrons. Two types of magnetic dynamics excited by the spin transfer torque from a direct current were predicted in 1996: magnetization reversal and steady-state precession. The physics of spin-torque-induced magnetization reversal of a single magnetic domain is now relatively well understood, but study of spin-torque-induced high frequency oscillation is still at an early stage. The electronic transport properties of this type of oscillation have been the subject of a lot of recent work, but no direct imaging has been reported. Another trend in the research of spin-torque dynamics is the focus shifting from the simplest uniform magnetization distribution, the so-called "macrospin", to non-uniform distributions, among which magnetic vortices attract a lot of attention due to their rotational symmetry and application possibilities.;This thesis describes the results of x-ray magnetic imaging recently carried out to study spin-torque-induced steady-state oscillation of an inhomogeneous magnetization distribution in a spin valve structure. Static magnetic images deduced from x-ray transmission signals confirm that the ground state of the inhomogeneous magnetization is a magnetic vortex and reveal an interesting vortex profile. Micromagnetic simulations were conducted to understand the nontrivial vortex profile we observed. To study the vortex oscillation induced by a direct current, we developed a synchronous detection technique using the injection locking of spin torque oscillators so that the gigahertz sample oscillation is synchronized to the probing x-ray pulses. The imaging results of the dynamic experiments confirm that the microwave frequency oscillation observed in transport measurements comes from a translational vortex core motion. A simple model is proposed to explain the observed dc-driven vortex oscillation.