Time-resolved X-ray Imaging of Spin-torque-induced Magnetic Vortex Oscillation a Dissertation Submitted to the Department of Applied Physics and the Committee on Graduate Studies of Stanford University in Partial Fulfillment of the Requirements for the Degree of Doctor of Philosophy

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 v observed in transport measurements comes from a translational vortex core motion. A simple model is proposed to explain the observed dc-driven vortex oscillation.