UC Berkeley

Understanding and controlling the manipulation of spin structures on a nanometer length scale is of primary importance to contemporary scientific investigations and future technological applications of magnetic materials. Soft x-ray transmission microscopy offers imaging with a spatial resolution down to 10 nm, and a temporal resolution of about 70 psec, with element specific magnetic contrast. As such, it is an ideal method for studying fast nanoscale spin dynamics. The focus of this dissertation is on imaging the response of magnetic vortex cores to external applied fields using soft x-ray transmission microscopy.

A technique for pinpointing vortex dynamics without time resolution was used to survey the response of a vortex core to RF fields of varying frequency and amplitude. This investigation revealed an unexpected nonlinear response of the vortex core at higher amplitude driving frequencies. Time-resolved images of the vortex core response in the linear regime were taken using a technique which phase-locked the RF driving field to the x-ray imaging pulses. These experimental images probing the frequency response of the vortex core motion are the initial step towards a deeper understanding of the behavior of the vortex core when driven by magnetic fields with frequencies near its resonant frequency.

In addition to investigating the dynamic response of a single vortex core, a magnetic system with two interacting vortex cores was studied. A patterned rectangular magnetic sample with a 7-domain magnetization configuration and two vortex cores at remanence was used in this experiment. The magnetization of this sample was imaged after the application of a fast magnetic field pulse using a pump-probe time-resolved imaging technique. The images taken showed a distortion of the vortex cores after the pulse was applied, and combined with simulation, these results provide strong evidence for the creation and annihilation of vortex-antivortex core pairs during the excitation of the initial vortices. This lends support to the many theories suggesting that a vortex-antivortex core pair is created and annihilated during the switching of vortex core polarity.

The studies outlined in this dissertation demonstrate the strength of transmission soft x-ray microscopy as a tool for investigating spin dynamics.