Time-resolved X-ray imaging of magnetization dynamics in Spin Transfer Torque devices.

摘要

Study of spin-dependent transport phenomena in ferromagnetic metals has gained a lot of interest amongst both theoretical and experimental physicists in the last two decades. In addition to being a rich field of study in terms of basic research, these studies have led to some very important technological applications. Discovery of the Giant Magneto Resistance (GMR) effect in 1988--89 revolutionized the read head technology for hard drives, thus making very high storage densities possible. The proposal and the subsequent experimental conformation of the existence of Spin Transfer Torque (STT) effect in ferromagnetic multi-layers promises to be a good candidate for developing high density, non-volatile, magnetic random access memory (MRAM) and tunable DC driven microwave oscillators. The subject matter of study in this thesis is investigation of the magnetization switching mechanism in STT devices.;Most of the experimental advances in the study of STT phenomena have been made via measurements of various electrical transport properties. The primary contribution of the work being presented in this thesis is an introduction of a complementary technique to study STT phenomena, viz., X-ray imaging based time resolved study of magnetization dynamics. The possibility of imaging in the sub-100 nm regime, combined with 100 ps time resolution provides a unique way to study the magnetization dynamics in STT devices.;We have performed pump-probe experiments by using the Scanning Transmission X-ray Microscope (STXM) at the Advanced Light Source (ALS) in Berkeley. Our experiments have revealed for the first time, the details of magnetization switching dynamics under the influence of STT. For our samples, we have identified two types of switching that primarily occur, viz., vortex-driven switching (VDS) and switching by C-state flip-over (CSF). Both these switching mechanisms can be described in a generic way in terms of the motion of magnetic vortex cores. In the last part of this work, we have developed a simple phenomenological model to consistently explain the switching trends observed in experiments. An expression for the critical sample size has been derived and its predictions match reasonably well with experimental results obtained so far. The model also gives another prediction for temperature dependence of the switching behavior, which can be tested by future experiments.