| Spin-torque (or spin transfer torque) is a novel phenomenon involving the transfer of angular momentum from a spin-polarized current to a ferromagnet. There is much excitement in the use of this effect for developing non-volatile, high density magnetic RAM, as well as for DC current-driven microwave oscillators. Indeed, steady-state precessional modes as well as full magnetization reversal of nanoscale magnetic elements driven by spin-torque have been observed. These observations have been via giant magneto-resistance measurements, using a reference "fixed" magnetic layer, which also serves as the spin-polarizer. Given the experimental challenges in probing thin, buried nanomagnets, the detailed magnetic configuration of the element has remained unknown.; This thesis describes four major contributions: (1) Development of a high resolution, time-resolved x-ray microscopy technique sensitive to the magnetic signal from thin, buried nanostructures. A spatial resolution of 35 nm and time resolution of 100 ps is achieved. (2) Demonstration of this technique in the study of spin-torque driven reversal of nanopatterned magnetic pillars. An assortment of samples are studied varying in size and aspect ratio. (3) Observation of two types of behavior during magnetization reversal. One unexpected process is seen to be based on the transient formation of a vortex configuration and the subsequent motion of this vortex across the magnetic element. The second process is less determined but does not involve the formation of a vortex. Instead, some form of stochastic mechanism drives the magnetization reversal. (4) Numerical modeling is performed which reproduces the above types of switching behavior. A physical picture of the underlying mechanism is given, along with its dependence on sample dimensions. A discussion of the ramifications for spin-torque driven reversal in general is also given. |
