Characterization Of Magnetic Dynamics Excited By Spin Transfer Torque In A Nanomagnet
This dissertation presents our investigation of the effects of spin transfer torque on a nanoscale ferromagnet. Spin transfer torque is generated by the transfer of angular momentum from spin polarized electrons to a ferromagnet. This torque provides an efficient means to manipulate the dynamic motion of the magnetization of a nanomagnet, and can be strong enough to induce magnetization reversal or steady-state precession. We have developed new techniques to characterize such dynamics induced by spin transfer torque. In the first study, we perform an experiment demonstrating that spin transfer from a microwave current pulse can produce a resonant excitation of a nanomagnet and improve switching characteristics in combination with a square current pulse. With the assistance of a microwave-frequency pulse, the switching time is reduced and achieves a narrower distribution than when driven by a square current pulse alone, and this can permit significant reductions in the integrated power required for switching. In the second study, we develop a single-shot electrical technique to capture the magnetic dynamics during the spin torque switching of a magnetic tunnel junction in real time. With substantially improved sensitivity compared to previous experiments, we directly resolve the resistance oscillations associated with magnetic dynamics, yielding detailed views of switching modes and variations between events. This also enables us to analyze the coherence times and non-equilibrium spectra of the magnetic dynamics under the effects of thermal fluctuations. In the last study, we use X-ray microscopy to image the magnetic normal modes which serve as the basis states for the magnetic dynamics. By applying a microwave current at the normal mode frequency, we selectively excite a particular normal mode and perform time resolved X-ray microscopy measurement to image its spatial structure. We observe different degrees of spatial non-uniformity for two mode branches we imaged. The branch with higher frequency is more spatially uniform than the other. At low magnetic fields where the two modes are close in frequency and excited at the same time, the more non-uniform mode dominates the overall behavior of the dynamics.
Ralph, Daniel C
Van Dover, Robert B.; Buhrman, Robert A
Ph.D. of Applied Physics
Doctor of Philosophy
dissertation or thesis