UC Irvine

This dissertation mainly describes the study of the spin-torque induced magnetic dynamics in patterned nanostructures from two aspects. The first study discusses the nonlinear damping effect in spin-torque oscillators(STOs). The direct time domain measurements on the stochastic STO dynamics will be described. We apply the time domain data to reconstruct statistical distributions of the STO free layer trajectories and analyze them in the framework of the effective Fokker-Planck energy approach. The prior work has been investigated for the dynamics near critical current. This thesis will focus on the regime far above critical current and explain the nonlinear damping effect on the dynamics in this regime.

The second session presents detection of microwave signals by magnetic tunnel junctions(MTJs) based on the spin-torque diode effect. We show a wireless detection of microwave signals using a MTJ based detector. This MTJ detector is integrated with compact coplanar waveguide antennas and non-magnetic, microwave-transparent, reusable antenna holder. We compare the experimental results with MTJs of different magnetic layer structures. The tested structures can achieve comparable sensitivities to those of commercial semiconductor, diode-based microwave sensors. The detection frequencies can be tuned by a permanent magnet attached to the detector. In addition, we demonstrate a microwave frequency determination method by a pair of MTJs as microwave detectors. A resonance-type spin-torque microwave detector (STMD) can be used to determine the frequency of an input microwave signal. But the

accuracy is limited by the STMD's ferromagnetic resonance linewidth. By applying a pair

of uncoupled STMDs connected in parallel to a microwave signal source, we show that the accuracy of frequency measurement is improved significantly.