Current-driven magnetization dynamics

Shufeng Zhang, University of Arizona

Utilizing spin-polarized current to manipulate magnetic states and dynamics has become increasingly important for nano-scale magnetic devices. After briefly reviewing the status of spin transfer research, we present our recent theoretical and modeling efforts of the subject. Two particular issues will be discussed. First, in magnetic tunnel junctions, the voltage dependence of perpendicular spin torques remains experimental and theoretically controversial. We will critically analyze experimental data from the spin-FMR and thermal-assisted reversal. We find that the perpendicular torques involve both spin and energy transfer at the finite voltage bias. Second, we introduce a concept of current-driven ferrodynamics where the ferromagnetism undergoes a dynamic phase transition. At a critical current, a uniformly magnetized ferromagnet becomes unstable and a new magnetic state is excited. We determine the critical exponents and predict resistance anomaly at the critical point.