Quantum theory of spin-torque driven magnetization switching

Magnetization dynamics driven by the current-induced spin torque is conventionally determined by the classical Landau-Lifshitz-Gilbert-Slonczewski equation in which the spin (magnetization) fluctuation at finite temperature is modeled by a white-noise random field. We propose a quantum approach for current driven magnetization switching that explicitly includes the spin fluctuation by the quantum statistics of magnon excitations. We find that the spin fluctuation substantially reduces the critical spin torque at high temperatures. Since the spin fluctuations are fundamentally stronger in lower-dimensional systems, this reduction is stronger in two-dimensional (2D) than in three-dimensional magnets. The result implies that the 2D magnets may have an advantage in terms of electrically manipulating magnetization states for spintronic applications.