Quantum spin-transfer torque induced nonclassical magnetization dynamics and electron-magnetization entanglement

TL;DR

This paper explores quantum effects in spin-transfer torque (STT) using a many-body quantum approach, revealing nonclassical magnetization dynamics and electron-magnetization entanglement that explain recent experimental observations at cryogenic temperatures.

Contribution

It introduces a fully quantum many-body framework for STT, highlighting the role of entanglement and decoherence in magnetization dynamics beyond classical descriptions.

Findings

Quantum STT causes entanglement between electron and local spins.

Decoherence leads to shrinking magnetization without rotation.

Quantum effects explain magnetization dynamics at cryogenic temperatures.

Abstract

The standard spin-transfer torque (STT)---where spin-polarized current drives dynamics of magnetization viewed as a classical vector---requires noncollinearity between electron spins carried by the current and magnetization of a ferromagnetic layer. However, recent experiments [A. Zholud et al., Phys. Rev. Lett. 119, 257201 (2017)] observing magnetization dynamics in spin valves at cryogenic temperatures, even when electron spin is collinear to magnetization, point at overlooked quantum effects in STT which can lead to highly nonclassical magnetization states. Using fully quantum many-body treatment, where an electron injected as spin-polarized wave packet interacts with local spins comprising the anisotropic quantum Heisenberg ferromagnetic chain, we define quantum STT as any time evolution of local spins due to initial many-body state not being an eigenstate of electron+local-spins system. For time evolution caused by injected spin-down electron scattering off local up-spins, entanglement between electron subsystem and local spins subsystem takes place leading to decoherence and, therefore, shrinking of the total magnetization but without rotation from its initial orientation which explains the experiments. Furthermore, the same processes---entanglement and thereby induced decoherence---are present also in standard noncollinear geometry, together with the usual magnetization rotation. This is because STT in quantum many-body picture is caused only by electron spin-down factor state, and the only difference between collinear and noncollinear geometries is in relative size of the contribution of initial many-body state containing such factor state to superpositions of separable many-body quantum states generated during time evolution.