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

Spin transfer torque (STT) is usually known to appear from non-collinearity of two spins but recent experiments [A. Zholud et al., Phys. Rev. Lett. 119, 257201 (2017)] have shown change in magnetization in spin vavles at cryogenic temperature even when electron spin is collinear to magnetization, pointing at overlooked quantum effects in STT ( which can lead to highly nonclassical magnetization states ). Using fully quantum many-body treatment, we showed that change in magnetization below cryogenic temperature come from entanglement between injected electron subsystem and anisotropic quantum Heisenberg ferromagnetic chain, caused by STT coming from non-collinearity of the two subsystems ( which explains the experiment ). 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-↓ factor state, and the only difference between collinear and noncollinear geometries is in relative size of the contribution of the initial separable state containing such factor state to superpositions of separable many-body quantum states generated during time evolution.