Quantum-kinetic theory of spin-transfer torque and magnon-assisted transport in nanoscale magnetic junctions

Abstract

We theoretically investigate the role of spin fluctuations in charge transport through a magnetic junction. Motivated by recent experiments that measure a nonlinear dependence of the current on electrical bias, we develop a systematic understanding of the interplay of charge and spin dynamics in nanoscale magnetic junctions. Our model captures two distinct features arising from these fluctuations: magnon-assisted transport and the effect of spin-transfer torque on the magnetoconductance. The latter stems from magnetic misalignment in the junction induced by spin-current fluctuations. As the temperature is lowered, inelastic quantum scattering takes over thermal fluctuations, exhibiting signatures that make it readily distinguishable from magnon-assisted transport.

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

“…Note added.-During the completion of this work, we became aware of two studies [36,37] where magnetization dynamics in collinear spin valves at cryogenic temperatures is attributed to current-enhanced quantum spin fluctuations. However, as discussed above, such fluctuations are forbidden in the ground state of ferromagnets [20], and if generated as random magnetic field [38] by the nonequilibrium spin shot noise [36], they would lead to rotation of magnetization away from the initial collinear direction that was excluded from the experiments of Ref.…”

Quantum many-body states and Green functions of nonequilibrium electron-magnon systems: Localized spin operators vs. their mapping to Holstein-Primakoff bosons

“…However, in the quantum state (â † i ) n |0 , the expectation value of localized spin operator is Ŝz i = S − n, so that the constraint 0 ≤ n ≤ 2S must be obeyed in order to remain in the subspace of physical states shown in Eq. (36). In other words, on a given site i one cannot create N mag > 2S HP bosons.…”

“…We note that Eq. ( 69) is much more complex that what is typically used in spintronics literature [33,36,[38][39][40][41]. Most importantly, it shows that MBPT or diagrammatic Monte Carlo simulations [86,87] for interacting electron-magnon system must deal with nonlinear [84] electron-boson interactions.…”

“…This is precisely the situation frequently encountered in transport phenomena within spintronics. The nonequilibrium MBPT [20,21] for such problems is virtually always conducted using HP transformation, but truncated, as exemplified by theoretical and computational modeling of inelastic electron tunneling spectroscopy in magnetic tunnel junctions [33]; spin-transfer [34][35][36] and spin-orbit torques [37]; ultrafast demagnetization [38]; and conversion of magnonic spin currents into electronic spin current or vice versa at magnetic-insulator/normal-metal interfaces [39,40]. Similarly, truncated HP transformation is chosen mapping from localized spin operators to bosonic operators for problems of quantum magnonics focused on nonequilibrium dynamics of localized spins within magnetic insulators which interact with each other [11][12][13]41] or with external electromagnetic fields [42].…”

Effects of the dynamical magnetization state on spin transfer

“…In the semiclassical approximation, the magnitude of magnetization in ferromagnets is fixed, so ST is forbidden by angular momentum conservation if the electron is spin-polarized collinearly with the magnetic order. However, recent experimental measurements [20,21] and theoretical studies [22][23][24][25][26] revealed a contribution to ST, termed the quantum ST, which persists in the collinear geometry. In ferromagnets, this effect becomes noticeable only at cryogenic temperatures.…”