Spintronics meets density matrix renormalization group: Quantum spin torque driven nonclassical magnetization reversal and dynamical buildup of long-range entanglement

Abstract

We introduce time-dependent density matrix renormalization group (tDMRG) as a solution to long standing problem in spintronics -- how to describe spin-transfer torque (STT) between flowing spins of conduction electrons and localized spins within a magnetic material by treating the dynamics of both spin species fully quantum-mechanically. In contrast to conventional Slonczewski-Berger STT, where the localized spins are viewed as classical vectors obeying the Landau-Lifshitz-Gilbert equation and where their STT-driven dynamics is initiated only when the spin-polarization of flowing electrons and localized spins are noncollinear, quantum STT can occur when these vectors are collinear but antiparallel. Using tDMRG, we simulate the time evolution of a many-body quantum state of electrons and localized spins, where the former are injected as a spin-polarized current pulse while the latter comprise a quantum Heisenberg ferromagnetic metallic (FM) spin-$\frac{1}{2}$ XXZ chain initially in the ground state with spin-polarization antiparallel to that of injected electrons. The quantum STT reverses the direction of localized spins, but without rotation from the initial orientation, when the number of injected electrons exceeds the number of localized spins. Such nonclassical reversal, which is absent from LLG dynamics, is strikingly inhomogeneous across the FM chain and it can be accompanied by reduction of the magnetization associated with localized spins, even to zero at specific locations. This is because quantum STT generates a highly entangled nonequilibrium many-body state of all flowing and localized spins, despite starting from the initially unentangled ground state of a mundane FM. Furthermore, the mutual information between localized spins at the FM edges remains nonzero even at infinite separation as the signature of dynamical buildup of long-range entanglement.

11 pages, 8 figures; four movie available from https://wiki.physics.udel.edu/qttg/Publications

References in corpus (14)

• Spin Transfer Torques
• Real time evolution using the density matrix renormalization group
• Mixed-state entanglement from local randomized measurements
• Quantum simulation of time-dependent Hamiltonians and the convenient illusion of Hilbert space
• Spin-Torque Driven Magnetization Dynamics: Micromagnetic Modelling
• Regional versus Global Entanglement in Resonating-Valence-Bond states
• Multi-scale modelling of current-induced switching in magnetic tunnel junctions using ab initio spin transfer torques
• Exact resummation of the Holstein-Primakoff expansion and differential equation approach to operator square-roots
• Magnon versus electron mediated spin-transfer torque exerted by spin currents across antiferromagnetic insulator to switch magnetization of adjacent ferromagnetic metal
• Chiral Spin Order in Kondo-Heisenberg systems
• Non-classical spin transfer effects in an antiferromagnet
• Quantum spin torque driven transmutation of antiferromagnetic Mott insulator
• Energy and momentum conservation in spin transfer
• Derivation of a time dependent Schrödinger equation as quantum mechanical Landau-Lifshitz-Bloch equation

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