Quantum Spin Torque Driven Transmutation of an Antiferromagnetic Mott Insulator

The standard model of spin-transfer torque (STT) in antiferromagnetic spintronics considers the exchange of angular momentum between quantum spins of flowing electrons and noncolfinear-to-them localized spins treated as classical vectors. These vectors are assumed to realize Neel order in equilibrium, up arrow down arrow ... up arrow down arrow, and their SIT-driven dynamics is described by the Landau-Lifshitz-Gilbert (LLG) equation. However, many experimentally employed materials (such as archetypal NiO) arc strongly electron-correlated antiferromagnetic Mott insulators (AFMIs) whose localized spins form a ground state quite different from the unentangled Neel state vertical bar up arrow down arrow ... up arrow down arrow > The true ground state is entangled by quantum spin fluctuations, leading to the expectation value of all localized spins being zero, so that LLG dynamics of classical vectors of fixed length rotating due to STT cannot even be initiated. Instead, a fully quantum treatment of both conduction electrons and localized spins is necessary to capture the exchange of spin angular momentum between them, denoted as quantum SIT. We use a recently developed time-dependent density matrix renormalization group approach to quantum STT to predict how injection of a spin-polarized current pulse into a normal metal layer coupled to an AFMI overlayer via exchange interaction and possibly small interlayer hopping-mimicking, e.g., topological-insulator/NiO bilayer employed experimentally-will induce a nonzero expectation value of AFMI localized spins. This new nonequilibrium phase is a spatially inhomogeneous ferromagnet with a zigzag profile of localized spins. The total spin absorbed by AFMI increases with electron-electron repulsion in AFMIs, as well as when the two layers do not exchange any charge.

Automated summary

The standard model of spin-transfer torque in antiferromagnetic spintronics does not capture the ground state entanglement of quantum spins, requiring a fully quantum treatment to understand the exchange of spin angular momentum. This new approach predicts a nonzero expectation value of localized spins in antiferromagnetic Mott insulators when subjected to spin-polarized current pulses, leading to a spatially inhomogeneous ferromagnetic phase with a zigzag profile. The total spin absorbed by AFMI increases with electron-electron repulsion, as well as when there is no charge exchange between layers.