Petrović Marko D, Mondal Priyanka, Feiguin Adrian E, Nikolić Branislav K
Department of Physics and Astronomy, University of Delaware, Newark, Delaware 19716, USA.
Department of Physics, Northeastern University, Boston, Massachusetts 02115, USA.
Phys Rev Lett. 2021 May 14;126(19):197202. doi: 10.1103/PhysRevLett.126.197202.
The standard model of spin-transfer torque (STT) in antiferromagnetic spintronics considers the exchange of angular momentum between quantum spins of flowing electrons and noncollinear-to-them localized spins treated as classical vectors. These vectors are assumed to realize Néel order in equilibrium, ↑↓⋯↑↓, and their STT-driven dynamics is described by the Landau-Lifshitz-Gilbert (LLG) equation. However, many experimentally employed materials (such as archetypal NiO) are strongly electron-correlated antiferromagnetic Mott insulators (AFMIs) whose localized spins form a ground state quite different from the unentangled Néel state |↑↓⋯↑↓⟩. 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 STT. 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.
反铁磁自旋电子学中自旋转移力矩（STT）的标准模型考虑了流动电子的量子自旋与被视为经典矢量的非共线局域自旋之间的角动量交换。这些矢量在平衡态下被假定实现奈尔序，即↑↓⋯↑↓，其由STT驱动的动力学由朗道-里夫希茨-吉尔伯特（LLG）方程描述。然而，许多实验中使用的材料（如典型的NiO）是强电子关联的反铁磁莫特绝缘体（AFMI），其局域自旋形成的基态与非纠缠的奈尔态|↑↓⋯↑↓⟩有很大不同。真实的基态被量子自旋涨落纠缠，导致所有局域自旋的期望值为零，因此由于STT而导致的固定长度经典矢量的LLG动力学甚至无法启动。相反，对传导电子和局域自旋进行完全量子处理对于捕捉它们之间的自旋角动量交换是必要的，这被称为量子STT。我们使用最近开发的用于量子STT的含时密度矩阵重整化群方法来预测，通过交换相互作用以及可能的小层间跳跃（模拟例如实验中使用的拓扑绝缘体/NiO双层）将自旋极化电流脉冲注入与AFMI覆盖层耦合的正常金属层时，如何诱导AFMI局域自旋的非零期望值。这个新的非平衡相是一个具有局域自旋锯齿形分布的空间不均匀铁磁体。AFMI吸收的总自旋随着AFMI中电子-电子排斥作用的增加而增加，并且当两层不交换任何电荷时也会增加。
