Special CPM Seminar

Multiscale time-dependent-quantum-transport/classical-micromagnetics approach to dynamics of noncollinear magnetic textures

Branislav Nikolić

University of Delaware

This talk introduces recently developed [1-3] multiscale and self-consistent computational tool which combines time-dependent nonequilibrium Green function (TDNEGF) algorithms, scaling linearly in the number of time steps and describing quantum-mechanically conduction electrons in the presence of time-dependent fields of arbitrary strength or frequency, with classical description of the dynamics of local magnetic moments based on the Landau-Lifshitz-Gilbert (LLG) equation. Such TDNEGF+LLG approach can be applied to a variety of problems where current-driven spin torque induces the dynamics of magnetic moments as the key resource for next generation spintronics. Previous approaches for describing such nonequilibrium many-body system have neglected noncommutativity of a quantum Hamiltonian of conduction electrons at different times and, therefore, the impact of time-dependent magnetic moments on electrons which can lead to pumping of spin and charge currents. In turn, pumped spin current will self-consistently affect the dynamics of magnetic moments themselves, including generation of an effective non-Markovian damping and magnetic inertia terms into the LLG equation [2]. Thus, TDNEGF+LLG captures numerous effects missed by widely used classical micromagnetics based on solving the LLG equation alone. We use examples of current- or magnetic-field-driven motion of domain walls within magnetic nanowires, including their annihilation [3] observed in very recent experiments [4], to illustrate novel insights that can be extracted from TDNEGF+LLG simulations and visualizations. In particular, TDNEGF+LLG as a nonperturbative (i.e., numerically exact) framework allows us to establish [1,2] the limits of validity of perturbative theories, such as the so-called spin-motive force theory [5] for charge pumping by time-dependent noncollinear and noncoplanar magnetic textures, which turn out to be just the lowest order of the result predicted by TDNEGF+LLG.

References:
[1] M. D. Petrovi, B. S. Popescu, U. Bajpai, P. Plecháč, and B. K. Nikolić, Phys. Rev. Applied 10, 054038 (2018).
[2] U. Bajpai and B. K. Nikolić, Phys. Rev. B 99, 134409 (2019).
[3] M. D. Petrovi, P. Plecháč, and B. K. Nikolić, arXiv:1908.03194 (2019).
[4] S. Woo, T. Delaney, and G. S. D. Beach, Nat. Phys. 13, 448 (2017).
[5] S. E. Barnes and S. Maekawa, Phys. Rev. Lett. 98, 246601 (2007).