Revealing angular momentum transfer channels and timescales in the ultrafast demagnetization process of ferromagnetic semiconductors

Abstract

Ultrafast control of magnetic order by light provides a promising realization for spintronic devices beyond Moore’s Law and has stimulated intense research interest in recent years. Yet, despite 2 decades of debates, the key question of how the spin angular momentum flows on the femtosecond timescale remains open. The lack of direct first-principle methods and pictures for such process exacerbates the issue. Here, we unravel the laser-induced demagnetization mechanism of ferromagnetic semiconductor GaMnAs, using an efficient time-dependent density functional theory approach that enables the direct real-time snapshot of the demagnetization process. Our results show a clear spin-transfer trajectory from the localized Mn-d electrons to itinerant carriers within 20 fs, illustrating the dominant role of sp−d interaction. We find that the total spin of localized electrons and itinerant carriers is not conserved in the presence of spin-orbit coupling (SOC). Immediately after laser excitation, a growing percentage of spin-angular momentum is quickly transferred to the electron orbital via SOC in about 1 ps, then slowly to the lattice via electron–phonon coupling in a few picoseconds, responsible for the 2-stage process observed experimentally. The spin-relaxation time via SOC is about 300 fs for itinerant carriers and about 700 fs for Mn-d electrons. These results provide a quantum-mechanical microscopic picture for the long-standing questions regarding the channels and timescales of spin transfer, as well as the roles of different interactions underlying the GaMnAs demagnetization process.

Algorithm advances and applications of time‐dependent first‐principles simulations for ultrafast dynamics

“…We have used our rt‐TDDFT method to study the laser‐induced ultrafast demagnetization process in GaMnAs 49 . Our simulations well reproduced the experimental femtosecond demagnetization.…”

“…In the dynamic simulation, we have included all the effects of noncollinear magnetic moment, light–spin, light–orbital, spin–orbit, electron–electron, electron–phonon (by nuclear movements) interactions and external potential as well as the effect of initial finite temperature disorder in the magnetic moment 48 . This enables us to study the fundamental physics for many complicated phenomena 48,49 . Due to the treatment of laser field, the inclusion of multiple k ‐points, and the accuracy of plane‐wave basis, this rt‐TDDFT is capable of simulating the ultrafast phenomena for different types of systems, including periodic crystals, two‐dimensional materials, molecules, and so on.…”

“…τ is the angular momentum relaxation time, as an estimation of the transfer speed of these channels. The τ for sp–d coupling depends on the laser setting (reprinted with permission from Reference 49. Copyright 2019 PNAS)…”

Polarisation-dependent single-pulse ultrafast optical switching of an elementary ferromagnet

“…More details on each term can be found in Supplementary Note 1. The dipole approximation was used in the implementation of the vector potential, meaning that the spatial dependency is not included since the wavelength of the used light (hω = 1.55 eV → λ = 800 nm) is much larger than the lattice constant, and that the quadratic term as well as the other higher terms are zero 50 .…”

Phonon Chirality Induced by Vibronic-Orbital Coupling

“…Phononic pseudo angular momentum is key to several fundamental effects in physics, including the microscopic explanation of the phonon Hall effect 13 , Einstein-de Haas effect 14 , possible circular heat flow 15 , and zero point energies of chiral phonons 14 . The transfer channel from spin to phonon, on the other hand, is thought to be relevant in ultra-fast demagnetization processes 16,17 .…”