LMI Seminar: Teaching an old equation new tricks: unlocking an elementary interaction between light and magnetism using insights from Spintronics and Quantum technologies

Abstract:

The ferromagnetic resonance (FMR) experiment is pivotal for detecting spin currents in Spintronics technology. It occurs on Gigahertz timescales due the relatively slow relaxation time of the spins in ferromagnets. In contrast, optical fields oscillate much faster, at ∼ 400 − 800 THz. Therefore, it seems unlikely that such fast-oscillating fields may interact with magnetic moments. However, by combining principles from quantum optics, we have recently realized that the equations governing the FMR experiment are even relevant for magnetic fields that oscillate much faster, at optical frequencies. Namely, the interaction between optical beams and the magnetization is made possible. We find that the strength of the interaction is determined by an elementary efficiency parameter η=αγH/f_opt, where H is the amplitude of the optical magnetic field, α is the dissipation rate of spin angular momentum to the lattice, and f_opt and γ are the optical frequency and gyromagnetic ratio. Our results shed light on a variety of highly debated experimental observations on the interaction between optical fields and ferromagnets that have been reported in the last 25 years.