Manipulating Femtosecond Spin--Orbit Torques with Laser Pulse Sequences to Control Magnetic Memory States and Ringing

Abstract: Femtosecond (fs) coherent control of collective order parameters is important for non--equilibrium phase dynamics in correlated materials. Here we propose a possible scheme for fs control of a ferromagnetic order parameter based on non--adiabatic optical manipulation of electron--hole ($e$--$h$) photoexcitations between spin--orbit--coupled bands that are exchange--split by magnetic interaction with local spins. We photoexcite fs carrier spin--pulses with controllable direction and time profile without using circularly--polarized light, via time--reversal symmetry--breaking by non--perturbative interplay between spin--orbit and magnetic exchange coupling of coherent photocarriers. We manipulate photoexcited {\em fs spin--orbit torques} to control complex switching pathways of the magnetization between multiple magnetic memory states. We calculate the photoinduced fs magnetic anisotropy in the time domain by using density matrix equations of motion rather than the quasi--equilibrium free energy. By comparing to pump--probe experiments, we identify a "sudden" magnetization canting induced by laser excitation, which displays magnetic hysteresis absent in static magneto--optical measurements and agrees with switchings measured by Hall magnetoresistivity. The fs magnetization canting switches direction with magnetic state and laser frequency, which distinguishes it from nonlinear optical and demagnetization longitudinal effects. By shaping two--color laser--pulse sequences analogous to multi--dimensional Nuclear Magnetic Resonance (NMR) spectroscopy, we show that sequences of clockwise or counter--clockwise fs spin--orbit torques can enhance or suppress magnetic ringing and switching rotation at any desired time. We propose protocols that can provide controlled access to four magnetic states via consequative 90$^{o}$ switchings.