Magnetizing altermagnets by ultrafast asymmetric spin dynamics

Abstract: Laser pulses are known to induce symmetric demagnetization; equal loss of magnetic moments in the identical sublattices of antiferromagnets and ferromagnets at ultrashort timescale. This is due to their identical local electronic structures guided by the underlying symmetries. Using time-dependent density functional theory, we demonstrate that laser pulses can drive asymmetric demagnetization dynamics of identical sublattices in the d-wave compensated altermagnet RuO2, resulting in a photo-induced ferrimagnetic state with a net moment of ~0.2 {\mu}B per unit cell. This metastable magnetization is highly controllable; depends on the direction of the linear polarized laser. We identify the underlying mechanism as an anisotropic optical-induced intersite spin transfer (a-OISTR) effect, originating from the momentum-dependent spin splitting unique to altermagnets. This a-OISTR effect enables the polarization of light to drive direction-selective transient spin-dependent currents between sublattices, leading to a controllable ultrafast magnetic state transition in AM. These findings uncover novel laser-driven pathways to control magnetic order in altermagnets, enabling a phase transition from AM to ferrimagnetic state.