Light-induced nonlinear resonant spin magnetization

Abstract The optical generation of nonequilibrium spin magnetization enables ultrafast control of magnetization dynamics without external magnetic fields. Here, we develop a microscopic quantum kinetic theory of light-induced nonlinear spin magnetization (LNSM) of itinerant electrons, generalizing the inverse Faraday effect. The response comprises six distinct contributions, five of which arise from band geometric effects linked to interband coherence of Bloch electrons, capturing both Fermi sea and Fermi surface mechanisms. We perform a complete classification of the LNSM tensor across all 122 magnetic point groups and show that both polarization angle and helicity can tune spin responses in centrosymmetric and noncentrosymmetric materials. We demonstrate a significant LNSM response in the antiferromagnetic CuMnAs under THz illumination, which exhibits helicity-dependent switching. Our findings open up new possibilities for generating light-induced nonlinear spin-orbit torques and advancing opto-spintronic technologies.