Contrasting Light-Induced Spin Torque in Antiferromagnetic and Altermagnetic Systems

Light-matter interaction has become one of the promising routes to manipulating various physical features of quantum materials in an ultrafast kinetics. In this Letter, we focus on the nonlinear optical effects of the spintronic behavior in antiferromagnetic (AFM) and altermagnetic (AM) systems with compensated magnetic moments, which has been extensively attractive for their potential applications. With vanishing net magnetic moments, one of the main concerns is how to distinguish and disentangle AFMs and AMs in experiments, as they usually behave similarly in many susceptibility measurements. To address this challenge, we propose that linearly polarized light could trigger contrasting nonequilibrium local spin torques in these systems, unraveling hidden light-induced spintronic behaviors. In general, one could achieve light-induced spin canting in AMs, but only Néel vector torques in AFMs. We scrutinize and enumerate their symmetry constraints of all 122 magnetic point groups. We also adopt low energy Hamiltonian models and first-principles calculations on two representative materials to illustrate our theory. Our work provides a new perspective for the design and optimization of spintronic devices.