Ultrafast Spin Dynamics beyond s-Wave Magnets: A Universal Polarization Dependence.

Symmetry and hybridization yield anisotropic but nodal-less Fermi surfaces in s-wave ferromagnets (FMs) and antiferromagnets (AFMs), while they produce distinct momentum-space nodes in altermagnets (AMs). Both drive anisotropic femtosecond magnetization dynamics, but this link remains little explored. Here, we investigate laser-driven ultrafast spin dynamics in FMs, AFMs, and AMs with varying polarization angles using time-dependent density functional theory. We demonstrated, in FMs and AFMs, that laser polarization controls the amplitude of anisotropic yet symmetric demagnetization. In contrast, AMs─featuring spin nodal structures─exhibit sublattice-asymmetric demagnetization that is highly sensitive to laser incidence. This behavior arises from the anisotropy of the Fermi surface and band dispersion, which governs optical-induced intersite spin transfer (OISTR). We proposed a unified framework using the band-path-resolved local density of states to understand anisotropic OISTR and its impact on spin dynamics. Our results establish a direct connection between polarization-dependent ultrafast spin responses and the anisotropic electronic structure of materials.