Antiferromagnetic spintronics offers ultrafast dynamics and immunity to stray fields. The recent emergence of altermagnetism, which features zero net magnetic moment combined with momentum-dependent spin splitting, enables low-power and high-density information processing. However, manipulating this spin splitting in three-dimensional bulk materials is restricted by rigid lattice symmetries. While crystal surfaces naturally break spatial inversion and out-of-plane translation symmetries, potentially unlocking hidden surface spin splittings, such symmetry breaking is inherently static once the cleavage plane is determined.
We propose a theoretical mechanism for a bulk-surface decoupled Floquet magnetic phase transition. Circularly polarized light (CPL) incident on a collinear antiferromagnetic surface has a finite penetration depth, which selectively breaks time-reversal symmetry within the top surface layers while leaving the deep bulk intact. We establish a symmetry framework for this Floquet-engineered surface magnetism. By classifying all 1,421 collinear spin space groups projected onto the 001 surface, we identify symmetry classes where the surface transitions from a symmetry-protected spin-degenerate state to a dynamically induced odd-parity (p-wave or f-wave) altermagnetic state under CPL illumination.
To verify this mechanism without the artifacts of finite-thickness slab models, we construct a microscopic semi-infinite tight-binding model on a honeycomb lattice and compute the local density of states and topological evolution using surface iterative Green’s function methods. The layer-resolved results show that the Floquet drive induces f-wave spin splitting at the surface, with the splitting magnitude decaying exponentially with depth, mirroring the light attenuation. Evaluations of the anomalous Hall conductivity reveal that, under weak driving, the topological response is dominated by the top surface layers. Within the bulk bandgap, the system realizes a surface-dominated quantum anomalous Hall state with Chern number C = -2, while the deep bulk states remain topologically trivial and PT-protected.
This work bridges group-theoretical symmetry classification with microscopic nonequilibrium lattice models, providing a theoretical foundation for exploring dynamic surface altermagnetism, layer-resolved topological phase transitions, and the design of controllable antiferromagnetic spintronic devices at the two-dimensional limit.