Optical switching of antiferromagnetic domains by nonreciprocal heat current

What distinguishes front from back? In physics, such directionality emerges only when an underlying symmetry is broken. Antiferromagnets that inherently break both space-inversion and time-reversal symmetries provide a striking example, exhibiting nonreciprocal optical responses that depend on the direction of light propagation. Beyond distinguishing antiferromagnetic domains, we show that this nonreciprocity can deterministically create them. Using mid-infrared light, we demonstrate deterministic switching of antiferromagnetic domains in the magnetoelectric antiferromagnet LiFePO4, where illumination from opposite sides selectively stabilizes opposite domain states. Remarkably, the switching persists over a broad wavelength range rather than being confined to a narrow transition-specific spectral region, overcoming the spectral and material constraints of resonance-based optical switching schemes. The broadband switching originates from the material's intrinsic nonreciprocity through optically generated heat currents. Our results establish nonreciprocity as a general principle for deterministically controlling symmetry-broken phases with light.