Giant negative photomagnetism in an alterlattice

Photomagnetism enables optical control of spin order for engineering non-equilibrium phases, yet studies remain largely confined to ferromagnetic systems. Here, we introduce the alterlattice, an emergent structural motif realized in the cation- and anion-deficient perovskite oxide La_0.4Ca_0.4TiO_3-δ. The alterlattice is a three-dimensional alternately patterned superstructural network arising from structural competition, in which average lattice is described by Ibmm symmetry while local superstructure relaxes toward Cmmm symmetry. Within this motif, photo-induced antiferromagnetic correlations develop below 19 K. Under 473 nm illumination, the susceptibility of the alterlattice host decreases by 51.7% at 2 K and 1 T and rapidly recovers in the dark. The observed giant negative photomagnetism is associated with anisotropic strain fields that guide defect-mediated photocarrier transport, thereby driving the system from a spin-dilute paramagnetic state toward a short-range antiferromagnetic state with quasi-one-dimensional characteristics. The alterlattice opens a route to three-dimensional defect-topology engineering for non-equilibrium phases and antiferromagnetic spintronics.