Competing magnetic interactions in a spin- 12 square lattice: Hidden order in Sr2VO4

With decreasing temperature, Sr2VO4 undergoes two structural phase transitions, tetragonal-to-orthorhombic-to-tetragonal, without long-range magnetic order. Recent experiments suggest that only at very low temperature, Sr2VO4 might enter a yet-unknown phase with long-range magnetic order, but without orthorhombic distortion. By combining relativistic density functional theory with an extended spin-1/2 compass-Heisenberg model, we find an antiferromagnetic single-stripe ground state with highly competing exchange interactions, involving a non-negligible interlayer coupling, which places the system at the crossover between the XY-model and Heisenberg-model regimes. Most strikingly, we find a strong two-site "spin-compass" exchange anisotropy which is relieved by the orthorhombic distortion induced by the spin stripe order. Based on these results, we discuss the origin of the hidden-order phase and the possible formation of a spin liquid at low temperatures.