Lattice distortions and magnetic interactions in single-layer VOCl

Atomically thin layers exfoliated from magnetic van der Waals layered materials are currently of high interest in solid state physics. VOCl is a quasi-two-dimensional layered antiferromagnet which was recently synthesized in monolayer form. Previous theoretical studies have assumed the high-temperature orthorhombic lattice symmetry also in the low-temperature range, where the bulk system is known to be monoclinic due to a strong magnetoelastic coupling. We demonstrate from ab initio calculations that this monoclinic distortion is prevalent also in monolayers, which is in line with recent experimental indications of monoclinic symmetry. Our calculations also show that competing ferromagnetic and antiferromagnetic interactions cause a frustrated twofold magnetic superstructure where higher-order magnetic interactions play a key role.