J1−J2 square lattice antiferromagnetism in the orbitally quenched insulator MoOPO4

Abstract

We report magnetic and thermodynamic properties of a $4{d}^{1}$ (${\mathrm{Mo}}^{5+}$) magnetic insulator ${\mathrm{MoOPO}}_{4}$ single crystal, which realizes a ${J}_{1}\ensuremath{-}{J}_{2}$ Heisenberg spin-$1/2$ model on a stacked square lattice. The specific-heat measurements show a magnetic transition at 16 K which is also confirmed by magnetic susceptibility, ESR, and neutron diffraction measurements. Magnetic entropy deduced from the specific heat corresponds to a two-level degree of freedom per ${\mathrm{Mo}}^{5+}$ ion, and the effective moment from the susceptibility corresponds to the spin-only value. Using ab initio quantum chemistry calculations, we demonstrate that the ${\mathrm{Mo}}^{5+}$ ion hosts a purely spin-$1/2$ magnetic moment, indicating negligible effects of spin-orbit interaction. The quenched orbital moments originate from the large displacement of Mo ions inside the ${\mathrm{MoO}}_{6}$ octahedra along the apical direction. The ground state is shown by neutron diffraction to support a collinear N\'eel-type magnetic order, and a spin-flop transition is observed around an applied magnetic field of 3.5 T. The magnetic phase diagram is reproduced by a mean-field calculation assuming a small easy-axis anisotropy in the exchange interactions. Our results suggest $4d$ molybdates as an alternative playground to search for model quantum magnets.