Zn₂VO(PO₄)₂: an<i>S</i>= 1/2 Heisenberg antiferromagnetic square lattice system

Abstract

We have reinvestigated the magnetic properties of Zn2VO(PO4)2 by means of magnetic susceptibility χ(T) and specific heat Cp(T) measurements performed on polycrystalline samples. At high temperatures χ(T) follows a Curie–Weiss law with the effective moment expected for a V4+, S = 1/2 ion, while a rounded maximum in χ(T) at 6.95 K and a hump in Cp(T) around 4.5 K indicate the onset of antiferromagnetic correlations at low temperatures. Finally, a kink in χ(T) and a well defined mean-field anomaly in Cp(T) at TN = 3.7 K evidence a transition into an ordered antiferromagnetic state. A comparison of χ(T) and of the magnetic contribution Cpmagn(T) to the specific heat with theoretical predictions indicates that the magnetic lattice of this compound corresponds to a square lattice rather than the spin chain proposed in an earlier report. The results of quantum Monte Carlo calculations for a square lattice agree very well down to TN with Cpmagn(T) extracted from our experiment, while finite size calculations overestimate the specific heat in the region of the maximum in Cpmagn(T).