High field properties of S = 1/2 square lattice J₁-J₂ magnetic compounds

We discuss the two-dimensional S = 1/2 J1-J2 frustrated Heisenberg model on a square lattice. This model provides a good description of two classes of quasi-two-dimensional vanadates, namely the Li2VOXO4 (X = Si,Ge) and AA'VO(PO4)2 (A,A' = Pb, Zn, Sr,Ba,Cd) compounds. We use the finite-temperature Lanczos method and spin wave analysis to study the high-field properties of this model in the whole J1-J2 plane. Of particular interest is the saturation field of the low-temperature magnetization. Together with the estimates for the average effective exchange constant from the zero-field heat capacity and ΘCW = (J1 + J2)/kB from the uniform magnetic susceptibility, the saturation field determines the position of a particular compound in the phase diagram uniquely. We give a summary of the results for the experimentally known compounds and show that our numerical findings agree very well with the experimental data for the magnetic susceptibility and the field dependence of the magnetization of the new compound BaCdVO(PO4)2. This compound is close to the quantum-critical point of the spin-nematic and collinear antiferromagnetic phase of the J1-J2 model as witnessed by a pronounced nonlinear magnetization curve.