Magnetocaloric effect in the frustrated square lattice J1-J2 model

We investigate the magnetocaloric properties of the two-dimensional frustrated J(1)-J(2) model on a square lattice. This model describes well the magnetic behavior of two classes of quasi-two-dimensional S=1/2 vanadates, namely, the Li2VOXO4 (X=Si,Ge) and AA ' VO(PO4)(2) (A,A '=Pb,Zn,Sr,Ba) compounds. The magnetocaloric effect (MCE) consists of the adiabatic temperature change upon changing the external magnetic field. In frustrated systems, the MCE can be enhanced close to the saturation field because of massive degeneracies among low-lying excitations. We discuss results for the MCE in the two distinct antiferromagnetic regimes of the phase diagram. The numerical finite temperature Lanczos method as well as analytical methods based on the spin-wave expansion are employed and results are compared. We give explicit values for the saturation fields of the vanadium compounds. We predict that at subcritical fields there is first a (positive) maximum followed by a sign change of the MCE, characteristic of all magnetically ordered phases.