The frustrated J1-J2 model in high magnetic fields

We investigate thermodynamic properties of the frustrated J1-J2 spin S = 1/2 model by using the finite-temperature Lanczos method for small clusters on a square lattice. We focus on the global properties of specific heat, susceptibility and spin structure factor as a function of the frustration angle φ = tan-1(J2/J1). Specifically, we discuss the high-field properties and the saturation field and its dependence on φ. We show that the asymmetry between collinear and Neel antiferromagnetic states gives an additional criterion to distinguish between these phases. Furthermore, we investigate the magnetocaloric effect and show that large peaks occur close to the saturation field. The J1-J2 spin model is realized in some layered quasi-2D vanadium perovskites. They have characteristic values for J1 and J2 of about 10 K. The associated saturation fields are of the order of 40 T and therefore easily accessible in pulsed-field experiments