Quantum effects in magnetization of J1-J2 square lattice antiferromagnet

The J(1)-J(2) square lattice Heisenberg model with spin S=1/2 has exchange bonds both along sides (J(1)) and along diagonals (J(2)) of the square. It exhibits three phases with long-range magnetic order and two unconventionally ordered phases depending on the ratio J(2)/J(1) of exchange constants. It describes a number of recently found layered vanadium oxide compounds. A simple means of investigating the ground state is the study of the magnetization curve and high field susceptibility. We discuss these quantities by using the second-order spinwave theory and the exact diagonalization in the whole J(1)-J(2) plane. We compare both results and find good overall agreement in the sectors of the phase diagram with magnetic order. Close to the disordered regions, the magnetization curve shows strong deviations from the classical linear behavior caused by large quantum fluctuations, and the spin-wave approximation breaks down. On the ferromagnet side (J(1) < 0), where one approaches the quantum gapless spin nematic ground state, this region is surprisingly large. This result is of great interest for the vanadium oxide compounds, which are found to lie in this region. We also investigate the effect of the interlayer coupling and find that the quasi-two-dimensional picture remains valid up to vertical bar J(perpendicular to)/J(1)vertical bar similar to 0.3.