We develop a general framework, which combines exact diagonalization in small clusters with a density matrix variational principle, to study frustrated magnets at finite temperature. This thermodynamic hierarchical mean-field technique is used to determine the phase diagram and magnetization process of the three-dimensional spin-1/2 J1-J2 antiferromagnet on a stacked square lattice. Its non-magnetic phase exhibits a thermal crossover from a quantum to a classical paramagnet at a temperature T=T0 which can be extracted from thermodynamic measurements. At low temperature an applied magnetic field stabilizes, through order-by-disorder, a variety of phases with non-trivial spin textures and a magnetization plateau at half-saturation which continuously disappears at T∼T0. Our results are relevant for frustrated vanadium oxides.
@article{arxiv.1204.2559,
title = {Frustrated magnets and quantum paramagnetic phases at finite temperature},
author = {L. Isaev and G. Ortiz},
journal= {arXiv preprint arXiv:1204.2559},
year = {2012}
}