Finite-temperature ordering in two-dimensional magnets

Details Finite-temperature ordering in two-dimensional magnets Finite-temperature ordering in two-dimensional magnets

2000-05-26

arxiv.org/abs/cond-mat/0005464v1

Phys. Rev. B 62, 3771 (2000)

Physics

Condensed Matter

Statistical Mechanics

To appear on Phys. Rev. B

Scientific paper

10.1103/PhysRevB.62.3771

We study the two dimensional quantum Heisenberg antiferromagnet on the square lattice with easy-axis exchange anisotropy. By the semiclassical method called pure-quantum self-consistent harmonic approximation we analyse several thermodynamic quantities and investigate the existence of a finite temperature transition, possibly describing the low-temperature critical behaviour experimentally observed in many layered real compounds. We find that an Ising-like transition characterizes the model even when the anisotropy is of the order of $10^{-2}J$ ($J$ being the intra-layer exchange integral), as in most experimental situations. On the other hand, typical features of the isotropic Heisenberg model are observed for both values of anisotropy considered, one in the {\it quasi}-isotropic limit and the other in a more markedly easy-axis region. The good agreement found between our theoretical results and the experimental data relative to the real compound Rb$_2$MnF$_4$ shows that the insertion of the easy-axis exchange anisotropy, with quantum effects properly taken into account, provides a quantitative description and explanation of the experimental data, thus allowing to recognize in such anisotropy the main agent for the observed onset of finite temperature long-range order.

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