The Two Dimensional S=1 Quantum Heisenberg Antiferromagnet at Finite Temperatures.

The temperature dependence of the correlation length, susceptibilities and the magnetic structure factor of the two-dimensional spin-1 square lattice quantum Heisenberg antiferromagnet are computed by the quantum Monte Carlo loop algorithm (QMC). In the experimentally relevant temperature regime the theoretically predicted asymptotic low temperature behavior is found to be not valid. The QMC results however, agree reasonably well with the experimental measurements of La 2 NiO 4 even without considering anisotropies in the exchange interactions.

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