Monte Carlo simulations of phase transitions in Rb2MnF4

Rb2MnF4 is believed to consist of very weakly coupled magnetic layers with isotropic superexchange between nearest neighbor Mn2+ ions in the same plane. To test this conjecture, Monte Carlo simulations were performed on a theoretical model consisting of stacked, nearest neighbor Heisenberg square lattices coupled by dipolar interactions. Simulation parameters such as the lattice constants and the superexchange constant were taken from experimental measurements, and periodic boundary conditions were used in all directions. We succeeded in achieving an algorithmic speed up from O(N2) to O(N ln N) through the combination of fast Fourier transforms for the evaluation of the dipolar interactions and global updates in the Monte Carlo steps. Our simulations reproduced very well the behavior observed in experiments with Rb2MnF4 crystals.