A MUON SPIN RELAXATION MEASUREMENT OF THE DYNAMIC CRITICAL-BEHAVIOR OF MNF2 ABOVE T-N

MnF2 has long been considered to be a three-dimensional Ising antiferr omagnet and its various static critical exponents are consistent with this description. However, the few measurements of its dynamic critica l behaviour have been indeterminate overall and have been interpreted sometimes as favouring the Heisenberg model. This apparent inconsisten cy has been an outstanding problem in critical behaviour. We report ze ro-field muon spin relaxation measurements just above the Neel tempera ture (T-N approximate to 67.3 K). The results point unequivocally to I sing behaviour over a narrow range of temperature (approximate to 0.2 K) above T-N.