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

The various static critical exponents of MnF2 are all consistent with it being a three-dimensional Ising antiferromagnet. The few previous s tudies of its dynamical critical behaviour, however, have been indeter minate or have given critical exponent values closer to those expected of a Heisenberg system. We report zero-field muon spin relaxation mea surements just above the Neel temperature (T-N approximate to 67.3 K) and a critical exponent for the depolarization rate w=0.68+/-0.07. Thi s exponent is appreciably sensitive to dynamic critical behaviour and theoretical predictions fall in the range 0.63-0.72 (Ising) and 0.33-0 .35 (Heisenberg). We conclude that MnF2 does indeed have Ising dynamic behaviour just above T-N.