A muon spin relaxation measurement of the critical dynamics of MnF2 above TN

The various static critical exponents of MnF2 are all consistent with it being a three-dimensional Ising antiferromagnet. The few previous studies of its dynamical critical behaviour, however, have been indeterminate or have given critical exponent values closer to those expected of a Heisenberg system. We report zero-field muon spin relaxation measurements just above the Ne el temperature {TN 67.3K) and a critical exponent for the depolarization rate w=0.68+/-0.07. This 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 TN.