Random field effects in a diluted antiferromagnet: MnxZn1-xF₂

Neutron scattering measurements have been made of the effect of applying a field of 4.0 T on the long-range order and critical fluctuations of the dilute antiferromagnet Mn x Zn 1-x F 2 with x=0.78. The long-range order on a scale of 1200 lattice spacings at low temperatures is not destroyed on application of the field, unlike the recent results for the Co x Zn 1- x F 2 system, and in contradiction to current theories. The critical fluctuations within 2K of T N are very different in character from those obtained in the absence of the field. The results suggest that there are unusually long-range spin correlations and that the results are dependent on the prior history of the sample.