Critical magnetic fluctuations in MnF2

The critical spin fluctuations in MnF 2 have been studied near the critical temperature T N using a triple axis neutron spectrometer with high resolution. Due to the anisotropy in MnF 2 , the longitudinal and the transverse component of the spin fluctuations can be separated experimentally. The longitudinal component of the static susceptibility diverges at T N , while the time fluctuations undergo the expected thermodynamic slowing down. The spectral width above T N varies in proportion to the temperature power law {( T-T N )/ T N } 0·62±0·02 . The temperature variation of the static susceptibility and the inverse correlation range of the longitudinal fluctuations are in agreement with the predictions of the Heisenberg model. The divergence of the transverse component is suppressed by the anisotropy, and this component is therefore finite at T N . The variation above T N was interpreted in terms of the transverse transition temperature T perpendicular , introduced by Moriya. In relation to this temperature, the static properties behave in an almost classical manner, that is in agreement with the molecular field theory. The transverse spectral width above T N varies as {( T-T perpendicular )/ T perpendicular } 0·53±0·05 . Below T N the transverse fluctuations consist of two spin waves. The spin-wave renormalization with temperature was measured in the long wavelength limit, and was found to vary in proportion to the staggered magnetization. The dynamical scaling law prediction for antiferromagnets of a q 3/2 dependence of the spectral width at T N was verified for the transverse component.