Critical Magnetic Fluctuations in MnF2

The critical spin fluctuations in MnF2 have been studied near the Néel temperature using a triple axis spectrometer with high resolution. The experimental data were analysed in terms of the static susceptibility and the spectral shape function. The results can be summarized as follows: The longitudinal component χ∥ (q) of the static susceptibility behaves as expected for a Heisenberg magnet, χ∥ (q = 0) ∝ [(T - TN) / TN]−1.35±0.01 and κ1∥ = 0.27[(T - TN) / TN]0.68±0.03 where κ1∥ is the longitudinal inverse correlation length. The corresponding transverse component χ⊥(q) is suppressed by the anisotropy. In terms of the suppressed Néel temperature T⊥ introduced by Moriya (1962) the temperature dependences were χ⊥(q=0)∝[(T−T⊥)/T⊥]−0.85±0.03 and κ1⊥=0.21[(T−T⊥)/T⊥]0.45±0.01. The characteristic widths of the spectral shape functions varied as Γ∥ (q = 0) = 1.5[(T - TN) / TN]0.62±0.02 and Γ⊥(q=0)=1.4[(T−T⊥)/T⊥]0.53±0.05 indicating thermodynamic showing down for the longitudinal component. The exponent 0.62±0.02 of Γ∥ (q = 0) is in bad agreement with the existing theories but is consistent with the similar exponent found in RbMnF3 by Nathans, Menzinger, and Pickart (1968). The q-variation of the transverse width Γ⊥(q) was consistent with the q3/2-law predicted by dynamic scaling.