Critical Magnetic Scattering from RbMnF3

Critical magnetic scattering of neutrons and sublattice magnetization have been measured for RbMnF₃ in the vicinity of the Néel point. The following critical indices have been obtained: $$\begin{array}{ccc}\text{β=0.316±0.008,}& {\mathrm{M\sim (\Delta T)}}^{\beta }& {\text{0.6\%<ΔT/T}}_N\text{<5\%}\\ \text{ν=0.724±0.008,}& {\kappa }_1{\mathrm{\sim (\Delta T)}}^{\nu }& {\text{0.04\%<ΔT/T}}_N\text{<18\%}\\ \text{γ=1.397±0.034}& {\mathrm{\chi \sim (\Delta T)}}^{\mathrm{-\gamma }}& {\text{0.04\%<ΔT/T}}_N\text{<4\%,}\end{array}$$ where M is the sublattice magnetization, κ₁ the inverse range parameter, and χ the susceptibility. ν and γ were obtained from the critical scattering assuming (1) the quasielastic approximation and (2) the existence of the parameter η in the cross‐section formula as proposed by Fisher and discussed below. For ΔT/T_N<0.1% the critical scattering exhibited a pronounced deviation from Lorentzian shape. Excellent agreement was obtained, however, using a cross section of the form ${\mathrm{(A/\kappa }}_1^2{\mathrm{+q}}^2{)}^{\text{1−η/2}}$ , where q is the momentum transfer relative to the (111) magnetic reciprocal lattice point. The best value of η, obtained by least‐squares curve fitting, was 0.067±0.01. Analysis of the data for q≫κ₁, where resolution corrections were negligible, gave the same result. It is to be noted, in addition, that the best values of ν and γ obey the relation γ=(2‐η).