Mössbauer study of the sublattice magnetization in the layer antiferromagnet CsFeF4

Abstract

Mössbauer-effect measurements of the hyperfine interaction of ${\mathrm{Fe}}^{57}$ nuclei in the layer antiferromagnet CsFe${\mathrm{F}}_4$ have been used to study the temperature dependence of the sublattice magnetization. At low temperatures ($T\le 0.4T_N$) the results agree with simple noninteracting two-dimensional spin-wave theory for a quadratic layer. The spin-wave fit yields a large anisotropy field of 18.7 kOe, only about 1/7 of which can be ascribed to magnetic dipole interactions. The fit also indicates a substantial zero-point spin deviation of 0.158. Data near $T_N$ can be fitted well by a power law ${(1-\frac{T}{T_N})}^{\beta }$ over temperature ranges as wide as $0.006<(1-\frac{T}{T_N})<\mathrm{}0.69\mathrm{}$. Weighted averages of fits over various temperature ranges yield $\beta =0.278\mathrm{}\pm 0.010$ and $T_N=(160.33\mathrm{}\pm 0.11)$ °K. This value for the critical exponent is closer to those found in three-dimensional magnetic compounds than to the value $\beta =\frac{1}{8}$ calculated for the two-dimensional spin-1/2 Ising model. Comparison with data for other layer systems suggests that this high $\beta$ is indicative of the presence of three-dimensional magnetic correlations above $T_N$ and is attributable to the absence of a staggered arrangement of the layers.