Mössbauer Studies ofFe57in Orthoferrites

The hyperfine interactions of the iron nuclei in the orthoferrites were studied between 85 and 770\ifmmode^\circ\else\textdegree\fi{}K using the M\"ossbauer effect. The temperature dependence of the sublattice magnetization ${\ensuremath{\sigma}}_{s}(T)$ is compared with various statistical-mechanical theories. It obeys approximately a $\frac{1}{3}$ power law in the temperature range $0.60l\frac{T}{{T}_{N}}l0.99$, and spin-wave theory and Callen decoupling at low temperature in the range $\frac{T}{{T}_{N}}l0.5$. From ${\ensuremath{\sigma}}_{s}(T)$, it is found that the canting angle in these weak ferromagnets is temperature-independent. The exchange integral calculated from the Oguchi spin-wave theory agrees with that calculated from Rushbrook and Wood's high-temperature series expansion and the Green's-function theories, while that calculated from the molecular field and Kubo's spin-wave theories is considerably lower. The quadrupole coupling constant is in good agreement with values calculated by point-charge lattice sums. The specific heat obtained from the temperature shift at high temperature agrees with the Dulong-Petit law. The value of the isomer shift is equal to that expected for a trivalent iron ion.