Mössbauer Studies of<mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"><mml:mrow><mml:msup><mml:mrow><mml:mi mathvariant="normal">Fe</mml:mi></mml:mrow><mml:mrow><mml:mn>57</mml:mn></mml:mrow></mml:msup></mml:mrow></mml:math>in Orthoferrites
Аннотация
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 $0.60<\frac{T}{{T}_{N}}<0.99$, and spin-wave theory and Callen decoupling at low temperature in the range $\frac{T}{{T}_{N}}<0.5$. 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.
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