Mössbauer Spectral Evidence for Rhombohedral Symmetry in R₃Fe₅O₁₂ Garnets with R = Y, Eu and Dy


The iron-57 Mossbauer spectra of R3Fe5O12, where R is Y, Eu and Dy, have been measured between 4.2 and 550 K. The substantial quadrupole splittings observed in the paramagnetic spectra confirm that the local symmetry at both the tetrahedral and octahedral iron(III) sites is not cubic. The low temperature Mossbauer spectra of Dy3Fe5O12 clearly confirm the spin reorientation between 10 and 15 K and the 4.2 and 10 K spectra are consistent with the known orientation of the magnetization at 14 K in the cubic Ia3̄d unit cell. The Mossbauer spectra of R3Fe5O12, where R is Y, Eu and Dy, obtained between 45 and 295 K, reveal four different tetrahedral iron(III) Mossbauer spectral components, four components which are inconsistent with a magnetization oriented along the [111] axis of a cubic Ia3̄d unit cell. In contrast, these four components are consistent with a crystal symmetry which is reduced from cubic to rhombohedral R3̄. The temperature dependence of the hyperfine fields in Dy3Fe5O12 indicates a small biquadratic exchange contribution to the magnetic exchange. The temperature dependence of the isomer shifts in Dy3Fe512 gives Mossbauer lattice temperatures of 405 and 505 K for the 16a and 24d sites, respectively, values which are in excellent agreement with the Debye temperature measured for Y3Fe5O12.



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