Mossbauer 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 be en measured between 4.2 and 550 K. The substantial quadrupole splittings ob served in the paramagnetic spectra confirm that the local symmetry at both the tetrahedral and octahedral iron(III) sites is not cubic. The low temper ature 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 k nown orientation of the magnetization at 14 K in the cubic Ia (3) over bard unit cell. The Mossbauer spectra of R3Fe5O12, where R is Y, Eu and Dy, obt ained between 45 and 295 K, reveal four different tetrahedral iron(III) Mos sbauer spectral components, four components which are inconsistent with a m agnetization oriented along the [1 1 1] axis of a cubic Ia (3) over bard un it cell. In contrast, these four components are consistent with a crystal s ymmetry which is reduced from cubic to rhombohedral R (3) over bar. The tem perature 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 Dy3Fe5O12 gives Mossbauer lattice tempe ratures of 405 and 505 K for the 16a and 24d sites, respectively, values wh ich are in excellent agreement with the Debye temperature measured for Y3Fe 5O12.