MOSSBAUER SPECTRAL INVESTIGATIONS OF ULTRAFINE RARE-EARTH IRON GARNETS

Ultrafine rare-earth iron garnets (RIG) in the crystallite size range 1.0-35 nm, prepared by the citrate precursor method have been investig ated by Mossbauer spectroscopy at room temperature. The crystallites o f 10-35 nm size show two six-line spectra corresponding to tetrahedral and octahedral Fe3+ with isomer shifts, delta, and magnetic hyperfine fields, H(hf), comparable to the bulk polycrystalline garnets. Howeve r, 1.0-1.5 nm crystallites (X-ray amorphous) show a quadrupole split d oublet due to superparamagnetic nature with isomer shifts (0.3 mm s-1) intermediate between tetrahedral and octahedral values of bulk garnet s. The relative increase in the tetrahedral delta and the decrease in the octahedral delta indicate, respectively, an increase of tetrahedra l Fe-O bond distance and a decrease of octahedral Fe-O bond distance i n the ultrafine state, in complete agreement with the observations mad e earlier in the spectrochemical investigations. The lines are broad ( linewidth = 0.65 mm s-1) and the quadrupole splittings are relatively larger (DELTAE(Q) = 1.0 mm s-1) indicating large deviation from cubic symmetry at the Fe3+ sites.