57Fe Mössbauer Experiments of Doped (La0·67Ba0·33)(Mn0·9Fe0·1)O3

Abstract

We report on Mössbauer effect (ME) experiments on the compound (La_0·67 Ba_0·33)(Mn_0·9 Fe_0·1)O₃ with the simple perovskite-like structure in the range from 4·2 to 300 K. From 190 to 300 K the spectra are described by a single quadrupole split doublet with a total splitting of e² qQ/2 = 0·38 mms^-1. This indicates that the local environment of the Fe³⁺ ions is not cubic, in contrast to the X-ray diffraction result on the same sample. At 4·2 and 25 K complex hyperfine (hf) split spectra are obtained. At least eight magnetically split subspectra with different hf fields must be used for describing these spectra adequately. From about 50 K up to 150 K the ME spectra reflect a relaxation process, indicating that the Fe moments fluctuate with frequencies in the range of the Larmor frequency in this temperature range. By describing the latter ME spectra by a magnetically split six-line spectrum with average hf fields and strongly broadened linewidths, the Fe moments are found to order below about 110 K. This value is much lower than the Curie temperature of T_c = 190 K obtained from our magnetisation measurements. The fluctuations of the Fe moments observed down to low temperatures might be responsible for the fact that the metal-semiconducting transition, which usually occurs at T_c according to the double exchange model, is shifted from T_c = 190 K down to 140 K.