Nuclear Resonance Inelastic Scattering of Synchrotron Radiation in Oxides With Colossal Magnetoresistance

Abstract

Phonon anomalies near the critical temperature are studied in a number of itinerant ferromagnets, including (Fe,Co)-based perovskite-related oxides, the 57Fe-doped ruthenates and the manganite La0.7Sr0.3MnO3. The partial 57Fe phonon densities of states (DOS) in these oxides were derived from the resonant nuclear inelastic scattering spectra. The spectra were measured with energy resolution of 2.5meV by detecting the 6.3 keV Fe Ka X-rays following after Mossbauer effect on 57Fe transition excited by monochromatized 14.41 keV synchrotron radiation. The changes in the 57Fe phonon DOS g(E) are observed at cooling across T c . In the perovskites Sr2FeCoO6-δ and SrBaFeCoO6-δ, the lowest energy peak near 15 meV develops below Tc. We attribute this change to the narrowing of the phonon bands at the onset of the transport coherence. The non-metallic brownmillerite SrCaFeCoO5 exhibits a peak in the much lower energy region of g (E), In the g(E)/E 2 curve, a strong low-energy deviation from Debye behavior (g(E)/E 2= const) appears at 7 meV. This peak enlarges dramatically the vibrational amplitudes for brownmillerites relative to perovskites.