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 LaO.7SrO.3Mn03. 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 keY Fe Ka X-rays following after Mossbauer effect on 57Fe transition excited by monochromatized 14.41 keY synchrotron radiation. The changes in the 57Fe phonon DOS g(E) are observed at cooling across Te. In the perovskites Sr2FeCo06_3 and SrBaFeCo06_3, the lowest energy peak near 15 meV develops below Te. We attribute this change to the narrowing of the phonon bands at the onset of the transport coherence. The non-metallic brownmillerite SrCaFeCo05 exhibits a peak in the much lower energy region of g (E). In the g(E)/g curve, a strong low-energy deviation from Debye behavior (g(E)/g= const) appears at 7 meV. This peak enlarges dramatically the vibrational amplitudes for brownmillerites relative to perovskites.