Mössbauer spectroscopy of an insulating spin-glass

Abstract

Temperature-dependent ^"^Fe Mossbauer measurements have been performed between 4.2 and 300K on three powdered samples of the oxide spin-glass Fe2-xTii+x05 with x=0.00, 0.20, and 0.25. Our measurements show that the onset of spin-glass transition temperature, Tg, drops from 62K for x=0.00 sample to 41.5K for x=0.25 sample. The transition from the doublet paramagnetic state to the six lines spin-glass state covers a broad temperature range, in contrast to a sharp transition generally observed in magnetically order materials. A broadening in the background above the transition temperature may be interpreted as due to cluster freezing effects observed in the spin dynamics of the crossover region between the paramagnetic and spin-glass states. These results are consistent with earlier studies of the spin freezing dynamics of this system using ESR and |XSR. The Window method is employed to extract the hyperfine magnetic field distributions, p(H), from the 4.2K-Tg spectra. The resulting p(H) curves are not very sensitive to concentration, confirming the similarity of the glassy magnetic phases. The observed line broadening may be caused by the randomly distributed cations in Fe2-xTii+x05^ simulations of the spectra based on the Local Molecular Field Theory model, which calculates a distribution of hyperfine magnetic fields resulted from the randomly arranged nearest neighbor cations, is then performed to test this assumption. These simulations show that the randomly arranged cations are not the only source for the line broadening found in x=0.00 and x=0.2 samples. The simulation, however, describes most of the general features of x=0.25 data well. The Mossbauer spectra at high temperatures indicate the presence of both Fe^3+ and Fe^2+ ions in the two x>0 samples as expected, whereas only Fe^"*" is seen in x=0.00 sample. However, the low temperature magnetic spectra appear to be nearly identical for all three concentrations, implying very little change in the spin-glass phase. The Fe^2+ ions are not distinguishable from the Fe^3+ ions in these spectra below Tg, an unexpected observation.