Polaron physics and crossover transition in magnetite probed by pressure-dependent infrared spectroscopy.

The optical properties of magnetite at room temperature were studied by infrared reflectivity measurements as a function of pressure up to 8 GPa. The optical conductivity spectrum consists of a Drude term, two sharp phonon modes, a far-infrared band at around 600 cm(-1) and a pronounced mid-infrared absorption band. With increasing pressure both absorption bands shift to lower frequencies and the phonon modes harden in a linear fashion. Based on the shape of the MIR band, the temperature dependence of the dc transport data, and the occurrence of the far-infrared band in the optical conductivity spectrum, the polaronic coupling strength in magnetite at room temperature should be classified as intermediate. For the lower energy phonon mode an abrupt increase of the linear pressure coefficient occurs at around 6 GPa, which could be attributed to minor alterations of the charge distribution among the different Fe sites.

Point-defect interactions in electron-irradiated titanomagnetites--as analysed by magnetic after-effect spectroscopy on annealing within 80 K < Ta < 1200 K.

During high-temperature growing of titanomagnetite single crystals (Fe(2.8-Δ)Ti(0.2)O(4), Δ < 0.005) in oxygen enriched atmospheres, specific Ti(4+)- and vacancy-based defect configurations are induced, giving rise to magnetic after-effect (MAE) spectra with peaks near 450, 200 and 65 K. The atomistic mechanisms of these relaxations are checked by exposing the crystals to low-temperature (80 K) electron (e(-)) irradiation and subsequent analysis of the interactions between radiation-induced and lattice-inherent defects on annealing over the range 80 K ≤ T(a) < or equal 1200 K. Within this interval, three characteristic temperature ranges are distinguished: (a) 80 K < T(a) < 500 K, revealing vigorous interactions between radiation-induced and inherent defect configurations, thus demonstrating their common point-defect nature; (b) 500 K < T(a) < 900 K wherein the MAE spectra re-assume, qualitatively, their initial structure with, however, mutually modified amplitude ratios; (c) 900 K < T(a) < 1200 K, being characterized by the complete annihilation of all MAEs but, interestingly, also the thermally induced re-appearance of vacancies and related defect configurations. The recovery kinetics of all prominent processes are numerically analysed and discussed with respect to their underlying atomistic mechanisms.

Full polarization analysis of resonant superlattice and forbidden x-ray reflections in magnetite.

Despite being one of the oldest known magnetic materials, and the classic mixed valence compound, thought to be charge ordered, the structure of magnetite below the Verwey transition is complex and the presence and role of charge order is still being debated. Here, we present resonant x-ray diffraction data at the iron K-edge on forbidden (0, 0, 2n+1)(C) and superlattice [Formula: see text] reflections. Full linear polarization analysis of the incident and scattered light was conducted in order to explore the origins of the reflections. Through simulation of the resonant spectra we have confirmed that a degree of charge ordering takes place, while the anisotropic tensor of susceptibility scattering is responsible for the superlattice reflections below the Verwey transition. We also report the surprising result of the conversion of a significant proportion of the scattered light from linear to nonlinear polarization.