Neutron elastic, inelastic and high energy x-ray scattering techniques are used to explore the nature of the polaron order and dynamics in the colossal magnetoresistive (CMR) system La 0.7 Ca 0.3 MnO 3. Polaron correlations are known to develop within a narrow temperature regime as the Curie temperature is approached from low temperatures, with a nanoscale correlation length that is only weakly temperature dependent. The static nature of these short-range polaron correlations indicates the presence of a glass-like state, very similar to the observations for the bilayer manganite in the metallic-ferromagnetic doping region. In addition to this elastic component, inelastic scattering measurements reveal dynamic correlations with a comparable correlation length, and with an energy distribution that is quasielastic. The elastic component disappears at a higher temperature T*, above which the correlations are purely dynamic. These observations are identical to the polaron dynamics found in the bilayer manganite system in the CMR regime, demonstrating that they are a general phenomenon in the manganites.

Cold neutron triple-axis measurements have been used to investigate the nature of the long-wavelength spin dynamics in strongly-doped La1−xSrxMnO3 single crystals with x=0.2 and 0.3. Both systems behave like isotropic ferromagnets at low T , with a gapless (E0 < 0.02 meV) quadratic dispersion relation E = E0 + Dq 2 . The values of the spin-wave stiffness constant D are large (DT =0 = 166.77 meV·Å 2 for x=0.2 and DT =0 = 175.87 meV·Å 2 for x=0.3), which directly shows that the electron transfer energy for the d band is large. D exhibits a power law behavior as a function of temperature, and appears to collapse as T → TC . Nevertheless, an anomalously strong quasielastic central component develops and dominates the fluctuation spectrum as T → TC . Bragg scattering indicates that the magnetization near TC exhibits power law behavior, with β ≃ 0.30 for both systems, as expected for a three-dimensional ferromagnet. 75.25.+z, 75.30.Kz, 75.40.Gb, 75.70.Pa

Neutron scattering has been used to study the nature of the spin dynamics and charge correlations in a single crystal of the colossal magnetoresistive perovskite La0.7Ca0.3MnO3. Diffuse scattering from lattice polarons develops as the Curie temperature is approached from below, along with short range polaron correlations that are consistent with stripe formation. Magnetic fields are found to suppress this polaron formation. The temperature dependence of the polaron correlations follows the same behavior as both the resistivity and the anomalous quasielastic component in the magnetic fluctuation spectrum, indicating that they have a common origin.

We present magnetic-, resistivity-, and specific-heat measurements of the title compounds and found that in Eu0.7Sr0.3MnO3 an insulator-metal transition can be induced by an external field below 60 K while Eu0.7Ca0.3MnO3 remains insulating. EuMnO3 exhibits two magnetic transitions at 35 and 47 K which are associated with the appearance of a weak ferromagnetic component and antiferromagnetic ordering. With respect to the parent compound, Ca and Sr substitution enhance the ferromagnetic component significantly, however, for low fields and after zero-field cooling the antiferromagnetic ground state is preserved to a large extent.

We report on studies of the kinetics of magnetite oxidation, focusing on the early stages of the process. In order to achieve well-defined experimental conditions, we used low-index surfaces of single crystals of magnetite: (111), ( 110) and (100). For this study we have developed a new technique utilizing the in situ hot stage of a high-resolution x-ray diffractometer. Other techniques were also employed, including Raman spectroscopy and vibrating sample magnetometry. We found that on all three surfaces a layer of haematite, α-Fe 2 O 3 , was formed as a result of annealing at 300 • C in air. The layers of oxides formed on all three low-index surfaces of magnetite are non-epitaxial. The layer thickness was calculated as a function of annealing time. The oxidation rate for the (100) surface was significantly greater than for the other two surfaces. Parameters of the oxidation kinetics were obtained through a fitting procedure. The results suggest that the mechanisms of oxidation for these three orientations are different. The oxidation of the magnetite ( 110) and (111) surfaces is in line with the Wagner model, but this is not the case for the (100) surface. We consider possible mechanisms of oxidation and compare our results with the results of oxidation in powder magnetite.