The Hall coefficient of Gd-doped La 2/3 Ca 1/3 MnO 3 exhibits Arrhenius behavior over a temperature range from 2 T c to 4 T c , with an activation energy very close to 2/3 that of the electrical conductivity. Although both the doping level and thermoelectric coefficient indicate hole-like conduction, the Hall coefficient is electron-like. This unusual result provides strong evidence in favor of small-polaronic conduction in the paramagnetic regime of the manganites.

We explore the possibility that polaronic distortions in the paramagnetic phase of La 0.67 Ca 0.33 MnO 3 manganites persist in the ferromagnetic phase by considering the observed electrical resistivity to arise from coexisting fieldand temperature-dependent polaronic and band-electron fractions. We use an effective medium approach to compute the total resistivity of the twocomponent system, and find that a limit with low percolation treshold explains the data rather well. To test the validity of this model, we apply it to the thermoelectric coefficient. We propose a plausible mean-field model that reproduces the essential features of a microscopic model and provides a comparison with the experimental mixing fraction, as well as the magnetization and magnetic susceptibility.

Thin films (≊0.4 μm) of cobalt ferrite (CoFe2O4) have been grown on single-crystal (100) MgO substrates using pulsed laser deposition (PLD). The phase, orientation, and microstructure of the as-deposited films were investigated as a function of substrate temperature (i.e., 200–800 °C) at a constant oxygen deposition pressure of 30 mTorr. The as-deposited films were found to be single phase, well oriented, and approximately matching the stoichiometry of the target, but the cubic lattice constant of the films depended on the substrate temperature indicating that the films were strained. The greatest effect of the substrate temperature was on the magnetic properties of the as-deposited films. At 800 °C, 4πMs was measured to be 5370 G which is approximately the accepted bulk value for cobalt ferrite. In addition, PLD cobalt ferrite films grown at substrate temperatures of 600 and 800 °C exhibited a uniaxial magnetic anisotropy with an easy direction normal to the film plane. Films grown at 200 and 400 °C also exhibited a uniaxial magnetic anisotropy but possessed a planar easy direction.

Epitaxial Y,Fe,Oi, (YIG) films have been grown by the pulsed laser deposition (PLD) technique on (111) gadolinium gallium garnet substrates. The effect of substrate temperature and oxygen partial pressure on the structure, composition, and magnetic properties of the films was investigated and compared to liquid phase epitaxy YIG films. The results demonstrated that epitaxial YIG lilms could be prepared under a wide range of deposition conditions, but narrow linewidth (AH-1 Oe) films were producible only at low oxygen partial pressures (0, < 250 mTorr) and relatively high substrate temperatures (TS> 800 "C). Since the linewidth of single-crystal YIG is dominated by surface and volume defects and/or impurities, the narrow linewidth indicated that PLD is a viable technique for producing high-quality ferrite films for microwave device applications. In addition, under all deposition conditions (50-1000 mTorr and 700-850 "C!) there is a uniaxial axis perpendicular to the film plane. However, at low oxygen pressure the uniaxial anisotropy energy constant KU is negative while at high oxygen pressure K,, is positive.

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