We report on the optical properties of the hole-doped manganites Nd 0.7 Sr 0.3 MnO 3 , La 0.7 Ca 0.3 MnO 3 , and La 0.7 Sr 0.3 MnO 3 . The low-energy optical conductivity in the paramagnetic-insulating state of these materials is characterized by a broad maximum near 1 eV. This feature shifts to lower energy and grows in optical oscillator strength as the temperature is lowered into the ferromagnetic state. It remains identifiable well below T c and transforms eventually into a Drude-like response. This optical behavior and the activated transport in the paramagnetic state of these materials are consistent with a Jahn-Teller small polaron. The optical spectra and oscillator strength changes compare well with models that include both double exchange and the dynamic Jahn-Teller effect in the description of the electronic structure.
We are optimizing thin films of perovskite manganese oxides for bolometric applications. We have studied the relevant material characteristics of several members of this family namely, La0.7Ba0.3MnO3, La0.7Sr0.3MnO3, La0.7Ca0.3MnO3, and Nd0.7Sr0.3MnO3. Here, we discuss issues related to the choice of material, the influence of deposition parameters, and postdeposition heat treatments on the relevant characteristics such as the resistivity-peak temperature (Tp) and the temperature coefficient of resistance (TCR). For a given material, a higher peak temperature implies a larger temperature coefficient of resistance. In contrast, on comparing different material systems, the TCR tends to decrease as Tp increases.
Oxide heterostructures were used for studies of quasiparticle injection effects in high-Tc superconducting thin films. The effect of injection of spin polarized quasiparticles from a ferromagnetic gate layer was compared to that of unpolarized quasiparticles from a nonmagnetic metallic gate. Transport measurements of the superconducting layer showed strong suppression in the supercurrent by the injection of spin-polarized quasiparticles, and a current gain of as large as five was attained. This is 10 to 30 times larger than the gain of unpolarized injection devices. Such large effects could be useful in a variety of active high-Tc superconductor/colossal magnetoresistance heterostructure based devices.
We report our study of the effect of crystallinity on the magnetoresistance in epitaxial and polycrystalline La2/3Ba1/3MnO3 and La2/3Ca1/3MnO3 thin films. Magnetoresistance in epitaxial films exhibits field dependence and temperature dependence similar to bulk single crystals and sintered bulk ceramics. The polycrystalline films exhibit a markedly different behavior. The magnetoresistance in this case shows either a monotonic increase or saturation with decreasing temperature in contrast to that of epitaxial films in which the magnetoresistance peaks close to the ferromagnetic transition temperature. The field dependence in the polycrystalline films is also remarkably different. At low fields, we observe a sharp drop in resistance followed by a more gradual decrease at higher fields. Our data suggest that in addition to the intrinsic magnetoresistance, grain-boundary transport contributes significantly to the magnetoresistance in polycrystalline films.
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