Effect of crystallinity on the magnetoresistance in perovskite manganese oxide thin films

We report our study of the effect of crystallinity on the magnetoresistance in epitaxial and polycrystalline ${\mathrm{La}}_{\text{2/3}}{\mathrm{Ba}}_{\text{1/3}}{\mathrm{MnO}}_3$ and ${\mathrm{La}}_{\text{2/3}}{\mathrm{Ca}}_{\text{1/3}}{\mathrm{MnO}}_3$ 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.