Magnetoresistance in thin films and bulks of layered-perovskite La2−2xCa1+2xMn2O7

Epitaxial thin-film and polycrystalline bulk samples of ferromagnetic ${\mathrm{La}}_{\text{2−2x}}{\mathrm{Ca}}_{\text{1+2x}}{\mathrm{Mn}}_2{\mathrm{O}}_7$ $\text{(x=0.3)}$ with a layered perovskite structure have been examined with respect to their magnetotransport properties. In addition to a large magnetoresistance (MR) effect at temperatures around the metal–insulator transition, $\mathrm{a-}$ axis thin films exhibit unusual low-temperature MR behavior with apparent hysteresis in applied magnetic fields. The features in the bulk samples are a low-temperature MR effect without such hysteresis, and also an MR effect at high temperatures well above the metal–insulator transition temperature. The unique MR behavior depending on the sample form would reflect an anisotropic nature in the layered-perovskite ferromagnet with two-dimensional Mn-O networks.