Effect of in-plane epitaxy on magnetotransport properties of (La_0.5Sr_0.5)CoO₃ thin films

We have systematically studied the temperature- and magnetic field-dependent resistance in (La_0.5Sr_0.5)CoO₃ (LSCO) films with various degrees of in-plane texturing using the biaxially oriented LSCO on CeO₂/YSZ/SiO₂/Si substrates. We find that the ferromagnetic transition temperature, T_c, is much the same in all samples while the resistive transition exhibits metallic behavior (dR/dT>0) in the epitaxial LSCO to semiconducting behavior (dR/dT<0) in the poorly textured LSCO. The magnetoresistance (MR) in the epitaxial LSCO on a LaAlO₃ substrate displays two distinct regions; the low field hysteretic MR related to the magnetization hysteresis and the high field linear negative MR due to the suppression of spin fluctuations. In contrast, the MR in the biaxially textured LSCO sample shows only the hysteretic behaviors at low fields and no linear MR at high fields. Compared to the polycrystalline-doped LaMnO₃ which shows an enhanced MR at low fields, the MR in the biaxially oriented LSCO does not exhibit such characteristics. We attribute this to the differences in the spin polarization between LSCO and the doped LaMnO₃.