LOW-FIELD MAGNETORESISTANCE IN TETRAGONAL LA1-XCAXMNO3 FILMS

We have measured the low-field magnetoresistance of molecular-beam-epi taxy-grown tetragonal La0.7Ca0.3MnO3 films as a function of temperatur e, and both magnitude and direction of the applied magnetic field. We observed low-field anisotropic hysteresis that depends on the directio n of the applied field in the plane of the film. The hysteretic effect can result in a sharp drop in resistance during magnetization reversa l which is more than 10 times steeper than the already ''colossal'' ma gnetoresistance. We also present evidence of biaxial magnetocrystallin e anisotropy with easy axes along the Mn-O bond ([100],[010]) directio ns. We show that the low-field anisotropic hysteresis arises from the combined effects of magnetocrystalline anisotropy, anisotropic magneto resistance, and ''colossal'' magnetoresistance. Based on a comparison of the data with a simple phenomenological model for the magnetoresist ance, we argue that magnetization reversal must proceed by a domain pr ocess.