Low-field magnetoresistance in tetragonal ${\mathrm{La}}_{1\mathrm{-}\mathrm{x}}$ ${\mathrm{Ca}}_{\mathrm{x}}$ ${\mathrm{MnO}}_3$sfilms

We have measured the low-field magnetoresistance of molecular-beam-epitaxy-grown tetragonal ${\mathrm{La}}_{0.7}$ ${\mathrm{Ca}}_{0.3}$ ${\mathrm{MnO}}_3$ films as a function of temperature, and both magnitude and direction of the applied magnetic field. We observed low-field anisotropic hysteresis that depends on the direction of the applied field in the plane of the film. The hysteretic effect can result in a sharp drop in resistance during magnetization reversal which is more than 10 times steeper than the already 'colossal' magnetoresistance. We also present evidence of biaxial magnetocrystalline anisotropy with easy axes along the Mn-O bond ([100],[010]) directions. We show that the low-field anisotropic hysteresis arises from the combined effects of magnetocrystalline anisotropy, anisotropic magnetoresistance, and 'colossal' magnetoresistance. Based on a comparison of the data with a simple phenomenological model for the magnetoresistance, we argue that magnetization reversal must proceed by a domain process.