Abstract
The grain-boundary magnetoresistance of various La0.7Ca0.3MnO3 samples was investigated by resistance, magnetoresistance, and dynamic conductance measurements. Data on five samples, namely an epitaxial film, a polycrystalline film, a step-edge array, a mechanically induced grain boundary, and a chromium-manganite tunneling contact, are presented. A model of spin-polarized inelastic tunneling through an insulating barrier with spin-glass-like characteristics is proposed that explains the experimental observations.