Microstructure and magnetoresistance of epitaxial films of the layered perovskite (formula presented) (formula presented) and 0.4)

We have grown c-axis oriented epitaxial thin films of the bilayered perovskite (formula presented) (formula presented) by laser molecular beam epitaxy on (formula presented) and (formula presented) substrates. X-ray diffraction and high-resolution transmission electron microscopy (TEM) revealed excellent epitaxial quality and phase purity of the films. However, a high density of stacking faults could also be detected by TEM. A comparison of magnetotransport measurements within the (formula presented) plane and along the c-axis direction showed an intrinsic c-axis tunneling magnetoresistance effect associated with nonlinear current-voltage characteristics for the (formula presented) compound. In addition to the colossal magnetoresistance effect around the Curie temperature (formula presented) at temperatures below about 40 K an additional high-field magnetoresistance was found probably due to a strain and disorder induced re-entrant spin glass state in both the (formula presented) and 0.4 compound. Our experiments show that the coherency strain in the high-quality epitaxial films results in pronounced differences in the magnetotransport behavior as compared to single crystals.