Microstructure and magnetoresistance of epitaxial films of the layered perovskite La_2-2xSr_1+2xMn₂O₇ (x=0.3 and 0.4)

We have grown c-axis oriented epitaxial thin films of the bilayered perovskite La_2-2xSr_1-2xMn₂O₇ (x=0.3,0.4) by laser molecular beam epitaxy on NdGaO₃ and SrTiO₃ 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 ab plane and along the c-axis direction showed an intrinsic c-axis tunneling magnetoresistance effect associated with nonlinear current-voltage characteristics for the x=0.3 compound. In addition to the colossal magnetoresistance effect around the Curie temperature T_c, 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 x=0.3 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.