Growth and magnetotransport properties of epitaxial films of the layered perovskite La_2-2xSr_1+2xMn₂O₇

Epitaxial thin films of the bilayered perovskite La_2-2xSr_1+2xMn₂O₇ (x = 0.3, 0.4) have been grown by laser molecular beam epitaxy on NdGaO₃ substrates. Magnetotransport measurements with the current in 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. Besides the colossal magnetoresistance effect around the Curie temperature T_C, at temperatures below about 40 K an additional high-field magnetoresistance was found most likely due to a strain and disorder induced re-entrant spin glass state in both the x = 0.3 and 0.4 compounds. Our experiments show that the substrate induced coherency strain in the high quality epitaxial films results in magnetotransport properties that show markedly different behavior from those of single crystals.