Microstructure and magnetoresistance of epitaxial films of the layered perovskiteLa2−2xSr1+2xMn2O7(x=0.3and 0.4)

Abstract

We have grown c-axis oriented epitaxial thin films of the bilayered perovskite ${\mathrm{La}}_{2\ensuremath{-}2x}{\mathrm{Sr}}_{1+2x}{\mathrm{Mn}}_{2}{\mathrm{O}}_{7}$ $(x=0.3,0.4)$ by laser molecular beam epitaxy on ${\mathrm{NdGaO}}_{3}$ and ${\mathrm{SrTiO}}_{3}$ 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 $\mathrm{ab}$ 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.