Current-voltage characteristics and electrical transport properties of grain boundaries in La1-x(Sr/Ca)(x)MnO3

Grain boundaries in doped lanthanum manganites have been shown to have a la rge low-field magnetoresistance. However, the mechanism of electrical trans port across grain boundaries, and the origin of the low-field magnetoresist ance, are not well understood. Models based on scattering at domain walls, spin-polarized tunneling and depression of the Curie temperature due to str ain near the grain boundary have all been proposed. This article reports de tailed studies of the transport properties of artificial grain boundaries f ormed in a variety of thin films grown on bicrystal substrates. Resistance versus field sweeps on all grain boundary devices showed strong low-field m agnetoresistance and the effect of individual domain motion at the grain bo undary has been observed in single grain boundaries. In all cases, current versus voltage characteristics were highly non-ohmic, and reminiscent of an electron tunneling process. However, the magnetic dependence of the curren t-voltage characteristics implies that the magnetoresistance may be unrelat ed to tunneling. (C) 1999 American Institute of Physics. [S0021-8979(99)052 10-X].