Dependence on film thickness of grain boundary low-field magnetoresistancein thin films of La0.7Ca0.3MnO3

The magnetoresistance of grain boundaries in the perovskite manganites is b eing studied, both in polycrystalline materials, and thin films grown on bi crystal substrates, because of interest in low-field applications. In this article we show that epitaxial films grown on SrTiO3 bicrystal substrates o f 45 degrees misorientation show magnetoresistance behavior which is strong ly dependent on the thickness of the film. Thin films, e.g., 40 nm, can sho w a large low-field magnetoresistance at low temperatures, with very sharp switching between distinct high and low resistance states for fields applie d in plane and parallel to the boundary. Thicker films show a more complex behavior of resistance as a function of field, and the dependence on the an gle between the applied field and the grain boundary is altered. These chan ges in magnetoresistance behavior are linked to the variation in morphology of the films. Thin films are coherently strained, due to the mismatch with the substrate, and very smooth. Thicker films relax, with the formation of defects, and hence different micromagnetic behavior. (C) 2001 American Ins titute of Physics.