Grain-boundary effects on the magnetoresistance properties of perovskite manganite films

The role of grain boundaries in the magnetoresistance (MR) properties of the manganites has been investigated by comparing the properties of epitaxial and polycrystalline films of ${\mathrm{La}}_{0.67}{D}_{0.33}\mathrm{Mn}{\mathrm{O}}_{3-\delta } (D=\mathrm{C}\mathrm{a},\mathrm{S}\mathrm{r},\mathrm{o}\mathrm{r}\mathrm{v}\mathrm{a}\mathrm{c}\mathrm{a}\mathrm{n}\mathrm{c}\mathrm{i}\mathrm{e}\mathrm{s})$. While the MR in the epitaxial films is strongly peaked near the ferromagnetic transition temperature and is very small at low temperatures, the polycrystalline films show large MR over a wide temperature range down to 5 K. The results are explained in terms of switching of magnetic domains in the grains and disorder-induced canting of Mn spins in the grain-boundary region.