Effects of the microstructure on the magnetotransport properties of polycrystalline manganite films grown by metalorganic chemical vapor deposition

The effects of the microstructure on the magnetoresistance of polycrystalli ne manganite (La1-xSrxMnO3) thin films have been investigated. The films ha ve been grown by metalorganic chemical vapor deposition on silicon (001) su bstrates covered with a native silica layer. Different microstructures were obtained by varying the substrate temperature in the range (550-750 degree s C). The films exhibit a ferromagnetic/paramagnetic transition around 325 K. The grain size is in the range 20-100 nm and, for deposition temperature lower than 650 degrees C, weak links appear in the films as the grains are mostly noncohesive. The low-field contribution to the magnetoresistance is independent of the grain size in the studied range. A factor of similar to 1.8 is gained in this contribution as the substrate temperature is lowered below 650 degrees C, which can be related to the occurrence of noncohesive grains in the films. The largest low-field magnetoresistance obtained is a bout 20% at 22 K (under 0.2 T). The high-field contribution to the total ma gnetoresistance appears to be independent of the microstructure of the film s. Our results are discussed in the framework of spin-dependent transport m echanisms (tunneling or scattering). Finally, the low-temperature (T less t han or equal to 20 K) resistivity upturn observed for all the polycrystalli ne films is discussed. It can be attributed to activated transport phenomen a (Coulomb blockade tunneling or tunneling through impurities, with activat ion energies on the order of 0.05 meV) or to localized states in the materi al. (C) 1999 American Institute of Physics. [S0021-8979(99)04024-4].