Microstructure dependent magnetotransport properties in polycrystalline La_(0.7)Sr_(0.3)MnO_3 ultrathin films on thermally oxidized Si substrates

摘要

The magnetotransport properties of polycrystalline La_(0.7)Sr_(0.3)MnO_3 (LSMO) ultrathin films depend strongly on their thickness and microstructure. In particular, the resistivity and magnetoresistance can be tuned by varying the film thickness and grain size. Here, we have deposited LSMO films with thicknesses ranging from 100 nm down to 10 nm by pulsed laser deposition on thermally oxidized Si substrates, retaining the films' metallic nature. To avoid reaction of the films with SiO_2 at high temperatures, we have introduced a two-step deposition method (half the film thickness grown at 400 °C and the remaining half at 800 °C). X-ray diffraction (XRD) revealed the polycrystalline nature, while atomic force microscopy (AFM) revealed the granular structure of the films from which, surface roughness and grain size are found to decrease with decreasing film thickness. However, low temperature resistivity upturn increases with decreasing film thickness and grain size. The variations of low temperature resistivity minima and magnetoresistance (MR) with film thickness and grain size have been interpreted in the regime of quantum interference effects (viz. weak localization and e-e interaction), intergranular spin polarized transport (SPT) phenomena and their dominance. It is found that SPT was the dominating phenomena. The signature of SPT was also evidenced clearly from low field MR at low temperatures. These kinds of ultrathin films are suitable as ferromagnetic electrode for spintronic device applications.