Spatially resolved study of electronic transport through grain boundaries in nanostructured films ofLa0.67Sr0.33MnO3

Abstract

We present an investigation on spatially resolved studies of electronic transport in nanostructured (NS) films of the colossal magnetoresistive material ${\mathrm{La}}_{0.67}{\mathrm{Sr}}_{0.33}{\mathrm{MnO}}_{3}$. The NS films (thickness $\ensuremath{\approx}500\phantom{\rule{0.3em}{0ex}}\mathrm{nm}$) were grown on fused quartz and Si substrates using the chemical solution deposition process. In addition to structural characterization, average transport properties of the films were characterized by conventional resistivity and magnetoresistance measurements ($1.5\phantom{\rule{0.3em}{0ex}}\mathrm{K}<T<300\phantom{\rule{0.3em}{0ex}}\mathrm{K}$ and $H\ensuremath{\le}10\phantom{\rule{0.3em}{0ex}}\mathrm{T}$). The presence of a large number of natural grain boundaries (GB) makes the electronic transport in NS films markedly different from that seen in epitaxial/textured films of the same material, such as the occurrence of low field magnetoresistance (LFMR) which increases at lower temperatures. The spatially resolved local electronic properties of the NS films as a function of temperature and low magnetic field were investigated by local conductance mapping using a variable temperature scanning tunneling microscope. These measurements reveal the extent of variation of the density of states (DOS) at and close to the Fermi level $({E}_{F})$ at the grain boundaries and how it evolves as a function of temperature and in a magnetic field. The DOS in the GB region gets depleted as the temperature is reduced below $100\phantom{\rule{0.3em}{0ex}}\mathrm{K}$. This eventually leads to an activated transport in the film below $10\phantom{\rule{0.3em}{0ex}}\mathrm{K}$. We were also able to establish that a part of the LFMR seen in the GB region arises because the DOS of the material in the GB region is enhanced on application of magnetic field.