Investigation of the effect of microstructure and grain boundaries in nanostructured CMR thin films using scanning tunneling microscopy (STM) and local conductance map (LCMAP)

Abstract

We have investigated the spatially resolved local electronic properties of a nanostructured film of a colossal magnetoresistive (CMR) material by local conductance mapping (LCMAP) using a variable temperature scanning tunneling microscope (STM) operating in a magnetic field. The nanostructured thin films (thickness ≈500 nm) of the CMR material La_0.67Sr_0.33MnO₃ (LSMO) on silicon substrates were prepared using chemical solution deposition (CSD) process. These films have a large density of natural incoherent grain boundaries (GBs) which leads to significantly different behavior compared to oriented and epitaxial films of the same composition. Due to the presence of the GBs, these films show substantial low field magnetoresistance (LFMR) followed by a slower and almost linear decrease at higher fields and this is found to be strictly dependent on particle size. Most of the mechanisms proposed to explain the LFMR in the GB are based on tunneling through the GB. The purpose of this study is to use different STM based techniques to image these inhomogeneities and quantify them to the extent possible. In particular, we study the effect of grain size and the grain boundaries and their role in the electrical transport in nanostructured films of CMR materials.