Scanning tunnelling microscopic and spectroscopic investigation of the microstructural and electronic properties of the grain boundaries of giant magnetoresistive manganites

Scanning tunnelling microscopic (STM) and spectroscopic (STS) investigation s have been carried out on the grain boundaries (GBs) bf sintered pellets o f giant magnetoresistive perovskite manganites La0.67Ca0.33MnO3 (LCMO), La0 .60Y0.07Ca0.33MnO3 (LYMCO) and La0.67Pb0.10Ca0.23MnO3 (LPMCO) Based on spec troscopic data obtained and estimation of band gap, it has been concluded t hat these materials possess some sort of semiconducting intergranular layer (IGL) whose thicknesses are in the range of a few nm to about 100 nm and t he band gap is in the range of 0.3-0.45 eV. IGLs are usually more resistive than the grains. For semiconducting samples like LCMO and LYCMO (room temp erature band gap = 0.23 and 0.27 eV respectively), IGLs bend the energy ban d near GBs. This bending has been estimated to be about 40-50 meV with the depletion depth of few tens of nm extending on both sides of the IGL. The d ecrease in conductivity near the GB is due to the disorder induced carrier scattering and the bending of the band. LPCMO is almost conducting at room temperature. The GBs in this material sometimes exhibit conducting behaviou r which may be due to the accumulation of some conducting material or the t rapping:centres in the IGL. Scanning electron microscopic and electrical me asurements also justify the STM/STS results.