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) investigations have been carried out on the grain boundaries (GBs) bf sintered pellets of giant magnetoresistive perovskite manganites La0.67Ca0.33MnO3 (LCMO), La0.60Y0.07Ca0.33MnO3 (LYMCO) and La0.67Pb0.10Ca0.23MnO3 (LPMCO) Based on spectroscopic data obtained and estimation of band gap, it has been concluded that 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 the 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 temperature band gap = 0.23 and 0.27 eV respectively), IGLs bend the energy band 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 decrease 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 behaviour which may be due to the accumulation of some conducting material or the trapping:centres in the IGL. Scanning electron microscopic and electrical measurements also justify the STM/STS results.