Enhanced grain surface effect on the temperature-dependent behavior of spin-polarized tunneling magnetoresistance of nanometric manganites

We have investigated the effects of nanometric grain size on magnetoresistance (MR), especially on its temperature-dependent behaviors of single-phase nanocrystalline granular La0.7Sr0.3MnO3 and La0.7Ca0.3MnO3 samples with an average grain size in the nanometric regime (12 and 17 nm). Most interestingly, we observed that the magnitude of low-field MR, arising from spin-polarized tunneling of conduction electrons, as well as of high-field MR remains constant up to a sufficiently high temperature (∼200K), and then drops sharply with temperature. With the application of a magnetic field, strong freezing of surface spins occur at the defect sites (having strong pinning strength of spins) of disordered grains surface as a consequence of competitive interactions between grain-boundary pinning strength (k) and magnetic field. Thermal energy (kBT), up to a considerably high temperature, remains unable to flip them from their strained condition, resulting in such a temperature insensitive behavior of MR as well as of surface spin susceptibility (χb).