Effect of nanosize modulation of granular La_0.67Sr_0.33MnO₃ manganites on temperature-dependent low-field spin-polarized tunneling magnetoresistance

We have investigated magnetotransport behaviors of a series of single-phase, nanocrystalline La_0.67Sr_0.33MnO₃ (LSMO) samples having grain sizes in the nanometric regime (14, 22, and 26 nm), all synthesized through chemical route “pyrophoric reaction process.” The motivation behind the present investigation is to study the effects of nanometric grain size on magnetoresistance (MR), specially its temperature and magnetic-field dependences. Magnetoresistance measurements show that in all samples there is a large negative MR at very low fields (LFMR), followed by a slower varying negative MR at comparatively high fields (HFMR), in the ferromagnetic regime. Surprisingly, we observed that at both low- and high-field regimes, the magnitude of MR remains constant up to sufficiently high temperature and then drops sharply with temperature. This temperature-dependent MR behavior gets enhanced with the decrease in particle size. Most interestingly, we found a considerable low-field MR (14%) persisting even at 200 K, which is an appreciable improvement on the results of previous workers. In order to explore the basic physics behind this unusual temperature dependence of MR, we analyzed our data in the light of a phenomenological model [P. Raychaudhuri etal, J. Appl. Phys. 84, 2048 (1998)], based on spin-polarized transport of conduction electrons at the grain boundaries, with major attention being paid to the gradual slippage of domain walls across the grain-boundary pinning centers in an applied magnetic field. Finally, we have attributed this feature of LFMR to the surface magnetization of our nanosize granular LSMO samples, which is crucial for nanodimensional systems.