Effect of grain size modulation on the magneto- and electronic-transport properties ofLa0.7Ca0.3MnO3nanoparticles: The role of spin-polarized tunneling at the enhanced grain surface

Abstract

We have investigated the effect of nanometric grain size on magneto- and electronic-transport properties of single-phase, nanocrystalline, granular ${\mathrm{La}}_{0.7}{\mathrm{Ca}}_{0.3}\mathrm{Mn}{\mathrm{O}}_3$ samples having an average grain size in the nanometric regime $(14–27\mathrm{nm})$. Based upon a spin-polarized tunneling mechanism, we have proposed a phenomenological model to explain the observed electronic transport behavior over the whole temperature range $(20–300\mathrm{K})$, especially the gradual drop of metal-insulator transition temperature with a decrease in grain size, while ferromagnetic-paramagnetic transition temperature remains almost constant. We have attributed the steeper low-temperature $(\sim 40\mathrm{K})$ resistivity upturn in the smaller grain size sample rather than that of the larger grain size sample below their respective resistivity minima at $T_{\min }$ to the increased value of charging energy, which has been estimated to be $13\mathrm{K}$ for a $17\mathrm{nm}$ sample and $0.026\mathrm{K}$ for a $27\mathrm{nm}$ sample. Most interestingly, magnetotransport measurements show that the magnitude of low-field magnetoresistance, as well as of high-field magnetoresistance remains constant up to sufficiently high temperature $(\sim 220\mathrm{K})$ and then drops sharply with temperature. The effect gets more pronounced with the decrease in particle size. In order to explore the basic physics behind this unusual temperature dependence of magnetoresistance (MR), we have analyzed our data in light of a phenomenological model [P. Raychaudhuri et al., Phys. Rev. B 59, 13919 (1999)], based on the spin-polarized transport of conduction electrons at the grain boundaries. Analyzing our data following the theoretical perspective as proposed by S. Lee et al. [Phys. Rev. Lett. 82, 4508 (1999)], we found that this strange temperature dependence of MR is decided predominantly by the nature of the temperature response of surface magnetization of nanosize magnetic particles. With the application of a magnetic field, strong freezing of surface spins occur at the defect sites [having strong pinning strength $\left(k\right)$ of spins] of disordered grains surface as a consequence of competitive interactions between the grain boundary pinning strength $\left(k\right)$ and the magnetic field. Thermal energy $\left(k_{\mathrm{B}}T\right)$, 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.