Magnetic cluster glass behavior and grain boundary effect in Nd0.7Ba0.3MnO3 nanoparticles

Abstract

The magnetic and transport properties of Nd0.7Ba0.3MnO3 nanoparticles were explored by transmission electron microscopy, x-ray powder diffraction, resistivity, magnetoresistance, thermopower (S), and magnetic measurements. The metal-insulator transition behavior of the temperature dependence of resistivity for the sample with the largest particle size changes to insulating nature with the decrease in the particle size due to the enhancement of the grain boundary effect. The magnetoresistance of the nanoparticles is analyzed in the light of a phenomenological model based on the spin polarized tunneling at the grain boundaries. The thermopower of the samples shows a crossover from negative to positive values and at high temperatures S follows adiabatic small polaron hopping theory. The zero field cooled and field cooled (FC) magnetizations display broad ferromagnetic transition. The Curie temperature (TC) and the irreversibility temperature (Tirr) decrease considerably with the decrease in the particle size. During cooling the ac susceptibility of the nanoparticles exhibits two magnetic phase transitions with paramagnetic, ferromagnetic, and glassy phases. The frequency dependent peak in the out of phase part (χ″) of the ac susceptibility is the signature of cluster glass behavior. Large thermomagnetic irreversibility, monotonic increase in the FC magnetization, nonsaturation of the magnetization, and the observation of two distinct magnetic transitions in ac susceptibility give evidence for the cluster glass nature of the nanoparticles.