Effects of grain size and grain boundaries on the transport and magnetic properties of charge-ordered Nd0.5Sr0.5MnO3 material

The effects of grain size and grain boundaries on the transport and magnetic properties of Nd0.5Sr0.5MnO3 polycrystalline material have been studied. Polycrystalline Nd0.5Sr0.5MnO3 samples were prepared at different maximum final sintering temperatures Tm ranging from 1100 to 1550 deg. C and characterized by x-ray diffraction and transport and magnetic measurements. All samples were determined to be single phase and crystallized in an orthorhombic structure. The grain size increased with increasing Tm as observed by SEM. Transport and magnetic measurements showed that the charge-ordering temperature Tco decreased as the grain size increased. The increase in resistivity at Tco, that is typically seen in transport measurements as the temperature drops is completely absent in all samples with sintering temperatures less than 1350 deg. C or higher than 1500 deg. C. The very large resistivity increases in small grain size samples for sintering temperatures less than 1300 deg. C and in large grain size samples for sintering temperatures higher than 1500 deg. C are the result of an increase in the grain boundary portion of resistivity. At the same time, the magnetization is at a maximum for very large grain size samples with Tm values of 1400 and 1450 deg. C. In the case of small grain size samples, it is proposed that the negative effects on double exchange of random spins located in the grain boundaries are responsible for the increase in resistivity and the disappearance of features associated with charge ordering in transport measurements. It is also proposed that the lesser magnetization seen in these samples is due to the lack of contribution to the magnetic properties by the random grain boundary spins. (author)