Giant Magnetoresistance Effect in La-Pb-Mn-O Polycrystalline Bulk Materials

Cubic polycrystalline bulk materials of L-aPb-Mn-O were prepared by solid state reaction method. Magnetic measurements indicated that the samples are ferromagnetic, and TC is 257K. Magnetotransport measurements showed that the metal-semiconductor transition temperature (TP) of the samples is 251K, and near Tp the giant magnetoresistance (GMR) peak vaues reach 72% and 85% under 5T and 13T magnetic field, respectively. It was noticed that the samples present GMR effect over a wide temperature range from 77K to room temperature. The GMR effect is related to the behavior of the spin polaron, which is affected by both temperature and applied field. Besides the influence of the spin polaron, the GMR peak effect near Tp is also ascribed to the strong decline of spin-disorder scattering of carriers caused by the applied field. Furthermore, the GMR effect over the whole measured temperature, especially at low temperatures far below TP, is also resulted from the spin-dependent scattering of carriers near grain boundary associated with the polycrystallinity of the samples. In addition, the inhibition of the spin-polarized intergrain tunneling at TP leads to the disappearance of the low-field GMR effect caused by the Hund exchange.