多晶La-Pb-Mn-O体材料的巨磁电阻效应

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

利用固相反应法制备了多晶立方结构的La－Pb－Mn－O体材料．材料的铁磁相变温度Tc为257K，其金属－半导体转变温度Tp为251K．外加磁场分别为5T和13T时，材料的巨磁电阻（GMR）峰值分别达到72％和85％．在77K～室温的温度范围内，材料都具有GMR效应，Tp附近具有GMR峰值效应．GMR效应与自旋极化子行为有关，而Tp附近的GMR峰值效应除了与自旋极化子行为有关外，与磁场作用下载流子自旋无序散射的急剧下降也有关．另外，多晶材料丰富的晶界结构对材料在整个测量温度范围内，尤其是温度远低于Tp时的GMR效应也有贡献．在Tp处；晶粒间自旋极化隧道效应被抑制，导致低场GMR效应的消失．

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.