Lal／3Ndl／3Bal／aMn03的电阻率双峰和低温电阻率反常现象

用固相反应法制备了二元稀土掺杂的La1／3Ndl／3Ba1/3Mn03多晶庞磁电阻块材．x射线衍射分析表明在1200℃烧结的样品具有立方对称的晶格结构．热磁曲线显示材料在低温处于自旋冰状态．自旋冰的熔点随着外磁场的增大而降低．在磁场达到40kOe时，自旋冻结现象消失．材料的居里温度为-250K，但室温磁化曲线中出现由微弱不均匀品格畸变造成的寄生铁磁性行为．电阻率一温度曲线出现双峰现象．在居里温度附近出现的电阻率峰由晶体的本征特性贡献，在低于居里温度的～190K附近出现的电阻率峰由界面隧穿效应贡献．电阻率在～40K附近出现极小值．经过曲线拟合证实，其行为与近藤效应相似．研究结果较为全面地分析了单相多晶CMR材料的基本磁电相关特性．

Double metal-insulator transitions and low-temperature resistivity minimum in La1/3Ndl/3Bal/3MnO3 manganite

La1/3Ndl/3Ba1/3MnO3 polycrystalline bulk samples in single phase form with colossal magnetoresistant effect were synthesized in air atmosphere by the conventional solid phase reaction process for ceramic, and their fundamental magnetoelectronic properties were discussed. X-ray diffraction pattem shows that the manganite substituted with two kinds of rare earth element has a cubic perovskite structure, instead of a rhombohedral. The temperature dependence curve of magnetization reveals the existence of spin ice due to the competition of ferromagnetism and antiferromagnetism in the manganite containing ions of Mn^3＋ and Mn^4＋. The melting point of spin ice shifts toward low temperature with the increase of applied field. At H = 40 kOe, the phenomenon of spin freezing disappears. At room temperature, above the Currie temperature （-250 K）, the hysterisis loop shows the behavior of parasitic ferromagnetism likely contributed by the fluctuation of lattice distortion. Double metal to insulator transitions, both strongly field dependent, were observed in the temperature dependence curve of resistivity. The resistivity peak near the Currie temperature is oriented from the intrinsic contribution of Lal/3Ndl/3Bal/3MnO3 crystal, while the other peak at -190 K sensitive to both applied field and sintering time is likely to be related to the tunneling of spin-polarized electrons at grain boundaries. An anomalous minimum of electrical resistivity at -40 K reveals the existence of local magnetic disorders. By a simulation with the traditional least square method, we believe that the anomalous minimum fits the Kondo theory for diluted magnetic alloy.