多晶La_(0.7)Sr_(0.3)MnO_3的低温输运性质和磁电阻效应

Transport properties and magnetoresistance effect of polycrystalline La _( 0.7) Sr_(0.3) MnO_3 at low temperatures

详细研究了由纳米晶粒组成的块体多晶La0 7Sr0 3MnO3(LSM)的电阻率和磁电阻效应 ,以及它们的温度依赖性 .随着温度从室温降低 ,电阻率 (ρ)在 2 5 0K附近存在一最大值 ,低于该温度后 ,样品表现为金属导电特性 ,随后在 5 0K附近存在一极小值 .也就是说在低于 5 0K的温度范围内 ,随着温度降低 ρ反而升高 ,表现为绝缘体性的导电特性 .经研究发现 ,这种随温度降低 ρ反而增加的现象与隧穿效应的理论模型 (lnρ∝T- 1 2 )符合得很好 .这种现象是由于传导电子在通过邻近LSM晶粒间表面 界面层时的隧道效应所导致的 .这种低温下电阻率的极小值现象来源于隧穿效应和LSM晶粒本征的金属导电特性的相互竞争 .还详细研究了相应的隧道磁电阻效应的温度依赖性 .

The resistivity and magnetoresistance of bulk polycrystalline La 0 7 Sr 0 3 MnO 3(LSM), deliberately fabricated to produce nanoscale grains, has been investigated as functions of temperature. With decreasing temperature below room temperature the resistivity ( ρ ) exhibits a maximum near 250K, below which it displays a “metallic' behaviour. However, ρ exhibits subsequently a minimum near 50K, below which it is well fit by the predictions for the tunneling of conduction electrons through insulating interfacial layers viz. ln ρ ∝ T -1/2 , a result interpreted as conduction-electron tunneling between adjacent LSM granules. The minimum of ρ occurs in this particular sample may result from the competition of the tunneling effect and the intrinsic transport mechanism of LSM. The temperature dependence of the associated tunneling magnetoresistance is also reported in this low temperature regime.