本文通过比较La0.7 Ba0.3 MnO3多晶和外延薄膜样品磁输运行为之间的差异,讨论了晶界对多晶样品输运特性的影响,并计算了晶界电阻率随温度和磁场的变化关系.与晶粒的本征电阻率相比,晶界的电阻率要大一个量级,并且其金属一绝缘转变温度低约50K.此外,在晶界中观察到了新奇的正磁阻效应.我们认为,晶界在其转变温度附近出现了相分离现象,并形成铁磁金属渗流通道.由于晶界的特殊构型,Ba掺杂锰氧化物材料中大的磁致伸缩效应对其输运行为影响很大,导致了晶界中正磁阻现象的出现.
In the present study we have measured the resistivity of polycrystalline bulk sample as well as epitaxial film of La0.7Ba0.3MnO3 as a function of temperature and applied magnetic field, and discussed the large discrepancies between them in details. Using an appropriate model, we have calculated the grain boundary (GB) contribution to the total resistivity of La0.7Ba0.3MnO3 bulk sample and obtained the temperature dependence and magnetic depend-ence of grain boundary resistivity. The resistivity of grain boundaries is much larger and its metal-insulator transi-tion temperature is lower than intrinsic resistivity of crystal grains. Moreover, the grain boundary resistivity pres-ents a novel positive magnetoresistance (MR) at temperatures above its metal-insulator transition temperature. We argue that the grain boundary is a phase separation system rather than a homogenous phase, and the giant negative volume magnetostriction (GVM) of the ferromagnetic-metallic clusters at lower magnetic fields leads to the positive MR effect.
参考文献
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