Sarkar Tapati, Kamalakar M Venkata, Raychaudhuri A K
DST Unit for Nanosciences, Department of Material Science, S. N. Bose National Center for Basic Sciences, Salt Lake Sector-III, Kolkata 700098, India.
J Nanosci Nanotechnol. 2009 Sep;9(9):5315-22. doi: 10.1166/jnn.2009.1140.
In this paper we report transport properties at low temperatures (3 K < or = T < or = 300 K) in nanostructured functional oxides. Electrical resistivity in the nanoparticles of La0.5CoO3 (LSCO), a ferromagnet, show a gradual change-over from a completely metallic state (in the bulk sample) to a completely insulating state (in the sample with the smallest particle size of approximately 35 nm), while still remaining a ferromagnet, albeit with a lower Tc. In the LSCO nanoparticles (diameter <60 nm) there is a change-over in the temperature dependence of the resistivity at the lowest temperature (T <10 K) which we could identify as arising from the Coulomb blockade in the conducting grains which are separated by insulating grain boundaries that allow transport by tunneling. However, nanoparticles of the nonmagnetic material, LaNiO3 (LNO) remain metallic (similar to the bulk sample) throughout the temperature range studied with the resistivity reaching a temperature independent value at the lowest temperature measured. We suggest that the essential difference between the electrical transport in these nanostructured materials arises due to the physical nature of the grain boundaries which is insulating in LSCO due to absence of long range spin order that is needed for the metallic state.
在本文中，我们报道了纳米结构功能氧化物在低温（3K≤T≤300K）下的输运性质。铁磁体La0.5CoO3（LSCO）纳米颗粒中的电阻率显示出从完全金属态（在块状样品中）到完全绝缘态（在最小粒径约为35nm的样品中）的逐渐转变，同时仍保持铁磁体状态，尽管居里温度较低。在LSCO纳米颗粒（直径<60nm）中，在最低温度（T<10K）下电阻率的温度依赖性发生了转变，我们可以确定这是由于被绝缘晶界分隔的导电晶粒中的库仑阻塞引起的，绝缘晶界允许通过隧穿进行输运。然而，非磁性材料LaNiO3（LNO）的纳米颗粒在整个研究温度范围内都保持金属态（类似于块状样品），在测量的最低温度下电阻率达到与温度无关的值。我们认为，这些纳米结构材料中电输运的本质差异是由于晶界的物理性质引起的，在LSCO中，由于金属态所需的长程自旋序不存在，晶界是绝缘的。
