Ferroelectrics of charge ordering type and its non-linear response in triangular mixed valence iron oxide

本文研究了三角形混合价态材料RFe2O4，该材料在受挫电荷相互作用中表现出极性电荷有序。在这两年中，我们已经建立了一个气密炉，样品重量测量系统。根据不同热处理过程中试样重量变化的详细信息，提出了制备RFe2O4细试样的几种方法。在研究过程中，我们发现了YbFe2O4， RFeCoO4， rfeco4和RGaCuO4 （r = Yb, Lu）的各种电子相和性质。近年来，由于电子的极性有序而产生的铁电性引起了人们的广泛关注，但在铁电材料上的实际应用仍然是一个悬而未决的问题。例如，相关电子系统中可能存在的高电子导电性可能会阻碍铁电体如绝缘体、电荷电容器等的通常应用。这篇报道是一个可能的搜索…更多这样的搜索。例如在某些样品处理条件下可能实现高电阻率状态的电子铁电体。有趣的是，具有较少氧空位的样品比非化学计量材料具有更高的活化能。解释了Rfe_2O_4介电色散的来源是极性畴的边界运动。高化学计量量样品的Q值越高，说明空氧越少样品的电荷序相干长度越大。实现了电子集体运动的层叠尺度。我们强调，这种由电子分布定义的铁电性与以前已知的铁电性有着至关重要的不同。铁电性起源的差异可能使在不久的将来开发一类新的器件应用成为可能。例如，在电子显微镜观察中存在异常报告，显示了一个极小的铁电畴，其相干尺寸为10 nm。这是x射线衍射实验得出的一致估计。超晶格轮廓比基本布拉格点宽。利用Schemer公式，从a-b平面峰剖面的半最大值的全宽度计算出相干长度约为10 nm。在这个铁电体中，Fe^<3+>和Fe^<2+>之间的库仑相互作用在畴尺寸的存在下主导了总能量，这与晶格畸变解耦。这表明了有序电子的铁电性的典型性质，没有晶格畸变。未来的研究可能会揭示这样一个极小的铁电元件在极小域的高密度电器件中。电子铁电材料在常规铁电体中的适用性仍然是一个悬而未决的问题。我们应该继续寻找更好的方法来实现高化学计量量的RFe_2O_4，或者寻找更多适合这种具有电极化和良好导电性的材料的特性应用。少

We made a research for triangular mixed valence material RFe2O4 which exhibits a polar charge ordering from the frustrated charge interaction. In this two years we had built up an air tight furnace having sample weight measurement system. From the detailed information of the sample weight change during the various heat treatment, we developed couple of method in obtaining fine sample of RFe2O4. During the course of the research we found the various electronic phases and properties for YbFe2O4, RFeCoO4, RFeCuO4 and RGaCuO4 (R.= Yb, Lu)Recently the ferroelectricity arise from the polar ordering of electrons have got much attention, however the usability for the actual application on the ferroelectric material have still been an open question. For example high electronic conductivity that possibly exist in the correlated electron system may prevent the usual application for the ferroelectrics like as insulator, charge condenser and so on. This report is one of the possibility search for s … More uch quests. For example in some sample treatment condition can possibly realize a high resistivity states of electronic ferroelectrics. It is also interesting that the sample having less oxygen vacancy shows high activation energy than non-stoichiometric material. It has been explained that the origin of the dielectric dispersion for Rfe_2O_4 is the boundary motion for polar domains. The higher Q value for higher stoichiometric sample indicates that the less oxygen vacant sample has larger charge ordering coherence length. Which realizes lamer scale of the collective electron motion.We emphasize that such ferroelectricity, defined by the electron distribution, is crucially different from previously known ferroelectrics. The difference in the origin of ferroelectricity may enable the development of a new class of device application in the near future. For example an anomaly report exists in electron microscope observation, which revealed an extremely small ferroelectric domain having a coherent size of 10 nm scale. This is a consistent estimate from the X-ray diffraction experiments. The superlattice profile is wider than the fundamental Bragg spot. Using the Schemer's formula the coherence length was estimated as about 10 nm from the full width of the half maximum of the peak profile in the a-b plane. In this ferroelectric the Coulomb interactions between Fe^<3+> and Fe^<2+> dominate the total energy for the presence of the domain sizes, that is decoupled from the lattice distortion. This indicates a typical property of the ferroelectricity by ordered electrons, free of lattice distortion. Future study may reveal such an extremely small ferroelectric element in high density electric devices in such extremely small domains.The applicability of electronic ferroelectric material for conventional ferroelectrics is still an open question. We should continue to find out better method to realize high stoichiometric RFe_2O_4, or also we should find out more characteristic application that may suit for such material having electric polarization and good conductivity. Less