Search for novel characteristics of triangular lattice antiferromagnets exhibiting dielectric anomaly

作为这项拨款的一项研究计划，作者提出了一项研究，旨在阐明结构相变引起的自旋受挫的减少对六方三角晶格ABX_3反铁磁体的磁相变和临界现象的影响。在本研究期间，对具有KNiCl3型晶格形变的体系的磁性、介电性质、光学性质和比热性质进行了一系列测量。产生这些扭曲晶格的原型晶体是CsNiCl3型晶格，空间群为P6_3/mm。因此，发现了与介电和磁性有关的新现象。例如，它们是相当复杂的连续结构相变，反铁磁有序和铁电有序共存，以及与磁性相变相关的介电异常。特别是，KNiCl3体系的介电性质可以归类为…将更多的IED分成三组。KNiCl3、RbMnBr3和TlFeCl3具有铁电性的特点，在一定的温度范围内，介电常数随温度的变化在磁相变点处没有异常。第二类是RbFeBr3，在低温下铁电性和反铁磁性共存。第三类是RbVBr3和RbCoBr3，其介电常数随温度的变化在磁连续相变点附近出现反常峰。RbCoBr3在37K同时具有铁电和弱铁磁序的发现似乎是最具挑衅性的。此外，垂直于Ising对称轴的弱磁矩的出现相当令人惊讶，因为到目前为止，已知六方Acox_3型系统中的Co^&lt；2+&gt；离子具有类Ising磁各向异性，并且横向分量与类Ising长程有序的建立无关。这种现象可以归因于在KNiCl3型晶格固有的晶体对称性所允许的六方c轴上，最近邻的Co^和lt；2+和gt；离子之间的Dzyaloshinskii-Moriya相互作用使XY组分倾斜。在此基础上，我们将测量外加磁场下的介电常数或电场下的磁化率，以一致地理解这些新颖而复杂的现象。较少

As a research program of this grant the present authors proposed a study of clarifying how the reduction of spin frustration produced by structural phase transitions influences on magnetic phase transitions and critical phenomena of hexagonal triangular lattice ABX_3 antiferromagnets. For the term of this investigation, couples of measurements have been carried out on magnetic, dielectric, optical and specific heat properties for systems with the KNiCl_3 type lattice deformation. The prototype crystal from which those distorted lattice are derived is the CsNiCl_3 type lattice with the space group P6_3/mmc.As a result, novel phenomena concerning both to dielectric and to magnetic properties have been found. For instance, they are rather complex successive structural phase transitions, coexistence of antiferromagnetic and ferrolectric orderings, and dielectric anomaly in association with magnetic phase transitions.In particular, dielectric properties of the KNiCl_3 systems can be classif … More ied into three groups. One to which KNiCl_3, RbMnBr_3 and TlFeCl_3 belong has a feature that ferroelectricity is observed in some temperature range and the temperature dependence of dielectric constant shows no anomaly at magnetic transition points. The second to which RbFeBr_3 belong has that ferroelectricity and antiferromagnetism coexists in low temperatures. The third to which RbVBr_3 and RbCoBr_3 belong has that the temperature dependence of dielectric constant displays anomalous peaks in the vicinity of magnetic successive phase transition points. A finding that RbCoBr_3 sets at 37K in ferroelectric and weak ferromagnetic orders simultaneously seems to be most provocative. Furthermore the appearance of weak magnetic moments perpendicular to Ising symmetry axis is quite surprising, because Co^<2+> ions in hexagonal ACoX_3 type systems have been so far known to have Ising-like magnetic anisotropy and the transverse components are irrelevant to establishing of Ising-like long range order. This phenomenon can be attributed to the canting of xy components through the Dzyaloshinskii-Moriya interaction between the nearest neighbor Co^<2+> ions along the hexagonal c axis allowed by the crystal symmetry intrinsic to the KNiCl_3 type crystal lattice.Based on the present results, a measurement of dielectric constant under an external magnetic field or that of magnetic susceptibility under an electric field will be performed for understanding those novel and complex phenomena consistently. Less