自旋轨道耦合钒氧化物的物性和相变研究

发布时间：2017-12-27 11:12

本文关键词：自旋轨道耦合钒氧化物的物性和相变研究　出处：《武汉大学》2016年博士论文　论文类型：学位论文

【摘要】：相变问题一直是凝聚态物理学中一个古老且十分重要的课题。近几十年来,过渡金属氧化物因在相变过程中的有序行为而受到广泛的研究关注。其中,具有尖晶石结构的钒氧化物AV204中相变过程往往会伴随着典型的轨道有序和自旋有序,同时其几何阻挫特性和自旋-轨道耦合作用也会带来丰富的物理现象。本文中,我们通过对钒氧化物AV204以及V203物性和相变的研究和比较,来进一步加深对过渡金属氧化物相变过程中的有序行为和相关物理现象的了解。主要内容包括：1.利用固相反应烧结法制备CdV2O4、ZnV2O4和V203的高质量多晶样品,并且通过x射线衍射谱对样品的结构进行表征。通过磁化率、比热测量的结果拟合计算出三种钒氧化物的居里温度、有效磁子数、电子比热系数、声子比热系数和德拜温度,并对这些物理参数进行比较和分析。同时,利用比热曲线的滞后现象确认了CdV2O4和ZnV2O4的结构相变为一级相变而磁相变为二级相变。对于结构相变,给出了相变过程中的剩余比热、焓变、熵变以及克拉伯龙方程。2.给出了CdV2O4和ZnV2O4磁相变的剩余比热。通过比热数据与高斯涨落理论比较得到CdV2O4磁相变的临界区域为0.5K|T-TM|1.5K,而ZnV2O4磁相变的临界区域为|T-TM|0.5K。在临界区域内,通过相变的重整化群理对数据拟合计算得到了临界指数和临界振幅比,进而判断出CdV2O4和ZnV2O4的临界行为均偏离3D-Heisenberg模型而偏向于3D-XY模型。这种奇特的临界行为是因为自旋矢量分量的空间自由度在轨道有序和自旋-轨道耦合的共同作用下被限制。通过金兹堡判据和相关理论估算了临界点附近CdV2O4和ZnV2O4的关联长度。比较发现在临界区域内ZnV2O4的关联长度比CdV2O4的关联长度大1-2个数量级。3.给出了ZnV2O4、CdV2O4和V203在不同温度下的等温磁化曲线的测量结果,并且计算出了磁熵变随温度的变化关系。对于CdV2O4和ZnV2O4而言,在结构相变温度之上,系统的磁有序来自于外场下的顺磁性。当温度靠近结构相变点,以及在结构相变和磁相变之间时,磁有序来自外场下系统顺磁性与非共线孤立自旋链的竞争。在磁相变温度以下,磁有序是来自于外场下杂质缺陷的顺磁贡献、非共线孤立自旋链以及共线反铁磁序的竞争。与CdV2O4相比,ZnV2O4中非共线的孤立自旋链在结构相变附近对磁有序结构影响较大,且ZnV2O4中杂质和缺陷在低温下的顺磁性响应对磁有序结构影响较大。而对于V203,在相变点以上的磁有序是来自于外场下系统的顺磁性；相变点以下至90K的磁有序主要来自于系统的反铁磁；90K以下的磁有序主要来自于杂质和缺陷在低温下的顺磁性。另一方面,对相变区域利用Arrott曲线的性质再次确认了ZnV2O4和CdV2O4结构相变以及磁相变的相变类型。
[Abstract]:The problem of phase transition has always been an old and very important subject in condensed matter physics. In recent decades, transition metal oxides have attracted much attention due to their orderly behavior in the process of phase transition. The phase transition process of vanadium oxide AV204 with spinel structure is often accompanied by typical orbital ordering and spin ordering. Meanwhile, its geometric frustration and spin orbit coupling will also bring rich physical phenomena. In this paper, we have further studied and compared the physical properties and phase transitions of vanadium oxides AV204 and V203, in order to further understand the orderly behavior and related physical phenomena in transition metal oxides. The main contents are as follows: 1.. High quality polycrystalline CdV2O4, ZnV2O4 and V203 samples were prepared by solid-phase reaction sintering, and the structure of the samples was characterized by X ray diffraction. The Curie temperature, effective magnetic field number, electron specific heat coefficient, specific heat coefficient and Debye temperature of three vanadium oxides are calculated by the results of magnetic susceptibility and specific heat measurements. At the same time, the phase transition of CdV2O4 and ZnV2O4 is confirmed by the hysteresis of the specific heat curve, and the phase transition of the magnetic phase is two phase transition. The structural phase transition, residual heat, enthalpy is given during the phase change process, entropy and clapyron equation. 2. the residual specific heat of the magnetic phase transition of CdV2O4 and ZnV2O4 is given. Compared with the Gauss fluctuation theory, the critical region of the CdV2O4 magnetic phase transition is 0.5K|T-TM|1.5K, and the critical region of the ZnV2O4 magnetic phase transition is |T-TM|0.5K. In the critical region, the critical exponent and critical amplitude ratio are obtained through data fitting and calculation by renormalization group theory of phase transformation. Further, it is judged that the critical behavior of CdV2O4 and ZnV2O4 deviates from 3D-Heisenberg model and is biased towards 3D-XY model. This peculiar critical behavior is due to the limitation of the space freedom of the spin vector component under the joint action of orbital order and spin orbit coupling. The Ginzburg criterion and related theory to estimate the correlation length near the critical point CdV2O4 and ZnV2O4. It is found that the correlation length of ZnV2O4 in the critical region is 1-2 orders of magnitude larger than that of CdV2O4. 3. the measurement results of the isothermal magnetization curves of ZnV2O4, CdV2O4 and V203 at different temperatures are given, and the relation between the change of magnetic entropy and the temperature is calculated. For CdV2O4 and ZnV2O4, the magnetic order of the system is derived from the paramagnetic field under the external field on top of the structural phase transition temperature. When the temperature is close to the structural transformation point, and between the structural phase transition and the magnetic phase transition, the magnetic order comes from the competition between the paramagnetic and the non collinear isolated spin chain in the external field. Under the magnetic phase transition temperature, the magnetic order is the paramagnetic contribution from the impurity defects in the external field, the non collinear isolated spin chain and the collinear antiferromagnetic order. Compared with CdV2O4, the non collinear isolated spin chain in ZnV2O4 has great influence on the magnetic ordered structure near the structural phase transition, and the paramagnetic response of impurities and defects at ZnV2O4 at low temperature has great influence on the magnetic ordered structure. For V203, the magnetic ordering above the transition point is derived from the paramagnetism of the system under the external field. The magnetic ordering from the 90K below the transition point mainly comes from the antiferromagnetic system. The magnetic ordering below 90K mainly comes from the paramagnetism of impurities and defects at low temperature. On the other hand, the structure phase transition of ZnV2O4 and CdV2O4 and the type of phase transition of magnetic phase transition are reconfirmed by using the properties of Arrott curve in the phase transition region.
【学位授予单位】：武汉大学
【学位级别】：博士
【学位授予年份】：2016
【分类号】：O469

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