發(fā)布時(shí)間：2017-12-27 11:12

  本文關(guān)鍵詞：自旋軌道耦合釩氧化物的物性和相變研究　出處：《武漢大學(xué)》2016年博士論文　論文類(lèi)型：學(xué)位論文

  更多相關(guān)文章： 尖晶石結(jié)構(gòu) 自旋-軌道耦合 相變 有序行為 臨界行為

【摘要】：相變問(wèn)題一直是凝聚態(tài)物理學(xué)中一個(gè)古老且十分重要的課題。近幾十年來(lái),過(guò)渡金屬氧化物因在相變過(guò)程中的有序行為而受到廣泛的研究關(guān)注。其中,具有尖晶石結(jié)構(gòu)的釩氧化物AV204中相變過(guò)程往往會(huì)伴隨著典型的軌道有序和自旋有序,同時(shí)其幾何阻挫特性和自旋-軌道耦合作用也會(huì)帶來(lái)豐富的物理現(xiàn)象。本文中,我們通過(guò)對(duì)釩氧化物AV204以及V203物性和相變的研究和比較,來(lái)進(jìn)一步加深對(duì)過(guò)渡金屬氧化物相變過(guò)程中的有序行為和相關(guān)物理現(xiàn)象的了解。主要內(nèi)容包括：1.利用固相反應(yīng)燒結(jié)法制備CdV2O4、ZnV2O4和V203的高質(zhì)量多晶樣品,并且通過(guò)x射線衍射譜對(duì)樣品的結(jié)構(gòu)進(jìn)行表征。通過(guò)磁化率、比熱測(cè)量的結(jié)果擬合計(jì)算出三種釩氧化物的居里溫度、有效磁子數(shù)、電子比熱系數(shù)、聲子比熱系數(shù)和德拜溫度,并對(duì)這些物理參數(shù)進(jìn)行比較和分析。同時(shí),利用比熱曲線的滯后現(xiàn)象確認(rèn)了CdV2O4和ZnV2O4的結(jié)構(gòu)相變?yōu)橐患?jí)相變而磁相變?yōu)槎?jí)相變。對(duì)于結(jié)構(gòu)相變,給出了相變過(guò)程中的剩余比熱、焓變、熵變以及克拉伯龍方程。2.給出了CdV2O4和ZnV2O4磁相變的剩余比熱。通過(guò)比熱數(shù)據(jù)與高斯?jié)q落理論比較得到CdV2O4磁相變的臨界區(qū)域?yàn)?.5K|T-TM|1.5K,而ZnV2O4磁相變的臨界區(qū)域?yàn)閨T-TM|0.5K。在臨界區(qū)域內(nèi),通過(guò)相變的重整化群理對(duì)數(shù)據(jù)擬合計(jì)算得到了臨界指數(shù)和臨界振幅比,進(jìn)而判斷出CdV2O4和ZnV2O4的臨界行為均偏離3D-Heisenberg模型而偏向于3D-XY模型。這種奇特的臨界行為是因?yàn)樽孕噶糠至康目臻g自由度在軌道有序和自旋-軌道耦合的共同作用下被限制。通過(guò)金茲堡判據(jù)和相關(guān)理論估算了臨界點(diǎn)附近CdV2O4和ZnV2O4的關(guān)聯(lián)長(zhǎng)度。比較發(fā)現(xiàn)在臨界區(qū)域內(nèi)ZnV2O4的關(guān)聯(lián)長(zhǎng)度比CdV2O4的關(guān)聯(lián)長(zhǎng)度大1-2個(gè)數(shù)量級(jí)。3.給出了ZnV2O4、CdV2O4和V203在不同溫度下的等溫磁化曲線的測(cè)量結(jié)果,并且計(jì)算出了磁熵變隨溫度的變化關(guān)系。對(duì)于CdV2O4和ZnV2O4而言,在結(jié)構(gòu)相變溫度之上,系統(tǒng)的磁有序來(lái)自于外場(chǎng)下的順磁性。當(dāng)溫度靠近結(jié)構(gòu)相變點(diǎn),以及在結(jié)構(gòu)相變和磁相變之間時(shí),磁有序來(lái)自外場(chǎng)下系統(tǒng)順磁性與非共線孤立自旋鏈的競(jìng)爭(zhēng)。在磁相變溫度以下,磁有序是來(lái)自于外場(chǎng)下雜質(zhì)缺陷的順磁貢獻(xiàn)、非共線孤立自旋鏈以及共線反鐵磁序的競(jìng)爭(zhēng)。與CdV2O4相比,ZnV2O4中非共線的孤立自旋鏈在結(jié)構(gòu)相變附近對(duì)磁有序結(jié)構(gòu)影響較大,且ZnV2O4中雜質(zhì)和缺陷在低溫下的順磁性響應(yīng)對(duì)磁有序結(jié)構(gòu)影響較大。而對(duì)于V203,在相變點(diǎn)以上的磁有序是來(lái)自于外場(chǎng)下系統(tǒng)的順磁性；相變點(diǎn)以下至90K的磁有序主要來(lái)自于系統(tǒng)的反鐵磁；90K以下的磁有序主要來(lái)自于雜質(zhì)和缺陷在低溫下的順磁性。另一方面,對(duì)相變區(qū)域利用Arrott曲線的性質(zhì)再次確認(rèn)了ZnV2O4和CdV2O4結(jié)構(gòu)相變以及磁相變的相變類(lèi)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.
【學(xué)位授予單位】：武漢大學(xué)
【學(xué)位級(jí)別】：博士
【學(xué)位授予年份】：2016
【分類(lèi)號(hào)】：O469

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