| 摘要: | 利用固態反應法成功製備出高純度且沒有雜項的錳碲氧化物，並利用鈷原子取代部份的錳原子而製備出樣品Mn3-xCoxTeO6 (x = 0, 1, 2)。我們利用變溫X光和變溫高解析中子散射實驗來檢視核結構是否有隨溫度改變，以及長程磁有序在樣品x = 1和2中的磁相轉變。結構精算軟體所分析出來的核結構顯示，鈷原子取代部份的錳原子，並不會改變其結構的對稱性，還是維持著六方晶系 的結構，即使環境溫度降至3 K，其結構的對稱性都維持不變。
從中子散射所得到的有序參數以及磁化率隨溫度的量測中，鈷的參雜 (x = 1改變至2)，將使得樣品的反鐵磁相轉變溫度從35 K升高至40 K，並且發現錳/鈷原子在反鐵磁有序溫度下，其原子的位置有發生偏移的情形。Mn3TeO6中的晶格常數熱膨脹行為，可以利用聲子震盪(隨溫度四次方變化)的貢獻來描述，但是在Mn2CoTeO6和MnCo2TeO6的樣品中，其晶格常數的熱膨脹行為則必須用傳導電子(隨溫度二次方變化)的貢獻來描述。從磁化強度對溫度與磁化率對溫度的量測中發現在溫度約為185 K時，有另一個由於鈷磁矩排列所造成的磁相變，並且整個系統的磁疇會隨著外加磁場的增加而改變。
當鈷的含量由x = 1增加至x = 2時，不相稱的磁傳遞向量則由k = [0, 0, 0.481]改變至k = [0, 0, 0.515]。而對整體而言，不相稱的磁傳遞向量由在樣品Mn3TeO6中k = [0, 0, 0.4302]，改變至在樣品MnCo2TeO6中k = [0, 0, 0.515]，不相稱的磁傳遞向量隨著鈷的含量增加，往相稱的磁傳遞向量k = [0, 0, 0.5]靠近。藉由擬和磁結構可以得知，在單一核位置上的錳/鈷自旋，必須設定成分裂的兩組不同的磁矩，才能成功描述此系列材料的磁結構。;High quality polycrystalline samples of Mn3-xCoxTeO6 (x = 0, 1, 2) perovskite were synthesized using the solid-state reaction technique, that a portion of the Mn2+ ions are substituted by the Co2+ ions for samples x = 1, 2. The temperature dependence of X-ray and high resolution neutron diffraction were performed to check the nuclear phase transition and to detect the long-range antiferromagnetic ordering of the samples x = 1, 2. From the results of General Structure Analysis System (GSAS) refinement, the substitution of Co2+ for Mn2+ in Mn3TeO6 will not alters the symmetry of crystal structure (hexagonal, ), even the temperature was cooled down to 3 K.
Order parameters analysis of neutron diffraction and χ-T measurement show that the transition temperature of antiferromagnetic order are changed from 35 to 40 K for x = 1 to 2, and relocations of Mn/Co ions are observed below 35 and 40 K for Mn2CoTeO6 and MnCo2TeO6, respectively. The thermal variation of lattice constants of Mn3TeO6 can be described by T4 phonon term, but that of Mn2CoTeO6 and of MnCo2TeO6 can be described by T2 conduction electron term. From magnetization and susceptibility measurements, another magnetic transition is revealed for Mn2CoTeO6 and for MnCo2TeO6 at the temperature ~ 185 K due to the order of moments of Co2+, and the magnetic domains in the system are changed with the increasing of applied magnetic fields.
The incommensurate propagation vectors are shifted from k = [0, 0, 0.481] to [0, 0, 0.515] as the concentration of Co2+ increased from x = 1 to 2, and it changes from k = [0, 0, 0.4302] in Mn3TeO6 to k = [0, 0, 0.515] in MnCo2TeO6, that a reduction of k with increased concentration of Co is toward the commensurate propagation vector k = [0, 0, 0.5]. The unique Mn/Co spin is split into two magnetically different orbits through the refinement of magnetic structure. |
