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Spin freezing into a disordered state in CaFeTi_2O_6 synthesized under high pressure

机译:在高压下合成的CaFeTi_2O_6中旋转冻结成无序状态

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摘要

Double perovskites exhibit a variety of interesting phenomena due to numerous combinations of cation orderings and distortions. CaMnTi_2O_6 and CaFeTi_2O_6, are the only two discovered double perovskites that have the columnar type A-site ordering associated with the a+a+c titling system. The former perovskile, synthesized recently, has a polar structure below T_c and shows an unusual ferroelectricity. The latter, while synthesized decades ago, received less investigation. Here, we have synthesized CaFeTi_2O_6, under high pressure and carried out thorough characterizations through measurements of x-ray powder diffraction (XRD), second harmonic generation (SHG), dc and ac magnetization, Moessbauer spectra, resistivity, thermoelectric power, and specific heat. Although the structural refinement of the XRD pattern cannot distinguish two possible tetragonal phases, SHG measurements help to rule out the possibility of a polar structure observed in CaMnTi_2O_6. Fitting the paramagnetic susceptibility to a Curie-Weiss law gives a negligible Weiss constant. The weak exchange interaction gives way to a dipole-dipole interaction and single-ion anisolropy. An anomaly at 1.2 K in the specific-heat is too broad for a magnetic transition, but suggestive of a spin-ice-like state as in the pyrochlores. Mossbauer spectroscopy shows two distinct iron sites, as expected for the crystal structure. Both sites are occupied by high-spin Fe~(2+) as indicated by the isomer shift. The sharply different temperature dependence of the quadrupole splitting at the two iron sites can be well explained based on the local structural distortions from the relinement of XRD patterns. The line broadening associated with long spin-relaxation limes is observed at 6 K as the spin freezing transition is approached from high temperatures.
机译:由于钙离子有序和畸变的多种结合,双钙钛矿表现出各种有趣的现象。 CaMnTi_2O_6和CaFeTi_2O_6是发现的仅有的两种钙钛矿,它们具有与a + a + c滴定系统相关的柱状A位点排序。最近合成的前钙钛矿具有低于T_c的极性结构,并显示出异常的铁电性。后者虽然是数十年前的综合研究,但受到的研究较少。在这里,我们在高压下合成了CaFeTi_2O_6,并通过测量X射线粉末衍射(XRD),二次谐波产生(SHG),直流和交流磁化强度,Moessbauer光谱,电阻率,热电功率和比热进行了全面表征。尽管XRD图案的结构细化无法区分两个可能的四方相,但SHG测量有助于排除在CaMnTi_2O_6中观察到极性结构的可能性。使顺磁磁化率与居里-魏斯定律相适应,得到的魏斯常数可忽略不计。弱交换相互作用让位于偶极-偶极相互作用和单离子苯环松解术。比热在1.2 K处的异常对于磁跃变而言太宽泛,但暗示着像烧绿石中的自旋冰状状态。莫斯鲍尔光谱法显示出两个不同的铁位点,正如晶体结构所预期的那样。如异构体位移所示,两个位点均被高自旋Fe〜(2+)占据。基于X射线衍射图描绘的局部结构变形,可以很好地解释两个铁位点处的四极分裂的温度依赖性明显不同。当从高温开始接近自旋冻结转变时,在6 K时观察到与长自旋弛豫石灰有关的线展宽。

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  • 来源
    《Physical review》 |2018年第6期|064201.1-064201.8|共8页
  • 作者单位

    Materials Science and Engineering Program, Department of Mechanical Engineering, University of Texas at Austin, Austin, Texas 78712, USA;

    School of Physics and Astronomy, Tel Aviv University, 69978 Tel Aviv, Israel;

    Materials Science and Technology Division, Oak Ridge National Laboratory, Oak Ridge, Tennessee, 37831, USA;

    Department of Physics, University of Texas at Austin, Austin, Texas 78712, USA;

    Department of Physics, University of Texas at Austin, Austin, Texas 78712, USA;

    Rudolf Peierls Centre for Theoretical Physics, University of Oxford, Oxford Oxl 3NP, United Kingdom;

    Materials Science and Engineering Program, Department of Mechanical Engineering, University of Texas at Austin, Austin, Texas 78712, USA;

    Materials Science and Engineering Program, Department of Mechanical Engineering, University of Texas at Austin, Austin, Texas 78712, USA;

    Beijing National Laboratory for Condensed Matter Physics and Institute of Physics, Chinese Academy of Sciences, Beijing 100190, China;

    Beijing National Laboratory for Condensed Matter Physics and Institute of Physics, Chinese Academy of Sciences, Beijing 100190, China;

    Materials Science and Engineering Program, Department of Mechanical Engineering, University of Texas at Austin, Austin, Texas 78712, USA;

    Materials Science and Engineering Program, Department of Mechanical Engineering, University of Texas at Austin, Austin, Texas 78712, USA;

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