Study of magnetic and electronic phase diagrams of novel Kondo Insulators, Heavy Fermions and complex oxides

具有双钙钛矿结构的过渡金属氧化物A_2B’B”O_6通常具有优异的磁性和电子性质。La 2-xCaxCoIrO 6和La 2-xAxCoMnO 6（A=Ca，Sr或Ba）体系中Co，Mn和Ir的电子价态与组成有关，形成复杂的磁性和电子相图。一个特殊的特性是强烈的所谓自发的交换偏差行为。磁滞曲线强烈不对称的原因被怀疑是铁磁和反铁磁以及自旋玻璃体积的共存。我们首次使用μ子自旋旋转（µSR）作为原子水平的局部探针对这些氧化物的磁性进行了研究，结果表明磁性相图比宏观磁化测量所预期的要复杂。特别是发现的电子自旋动力学的作用仍然存在，即使在低温和自旋玻璃体的体积分数需要澄清。我们计划使用µSR研究磁相图。电子性质将通过XPS、XANES和XMCD（与巴西小组合作）进行研究。我们的目标是更好地理解这些材料中的交换偏置效应，目的是设计具有改进性能的新材料。在该项目的第二部分中，研究了组成为Sr 2CuTe 1-xWxO 6（x在0和1之间）的双钙钛矿。这些材料在CuII与Spin 1/2之间具有强的准二维磁耦合。对于x=0和1 µSR，中子散射发现了长程反铁磁有序，理论上可以通过与下一个和下一个最近的Cu的磁耦合来很好地描述。对于x在0.5左右的固溶体，人们预期由于磁耦合和无序的挫败而抑制磁有序。我们对这个系统的第一次µSR测量已经证实了这一点（在至少0.25 K的温度下没有观察到磁序），人们可能会怀疑，对于x=0.5的混合物，量子自旋液体似乎是基态。我们计划跟踪自旋动力学的发展到最低温度（低于100 mK），研究各种成分x的磁相图，并将我们的结果与磁耦合的理论模型计算进行比较。

Transition metal oxides A2B’B‘‘O6 having double perovskite structure often possess extraordinary magnetic and electronic properties. The systems La2-xCaxCoIrO6 and La2-xAxCoMnO6 (A=Ca,Sr or Ba) show composition dependent electronic valency states of Co, Mn and Ir causing complex magnetic and electronic phase diagrams. A special peculiarity is a strong so-called spontaneous Exchange Bias behavior. Reason for the strong asymmetry in magnetic hysteresis curves is suspected to be the co-existence of ferro- and antiferromagnetic as well as spin-glassy volumes. Our first investigations of the magnetic properties of these oxides using muon spin rotation (µSR) as local probe at atomic level have revealed that magnetic phase diagrams are more complex than anticipated from macroscopic magnetization measurements. Especially the role of the found electronic spin dynamics still present even at low temperatures and of the spin-glassy volume fractions needs to be clarified. We plan to investigate the magnetic phase diagrams using µSR. The electronic properties will be studied by XPS, XANES and XMCD (cooperation with Brazilian groups). The objective is an improved understanding of the Exchange Bias effects in these materials with the aim of designing new materials with improved properties. The synthesis of the materials to be studied will also be done by the Brazilian groups.In a second part of the project there are studied double perovskites with composition Sr2CuTe1-xWxO6 (x between 0 and 1). These materials possess a strong quasi 2-dimensional magnetic coupling between CuII with Spin ½. For x=0 and 1 µSR an neutron scattering find long-range antiferromagnetic order that can be theoretically well described by magnetic coupling to next and next-nearest Cu. For the solid solutions with x around 0.5 one expects suppression of magnetic order due to frustration of magnetic couplings and disorder. Our first µSR measurements on this systems have confirmed this (no magnetic order observed down to at least 0.25 K) and one may suspect that for mixtures around x=0.5 a quantum spin liquid is appears as ground state. We plan to follow the development of spin dynamics down to lowest temperatures (below 100 mK), to investigate the magnetic phase diagrams for the various compositions x and to compare our results with theoretical model calculations for the magnetic coupling.