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

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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.