A key issue in the Solid State Chemistry department is the search for new magnetic materials with a potential for applications, e.g. in spintronics, as rare-earth-free hard magnets, as multiferroics or as magneto-caloric materials. For this purpose double perovskites (DPs) A2BB’O6, which feature an ordered rock-salt like arrangement of corner-sharing BO6 and B’O6 units in the crystal structure (Fig. 1) are a versatile class of compounds. Proper choice of the metal ions B and B’ and of the alkali, alkaline earth or rare earth ions A allows to realize a large variety of physical properties in DPs. Prototype compounds which have found much interest are Sr2FeMoO6 and Sr2FeReO6 as they are half-metallic ferromagnets with Curie temperatures above room temperature and large magnetoresistance effects at ambient conditions. The unique properties of these DPs arise from the combination of an electronically more localized 3d ion at the B with a more delocalized 4d or 5d ion at the B’ site. Another key member in the DP family is Sr2CrOsO6, which is a ferrimagnetic insulator with an unusually high Curie temperature TC of 720 K, which illustrates the potential for finding promising magnets among DPs. Our aims in this project are:
- to elucidate the interplay between crystal structure, electronic structure, magnetism, electrical transport, spin-orbit interactions, and electron-lattice coupling in selected DPs and
- to search for new functional materials in this class of oxides
We have studied extensively the DPs Sr2FeOsO6  and Sr2CoOsO6 , which in contrast to the above ferro- or ferrimagnetic compounds, turned out to be antiferromagnetic insulators. Both compounds show two magnetic phase transitions, namely at TN1 = 140 K, TN2 = 67 K for the FeOs and TN1 = 108 K, TN2 = 67 K for the CoOs compound, respectively. The crystal and spin structures have been determined from temperature dependent powder neutron diffraction experiments. As an example the results for Sr2FeOsO6 are shown in Fig. 2, the properties of which have also been studied by 57Fe Mössbauer spectroscopy. The two antiferromagnetic spin structures formed below TN1 (AF1) and TN2 (AF2), respectively, differ in the spin sequence along the c-direction. Whereas all Fe and Os spins show parallel alignment along c in AF1, an up-up, down-down, etc. sequence is observed in AF2. The magnetic transitions are accompanied by an increase of the tetragonal distortion. A peculiar feature is the step-like evolution of the magnetic moments at the transition metal sites in the magnetically ordered phases. A complex magnetic ordering pattern with two distinct phases AF1 and AF2 and a tetragonal – monoclinic phase transition coinciding with TN1 are found for Sr2CoOsO6. Mössbauer and X-ray absorption studies show that Fe3+ (t2g3eg2) and Os5+ (t2g3) ions are present in Sr2FeOsO6, whereas Co2+ (t2g5eg2) and Os6+ (t2g2) ions occur in Sr2CoOsO6. Our experimental and computational studies on Sr2FeOsO6 and Sr2CoOsO6 provide insights into the electronic structure, the balance of exchange interactions and the coupling to lattice degrees of freedom in these magnetic DPs.
The complex magnetic properties of insulating DPs with several spin structures of comparable energy reflect the large number of partly competing exchange interactions within and between the two interpenetrating fcc-like lattices of transition metal ions. We have studied the structural and magnetic properties as well as the spin structures of DPs Sr2BOsO6 with B = Y, In, where Os5+ (t2g3) ions are the sole magnetic component (Fig. 3) . Thus, only one of the fcc-like sublattices is magnetic and the number of possible exchange interactions is reduced. Both, Sr2YOsO6 and Sr2InOsO6 adopt the same type I antiferromagnetic spin structure as Sr2YRuO6 which has been recently reinvestigated as a model system for magnetic frustration in an fcc-like lattice with antiferromagnetic interactions. Considering in addition Sr2ScOsO6, which adopts the same spin structure, it is found that the trend in the Néel temperatures does not simply correlate with the chemical pressure effects by changing the size of the B ion. Instead, our results indicate that the detailed balance of exchange interactions is different in case of d0 and d10 ions on the B site of the DP structure.