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In the search for new spintronic materials with high spin-polarization at room-temperature, we have synthesized an osmium based double perovskite with a Curie-temperature of 725 K. Our combined experimental results confirm the existence of a sizable induced magnetic moment at the Os site, supported by band-structure calculations in agreement with a proposed kinetic energy driven mechanism of ferrimagnetism in these compounds. The intriguing property of Sr2CrOsO6 is that it is at the endpoint of a metal-insulator transition due to 5d band filling, and at the same time ferrimagnetism and high-spin polarization is preserved. PACS numbers: 61.12.Ld 75.50.Gg 75.50.Pp 75.50.Vv 81.05.ZxA so-called half-metal is a highly desired material for spintronics, as only charge carriers having one of the two possible polarization states contribute to conduction. In the class of the ferrimagnetic double perovskites such half-metals are well known, e. g. Sr 2 FeMoO 6 [1]. The here described compound Sr 2 CrOsO 6 is special, as it has a completely filled 5d t 2g minority spin orbital, while the majority spin channel is still gapped. It is thus at the endpoint of an ideally fully spin-polarized metal-insulator transition. At the metallic side of this transition we have the half-metallic materials Sr 2 CrWO 6 [2] and Sr 2 CrReO 6 [3,4]. Within the unique materials class of double perovskites, therefore, one can find high Curie-temperature ferrimagnets with spin-polarized conductivity ranging over several orders of magnitude from ferrimagnetic metallic to ferrimagnetic insulating tunable by electron doping. Note that Sr 2 CrOsO 6 , where a regular spin polarized 5d band is shifted below the Fermi level, is fundamentally different from a diluted magnetic semiconductor, where spin-polarized charge carriers derive from impurity states.While for simple perovskites as the half-metallic ferromagnetic manganites the Curie-temperature, T C , is in the highest case still close to room-temperature, halfmetallic ferrimagnetic double perovskites can have a considerably higher T C [5]. It has been suggested that ferrimagnetism in the double perovskites is kinetic energy driven [6,7,8]. In short, due to the hybridization of the exchange split 3d-orbitals of Fe 3+ (3d 5 , spin majority orbitals fully occupied) or Cr 3+ (3d 3 , only t 2g are fully occupied), and the non-magnetic 4d/5d-orbitals of * Electronic address: alff@oxide.tu-darmstadt.de Mo, W, Re or Os (N -sites), a kinetic energy gain is only possible for the minority spin carriers. This will lead to a corresponding shift of the bare energy levels at the non-magnetic site, and a strong tendency to half-metallic behavior. This mechanism is operative for the Fe 3+ and Cr 3+ (M sites) compounds [2], where all 3d majority spin states resp. all t 2g majority spin sates are fully occupied and represent localized spins. In agreement with band-structure calculations [1, 2,6,9,10,11] this mechanism is naturally associated with half-metallic behavior, as the spin-polarized conduction electrons mediate ...

The electronic structure of the spintronic material Sr2CrReO6 is studied by means of full-potential linear muffin-tin orbital method. Scalar relativistic calculations predict Sr2CrReO6 to be halfmetallic with a magnetic moment of 1 µB. When spin-orbit coupling is included, the half-metallic gap closes into a pseudo-gap, and an unquenched rhenium orbital moment appears, resulting in a significant increase of the total magnetic moment to 1.28 µB. This moment is significantly larger than the experimental moment of 0.9 µB. A possible explanation of this discrepancy is that the anti-site disorder in Sr2CrReO6 is significantly larger than hitherto assumed.The family of magnetic oxides with an ordered double perovskite structure are complex materials with high technological potential in the area of spin electronics. Double perovskites have the general formula A 2 BB'O 6 , where A can be an alkali metal such as strontium, calcium, or barium, or a lanthanide, and B and B' are transition metals. Each transition metal site is surrounded by an oxygen octahedron (sometimes heavily distorted), and the A atoms are situated in the holes produced by eight adjacent oxygen octahedra.In 1998, it was discovered that one such double perovskite, Sr 2 FeMoO 6 , possesses intrinsic tunneling-type magnetoresistance at room temperature -until then only observed in the mixed-valent manganese oxides 1 -making it a hot candidate material for spin-electronics applications. 2 The physical origin of the magnetoresistance in Sr 2 FeMoO 6 and in the mixed-valent manganese oxides is half-metalicity, i.e., the material is an insulator in one of the spin channels, but a metal in the other. This leads to a complete spin polarization at the Fermi level, which in turn results in strongly spin-dependent scattering of the charge carriers and thus a possibility to influence the resistance using relatively weak magnetic fields. Prerequisites for realizing high-performance devices using these materials are that the half-metalicity to a high degree is preserved at ambient temperature, and that high quality thin films of the material can be grown.In this letter, we investigate the electronic structure of the double perovskite Sr 2 CrReO 6 using density functional theory. This system is particularly interesting since it exhibits the hitherto largest Curie temperature T c of all known double perovskites, 635 K, 3 which is a couple of hundred Kelvin higher or more than for Sr 2 FeMoO 6 4 as well as for the mixed-valent manganese oxides. Sr 2 CrReO 6 is a metallic ferromagnet with a saturation magnetic moment of around 0.9 µ B per formula unit. At room temperature, the moment is only slightly reduced to around 0.8 µ B , and high quality thin films of Sr 2 CrReO 6 can be produced in quite a large temperature window. 3,5 Thus, this material appears to satisfy important technological criteria. The measured saturation moment is quite well reproduced by a simple ionic picture of the Sr 2 CrReO 6 system, although this model takes neither hybridization nor orbital momen...

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