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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 ...

Using a sol-gel method we have prepared monodispersive particles of La0.67Ca0.33MnO3 in the range from 20 to 110 nm at temperatures from 540 to 1000 °C. A magnetoresistance above 10% was obtained in a field of 1 kOe for all the particles sizes. These results may be relevant for future applications.

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Magnetic dc susceptibility (χ) and electron spin resonance (ESR) measurements in the paramagnetic regime, are presented. We found a Curie-Weiss (CW) behavior for χ(T) with a ferromagnetic Θ = 446(5) K and µ ef f = 4.72(9)µ B /f.u., this being lower than that expected for either F e 3+ (5.9µ B ) or F e 2+ (4.9µ B ) ions. The ESR g-factor g = 2.01(2), is associated with F e 3+ .We obtained an excellent description of the experiments in terms of two interacting sublattices: the localized F e 3+ (3d 5 ) cores and the delocalized electrons. The coupled equations were solved in a mean-field approximation, 1 assuming for the itinerant electrons a bare susceptibility independent on T .We obtained χ 0 e = 3.7 10 −4 emu/mol. We show that the reduction of µ ef f for F e 3+ arises from the strong antiferromagnetic (AFM) interaction between the two sublattices. At variance with classical ferrimagnets, we found that Θ is ferromagnetic. Within the same model, we show that the ESR spectrum can be described by Bloch-Hasegawa type equations. Bottleneck is evidenced by the absence of a g-shift. Surprisingly, as observed in CMR manganites, no narrowing effects of the ESR linewidth is detected in spite of the presence of the strong magnetic coupling. These results provide evidence that the magnetic order in Sr 2 F eM oO 6 does not originates in superexchange interactions, but from a novel mechanism recently proposed for double perovskites.PACS: 75.10. 76.60.Es, 75.30.Vn