The structural, transport, magnetic and optical properties of the double perovskite A2CrWO6 with A = Sr, Ba, Ca have been studied. By varying the alkaline earth ion on the A site, the influence of steric effects on the Curie temperature TC and the saturation magnetization has been determined. A maximum TC = 458 K was found for Sr2CrWO6 having an almost undistorted perovskite structure with a tolerance factor f ≃ 1. For Ca2CrWO6 and Ba2CrWO6 structural changes result in a strong reduction of TC. Our study strongly suggests that for the double perovskites in general an optimum TC is achieved only for f ≃ 1, that is, for an undistorted perovskite structure. Electron doping in Sr2CrWO6 by a partial substitution of Sr 2+ by La 3+ was found to reduce both TC and the saturation magnetization Ms. The reduction of Ms could be attributed both to band structure effects and the Cr/W antisites induced by doping. Band structure calculations for Sr2CrWO6 predict an energy gap in the spin-up band, but a finite density of states for the spin-down band. The predictions of the band structure calculation are consistent with our optical measurements. Our experimental results support the presence of a kinetic energy driven mechanism in A2CrWO6, where ferromagnetism is stabilized by a hybridization of states of the nonmagnetic W-site positioned in between the high spin Cr-sites.

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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 ...

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The electrical transport properties of grain boundaries in the epitaxial La 2/3 Ca 1/3 MnO 3−δ thin films have been studied as a function of temperature and applied magnetic field. Below the Curie temperature T C an additional grain boundary resistance, highly non-linear current-voltage curves, and a large magnetoresistive effect in the whole temperature regime below T C are found. The results can be explained consistently by the presence of a disordered, a few nm wide paramagnetic grain boundary layer that is depleted below T C due to an increase of the work function of the ferromagnetic grain material adjacent to this layer. The related band bending and space charge effects are important for the physics of grain boundaries in the manganites.