Structural and doping effects in the half-metallic double perovskite<mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"><mml:mrow><mml:msub><mml:mrow><mml:mi>A</mml:mi></mml:mrow><mml:mrow><mml:mn>2</mml:mn></mml:mrow></mml:msub></mml:mrow><mml:mrow><mml:msub><mml:mrow><mml:mi mathvariant="normal">CrWO</mml:mi></mml:mrow><mml:mrow><mml:mn>6</mml:mn></mml:mrow></mml:msub></mml:mrow></mml:math><mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"><mml:mo>(</mml:mo><mml:mi>A</mml:mi><mml:mo>=</mml:mo></mml:math>Sr, Ba, and Ca)

The structural, transport, magnetic, and optical properties of the double perovskite ${A}_{2}{\mathrm{CrWO}}_{6}$ with $A=\mathrm{S}\mathrm{r},\mathrm{B}\mathrm{a},\mathrm{C}\mathrm{a}$ have been studied. By varying the alkaline earth ion on the A site, the influence of steric effects on the Curie temperature ${T}_{C}$ and the saturation magnetization has been determined. A maximum ${T}_{C}=458\mathrm{K}$ was found for ${\mathrm{Sr}}_{2}{\mathrm{CrWO}}_{6}$ having an almost undistorted perovskite structure with a tolerance factor $f\ensuremath{\simeq}1.$ For ${\mathrm{Ca}}_{2}{\mathrm{CrWO}}_{6}$ and ${\mathrm{Ba}}_{2}{\mathrm{CrWO}}_{6}$ structural changes result in a strong reduction of ${T}_{C}.$ Our study strongly suggests that for the double perovskites in general an optimum ${T}_{C}$ is achieved only for $f\ensuremath{\simeq}1,$ that is, for an undistorted perovskite structure. Electron doping in ${\mathrm{Sr}}_{2}{\mathrm{CrWO}}_{6}$ by a partial substitution of ${\mathrm{Sr}}^{2+}$ by ${\mathrm{La}}^{3+}$ was found to reduce both ${T}_{C}$ and the saturation magnetization ${M}_{s}.$ The reduction of ${M}_{s}$ could be attributed both to band structure effects and the Cr/W antisites induced by doping. Band structure calculations for ${\mathrm{Sr}}_{2}{\mathrm{CrWO}}_{6}$ 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 ${A}_{2}{\mathrm{CrWO}}_{6},$ 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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