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 $T_C=458\mathrm{}\mathrm{K}$ was found for ${\mathrm{Sr}}_2{\mathrm{CrWO}}_6$ having an almost undistorted perovskite structure with a tolerance factor $f\simeq 1.$ 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 $f\simeq 1,$ 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.

We report first quantitative measurements by energy-selected imaging in a transmission electron microscope of In segregation within an uncapped islanded ${\mathrm{In}}_{0.25}{\mathrm{Ga}}_{0.75}\mathrm{As}$ layer grown epitaxially on GaAs. This layer has the lowest In concentration at which islanding occurs and, then, only after a flat $\sim 3\mathrm{nm}$ alloy layer has been formed. In buildup by segregation at the surface of this initial flat layer is considered the driving force for islanding and, importantly, the segregation process introduces the characteristic delay seen before the Stranski-Krastanow transition. We observe strong inhomogeneous In enrichment within the islands (up to $x\left(\mathrm{In}\right)\approx 0.6$ at the apex) and a simultaneous In depletion in the remaining flat layer.

A detailed examination is presented of the way in which the Stranski-Krastanow epitaxial islanding transition can be controlled by strain due to elemental segregation within the initially-formed flat “wetting” layer. Calculations using a segregation model are shown to accord well with experimentally measured critical wetting-layer thicknesses for the ${\mathrm{In}}_x{\mathrm{Ga}}_{1-x}\mathrm{As}/\mathrm{GaAs}$ system $(x=0.25\mathrm{}–1).$ The strain energy associated with the segregated surface layer is determined for the complete range of deposited In concentrations using atomistic simulations. The segregation-mediated driving force for the Stranski-Krastanow transition is considered also to be important for all other epitaxial systems exhibiting the transition.

The local Ge composition in undulating Si0.8Ge0.2 layers on Si has been studied in a scanning transmission electron microscope using electron energy-loss spectroscopy. We observe Ge enrichment of the SiGe layer near the free surface (vertical Ge segregation in the growth direction) as well as Ge depletion of the ripple troughs compared to the peaks (lateral segregation). These lateral compositional fluctuations are likely to retard the generation of misfit dislocations and might be relevant to the Stranski–Krastanov growth of strained epitaxial alloy layers as well as to the self-organized growth of quantum dot structures.