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.

We report first quantitative measurements by energy-selected imaging in a transmission electron microscope of In segregation within an uncapped islanded In0.25Ga0.75As layer grown epitaxially on GaAs. This layer has the lowest In concentration at which islanding occurs and, then, only after a flat approximately 3 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(In) approximately 0.6 at the apex) and a simultaneous In depletion in the remaining flat layer.

The atomic structure and the chemistry of basal-plane inversion boundaries in Sb 2 O 3 -doped ZnO were investigated using quantitative transmission electron microscopy techniques. Electron microdiffraction and high-resolution transmission electron microscopy were used to determine the orientation of the polar c-axis on both sides of the inversion boundary and the translation state between the inverted ZnO domains. Quantitative energy-dispersive X-ray spectroscopy combined with high-resolution transmission electron microscopy allowed us to determine the exact amount and the arrangement of antimony in the boundary layer. Inversion boundaries are head-to-head oriented with a displacement vector of the oxygen sublattice of R IB ‫؍‬ 1 ⁄3[011 0] -0.102[0001]. The boundary plane consists of a highly ordered SbZn 2 monolayer in which the cations occupy the octahedral interstices of the structure. In the octahedral boundary layer, zinc and antimony atoms constitute a honeycomb superstructure with a threefold (3m) in-plane symmetry.

No abstract

No abstract