Myun J. Cheon

Above-room-temperature ferromagnetism in GaSb/Mn digital alloys

Digital alloys of GaSb/Mn have been fabricated by molecular-beam epitaxy. Transmission electron micrographs showed good crystal quality with individual Mn-containing layers well resolved, no evidence of three-dimensional MnSb precipitates was seen in as-grown samples. All samples studied exhibited ferromagnetism with temperature-dependent hysteresis loops in the magnetization accompanied by metallic p-type conductivity with a strong anomalous Hall effect (AHE) up to 400 K (limited by the experimental setup). The anomalous Hall effect shows hysteresis loops at low temperatures and above room temperature very similar to those seen in the magnetization. The strong AHE with hysteresis indicates that the holes interact with the Mn spins above room temperature. All samples are metallic, which is important for spintronics applications.

Variations of long- and short-range-order structural and magnetic properties of thermally annealed Mn/GaAs digital alloys

The crystal structure and local environment surrounding Mn atoms in Mn/GaAs digital layers thermally annealed at different temperatures have been investigated using x-ray diffraction and extended x-ray absorption fine structure methods. As the annealing temperature is increased, a satellite peak near the GaAs (004) diffraction line systematically shifts towards higher angles, indicating an apparent decrease of lattice parameters in the Mn/GaAs layers. When the annealing temperature is increased to 550 °C the satellite peak position moves dramatically from below to above the GaAs (004) peak, accompanied by a corresponding increase of Mn–As bond length from 2.48 to 2.56 A, suggesting that the local structure around Mn changes from Ga substitution in GaAs to that of MnAs-like phase. Variations of the long- and short-range-order structures are believed to be related to the observed changes of magnetic properties including the disappearance of ferromagnetism in the sample annealed at 550 °C.

Transition from island formation to pseudomorphic growth in the submonolayer CdSe/ZnSe multilayer system

Transmission electron microscopy and photoluminescence measurements have been performed on samples with submonolayer (0.3 up to 0.9 ML) CdSe insertions in ZnSe in both single- and multilayer geometries. We observe a significant difference in Cd distribution in the layer plane between these two geometries for the same Cd coverage. While Cd-rich islands can be seen for a single layer of Cd, a layered growth is achieved in the multilayer samples. This effect is attributed to the reduction of lattice mismatch in the multilayer samples due to a high level of diffusion of Cd atoms along the growth direction.