Thin film quantum materials

Abstract:

The magnetic microstructure of single-crystalline MnAs films on GaAs(001) has been investigated. Magnetic force microscopy (MFM) reveals a three-dimensional magnetization pattern that is in disagreement with the simple domain picture observed by surface-sensitive magnetic imaging. Here, we present a consistent micromagnetic picture of MnAs thin films in the ferromagnetic stripe phase, which appears in the course of the phase transition. A number of equilibrium magnetization patterns of the stripes are found that are, in fact, based on flux-closure domain patterns in the basal plane of MnAs. The simulation of a stripe array yields excellent agreement with the measured surface magnetization. The experimentally observed stray field contrast was confirmed by MFM contrast simulations based on these equilibrium magnetization patterns. © 2007 American Institute of Physics.

Abstract:

We report the growth of single-walled carbon nanotubes (SWNTs) from silica aerogel by methane chemical vapor deposition (CVD). Bimetallic Fe/Mo supported by amorphous silica forms nanostructures that are catalytically active at the growth temperature. Despite the flexibility gained by using silica as a support matrix, the as-grown nanotube samples need to be further purified for applications in nanodevices. Our experiments show that the treatment in hydrofluoric acid selectively removes the silica matrix,while the amorphous carbon deposits can be selectively removed by oxidation. The influence of the oxidation and etching parameters on the SWNT yield is discussed. The optimized purification procedure renders SWNTs only. © 2006 Elsevier B.V. All rights reserved.

Abstract:

We have investigated the influence of anisotropic exchange on the micromagnetic domain structure. Three-dimensional simulations based on the Landau-Lifshitz-Gilbert equation were performed incorporating a generalized tensor representation of the exchange following a phenomenological approach. In comparison to isotropic exchange, which is usually used in micromagnetic simulations, anisotropic exchange significantly affected the equilibrium distribution of the magnetization. The formation of slanted domain walls aligning in the direction of stiffest exchange and the deformation of edge domains were the most prominent consequences. In general, we found that anisotropic exchange may have profound effects on magnetic nanostructures. © 2007 WILEY-VCH Verlag GmbH & Co. KGaA.

Abstract:

At the basis of future applications of spin electronics are ferromagnetic films that have a Curie temperature above room temperature, a crystal structure that allows for epitaxial growth on common semiconductor surfaces, and a high degree of spin polarization at the Fermi level. A class of ternary compounds, the so-called Heusler alloys, combine these requirements as they are lattice-matched to many compound semiconductors, have a compatible crystal structure (face-centered cubic), and show high Curie temperatures. Moreover, calculations suggested that some Heusler alloys may belong to the magnetic class of half-metals that is characterized by a 100% spin polarization at the Fermi level. We review the work on epitaxial-Heusler alloy films on semiconductor surfaces. Special emphasis is laid on molecular-beam epitaxy (MBE), as this growth method allows for an in situ control of the growth and structure of the material. Taking Co2FeSi on GaAs as an example, the structural and magnetic properties of MBE-grown samples will be discussed.