Thin film quantum materials

Abstract:

The catalytic growth by chemical vapor deposition is a well-established route to single-wall carbon nanotubes (SWNTs). In this process, the choice and preparation of the metal catalyst determines the nanotube growth. The system Fe/Mo is known to yield a large percentage of SWNTs. In order to make use of SWNTs in electronic or electromechanical devices, the patterned growth relies on lithography techniques like photolithography or electron beam lithography. Many standard lithographic processes, i.e. the combination of photoresist and lift-off procedure, are not compatible with Fe/Mo catalyst solutions, resulting in low SWNT yield. We present a systematic study of the influence of the catalyst solvent on the patterned SWNT growth. Most remarkably, the use of water as a solvent is the basis for integrating SWNT with the established processing techniques due to its compatibility with any lithographic process and the simultaneous high SWNT yield. © 2007 Elsevier B.V. All rights reserved.

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.

Abstract:

We present a systematic study of the micromagnetic properties of MnAs deposited by molecular-beam epitaxy on GaAs(001) and GaAs(111)B surfaces. In epitaxial MnAs films, the strain state in MnAs-on-GaAs(001) (anisotropic) and MnAs-on-GaAs(111)B (isotropic) has a strong influence on the magneto-structural phase transition and thus the micromagnetic properties. The ferromagnetic α and the β phase coexist over a wide temperature range exhibiting self-organized, magnetically coupled nanostructures. Independent of the substrate orientation, magnetic flux-closure domain patterns are formed in the basal plane of MnAs. The spatial distribution of the phases in equilibrium (stripes and quasi-hexagonal islands, respectively) stabilizes various magnetic states, which were found experimentally and confirmed by micromagnetic simulations. © 2007 WILEY-VCH Verlag GmbH & Co. KGaA.

Abstract:

Hybrid carbon nanotube-semiconductor systems offer unique properties by combining the advantages of one-dimensional conductors with the broad opportunities of semiconductor technology. Thus, it is desirable to incorporate the nanotube growth in III-V semiconductor systems. We present the directed growth of carbon nanotubes from prepatterned CrNi catalyst structures on GaAs. © 2007 Elsevier B.V. All rights reserved.

Abstract:

Magnetic logic gates based on magnetostatically coupled ferromagnetic structures show a great promise for future information processing. The absolute size and shape of the elementary structures determine their intrinsic and interaction properties and thus have to be chosen appropriately in order to allow for binary logic operations. Here, we present a systematic study of the switching behavior, stray field, and coupling of Permalloy nanostructures as a function of geometrical parameters using micromagnetic simulations. We show that bow-tie shaped elements are promising for magnetic logic gates. © 2007 American Institute of Physics.