Topological Magnetism Group

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.

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.