In magnetic materials with a lack of inversion symmetry in the crystal structure the weak Dzyaloshinsky–Moriya interaction can lead to helical and conical spin modulations. In B20 systems such as MnSi and Fe1-xCoxSi topologically non-trivial spin structures, which are referred to as skyrmions, novel electronic and magnetic transport phenomena open up an innovative type of spintronic data storage and handling.
Thin films and nanostructures are of increasing interest both for applications and from a fundamental point of view. Using molecular beam epitaxy we grow heterostructures of B20 transition metal silicides and germanides (MnSi, MnGe, FeGe, etc.) as well as superconductors and helical magnets. We are interested in the magnetic and transport properties of low dimensional structures and in proximity effects at the interface between superconductors and magnets. We aim to expand the skyrmion phase to a large region of the magnetic phase diagram and investigate the coupling between skyrmions and Abrikosov vortices.
For spintronic applications nanostructures of the functional materials are mandatory. Therefore, it is necessary to grow epitaxial thin films of high morphological quality and a surface roughness in the order of nanometers. In collaboration with the Physikalisch-Technische Bundesanstalt (PTB) nanostructures are precisely prepared using electron lithography. This allows for transport measurements in well-defined geometry and for investigation of surface and interface effects.
Many transition metal silicides have a crystal structure with a lack of inversion center regarding the magnetic ions. In theses systems the Dzyaloshinskii-Moriya interaction generates a helical spin structure, the chirality of which is coupled to the absolute crystal structure. From small angle neutron scattering and magnetization measurements the magnetic phase diagram of such systems can be determined and, thus, general concepts of chiral symmetries being very common in nature can be investigated. These projects are conducted in collaboration with the St.-Petersburg Nuclear Physics Institute and the Helmholtz Center Geesthacht.
The investigation of the intrinsic properties of materials is often connected to preferably perfect single crystals. Many intermetallic compounds can be synthesized by the tri-arc Czochralski crystal-growth method, in which electric arcs are set off using commercial welding equipment and heat up the melt in a water-cooled copper crucible. The temperature can be controlled via the electric power of the arcs.
In a first step, the starting materials are stoichiometrically weighted out and pre-melted by a single-arc melter resulting in a polycrystalline button of the aimed compound. Afterwards, a single crystal is grown from the melt in the tri-arc Czochralski setup. We produce single-crystalline species of MnSi, Fe1-xCoxSi, Fe1-xMnxSi, but also Co8Zn6+yMn8-y-zFez and perform transport and susceptibility measurements.
The binary compound FeSi has for a long time be regarded as first Kondo insulator, where the local magnetic moments are carried by d-electrons and not, as commonly, by f-electrons. However, using highly-resolved photoemission and optical spectroscopy the investigation of the electronic correlations in this material clearly reveals that FeSi is rather to be described within a band model and that electronic self-energy leads to renormalization of the density of states.