Functional Thin Films

In the framework of the fundamental work in the Ehresmann group, mainly polycrystalline exchange-shifted magnetic thin film systems are investigated, which exhibit a stable remanent magnetization independent of the magnetization history due to the combination of thin antiferromagnetic as well as ferromagnetic materials. The dependencies of the magnetic properties on growth parameters of the thin films as well as thermal and ion bombardment induced processes are investigated. The focus is on understanding the variability of inherent magnetic material constants, the shaping of domain structures and the behavior of the layers during remagnetization processes. Based on the results of these investigations, models for the complete description of the static and dynamic processes in thin magnetic layers can be newly or further developed.

Reaching from magnetism fundamentals to applications the exchange biased thin film systems can be magnetically structured to serve as a source for tailored magnetic stray field landscapes. Here, one- or two-dimensional patterns of remanent magnetic domains can be generated via a treatment called ‘ion bombardment induced magnetic patterning’. Helium ions generated in a Penning ion source are accelerated (usually by an acceleration voltage of 10 kV) and then hit the magnetic sample on chosen positions – either in absence or presence of a magnetic field. Hence, the sample’s magnetization can be locally modified and it is possible to design domain patterns with for example antiparallel magnetization in adjacent domains, which are separated by so-called 180° domain walls. The here emerging magnetic stray fields are then spatially characterized and used in various applications.

This research is focused on the engineering of microstructures as well as particulate systems with defined magnetic properties, mainly through the deposition of thin magnetic layers on top of  objects with different shapes and curvatures. Of special interest is the experimental and theoretical investigation of the interplay between the object’s geometry and magnetic anisotropies (e.g. the unidirectional anisotropy in exchange biased thin films) and their influence on three-dimensional magnetization distributions. Thus structures with defined magnetization states can be fabricated allowing their use in specific applications, e.g. biosensing devices. 

Combining our expertise regarding the tailoring of magnetic stray field landscapes on a microscale and the fabrication of novel classes of magnetic materials and particles we investigate several promising application opportunities. Our main focus lies herein on the development of a Lab-on-a-chip technology platform based on the controllable actuation of magnetic micro- and nanoobjects within the afore mentioned magnetic stray field landscapes by adding a time-dependent external macroscopic magnetic field. As a result a wide variety of interesting processes can be induced: from an active substance mixing in tiny fluid volumes to capturing, purifying and detecting disease specific biomolecules the induced motion of magnetic objects gives rise to a variety of possible applications in biomedicine. Key factors for the motion direction and velocity, that are investigated systematically, include the underlying magnetic substrate structure (1D, 2D or 3D), the present surface forces and a desired bio functionalization of the magnetic objects. Additionally it is our goal to make use of artificial magnetic stray fields for the controlled self-assembly of magnetic particles in various media in order to realize e.g. magnetically switchable optical gratings or biomimetic hybrid materials.