Creation of an artificial jellyfish

The magnetization direction in exchange bias thin layer systems can be locally modified by a combination of photolithography and He+ ion bombardment, producing micron-sized magnetic patterns. Due to the resulting stray field landscape, (super)paramagnetic core-shell particles can be directly positioned above the domain walls of the topographically planar substrate. By embedding these arranged microparticles into a polymer matrix, a smart material with externally controllable viscoelastic and dynamic mechanical properties is obtained (magnetorheological polymer). If a magnetic field is applied, the composite material stiffens because of the dipole-dipole interaction among the particles. Furthermore, a magnetostrictive effect is achieved, which is – depending on the magnetization orientation of adjacent beads - attributed to the repulsion or the attraction between the magnetic particles. When the magnetic field is switched off, the regained elasticity ensures that the original shape is recovered.

The aim of that work is to modify the magnetic particles and their special arrangement in a way that a microstructure is produced, which is able to move in fluids by fast contraction and slow regeneration similar to a jellyfish in water.

Therefore, the application of different magnetic materials as well as polymers respectively polymer blends will be analyzed. Additionally, an experimental setup for the detection of the composite’s mechanical changes when applying a magnetic field will be developed within this project.

Contact: Iris Koch

Guinier camera for crystallite size measurements

Exchange biased thin film systems are structurally characterised with the X-ray Guinier thin film camera Huber G 653. One advantage of this Guinier camera includes the use of a focussing monochromator between the X-ray tube and the thin film system for achieving strictly monochromatic X-rays. Another benefit lies in the very small angle of incidence (< 10°) between the incident X-rays and the surface of the sample to achieve a large effective distance in the thin films and thereby higher diffraction intensities. The measurement geometry behind the monochromator is called Seemann-Bohlin geometry. In this geometry the focal line of the monochromator, the surface of the sample and the detector entrance slit are located on a constant focussing cylinder. During one measurement the angle of incidence is constant and the detector entrance slit moves on the focussing cylinder. As a consequence the Guinier thin film goniometer is especially suitable for analysing thin polycrystalline films on a crystalline or amorphous substrate. From the diffraction spectrum e.g. the lattice parameter, the lattice type and the crystallite sizes can be calculated. In the future the crystallite sizes of exchange bias systems shall be measured in order to get a correlation between the exchange bias field and the crystallite sizes. Afterwards the deposition parameters of the layer systems shall be optimized for a maximum exchange bias effect.

Contact: Markus Meyl



Two dimensional magnetic patterning of exchange bias systems

Complex magnetic patterns within an exchange biased multilayer system consisting of an antiferromagnetic and an ferromagnetic layer are achieved by repetitive lithographic and helium ion induced magnetic patterning processes. The magnetic properties can thereby be tuned specifically. In this way the domains properties such as domain wall width or magnetic ripple wave length can be set to fit the applications requirements.

Design and characterization of such systems build the main topic of research. The field depended magnetization of the sample is analyzed by vibrating sample magnetometry as well as magneto optic Kerr magnetometry. The magnetic stray field landscapes are analyzed by magnetic force microscopy (MFM). The magnetic moments within the ferromagnetic layers domains or domain walls are analyzed by scanning electron microscopy with polarization analysis (SEMP) with a very high spatial resolution.

The SEMPA measurements are performed in the group for surface science physics of Prof. Dr. H.P. Oepen in the University of Hambu

Magnetically patterned three-dimensional Nanostructures

The ongoing miniaturization of functional systems in the nanoscale has already well proceeded. Especially the three-dimensional structuring is a challenge though. Magnetically patterned three-dimensional systems in the nanoscale have a particular importance in application fields like data storage, sensorics and transport of agents. The production as well as the patterning and the analysis of such structures require special methods.

In this project new magnetically structured systems in the nanoscales will be developed and characterized. For example topographically structured exchange bias systems with various geometries shall be fabricated enabled by the NanoImprint technique in the INA. In a second cooperation tubular magnetically patterned exchange bias systems, that roll up by themselves due to strain, are to be produced. The aim of this experiment is to guide superparamagnetic particles through these tubes simply by external alternating magnetic fields.

In addition to common analysis methods like VSM, MOKE or AFM/MFM, that are available at the facilities of the group, within this project also X-PEEM experiments with synchrotron radiation are facilitated in order to investigate positioning of molecules or particles and magnetic properties.

Magneto Optic Surface Plasmon Resonance (MOSPR)

Angularly resolved surface plasmon resonance (SPR) based biosensors are an outstanding tool for biomolecular interaction analysis. This method relies on reflectivity measurements at the interface between a dielectric and a noble metal behind a glass prism (Kretschmann configuration). If linear polarized light is reflected from the backside of the metal layer (through the prism) above the critical angle total reflection (glass-dielectric), collective electron density oscillations (surface plasmon) can be excited, observed by a minimum in the intensity of the reflected light. If the refractive index in front of the metal layer (dielectric) changes due to binding of a biomolecule to the surface, the resonance angle and therefore the angular position of the reflectivity minimum shifts. This effect can be utilized to study the binding behavior of biomolecules, since refractive index correlates with the concentration of biomolecules in a solution.

We have built an angularly and spatially resolved SPR set-up in Kretschmann configuration enabling magneto-optic SPR (MOSPR) measurements in transversal geometry. In order to perform MOSPR measurements a ferromagnetic layer, which is magnetized by an external magnetic field perpendicular to the plane of the incident light, needs to be part of the metallic layer system. Therefore the reflectivity depends on the magnetization of the ferromagnetic layer due to transversal magneto optic Kerr effect. It has been demonstrated that the sensitivity of SPR sensors can be increased employing the differential signal δ = R(H+)-R(H-)/R(0),where R is the reflectivity at the different magnetizations of the ferromagnetic layer[1,2].

Within the MOSPR project we investigate the SPR/MOSPR characteristics of different magnetic thin film layer systems theoretically and experimental. Recent calculations revealed that the enhanced sensitivity of an MOSPR biosensor can be exploited only by defined thicknesses of the metal layers for distinct refractive index regions [1].

[1] Kämpf K., S.Kübler, F. W Herberg, A. Ehresmann,.J. Appl. Phys. 112, 034505 (2012)
[2] D. Regatos, B. Sepulveda, D. Farina, L. G. Carrascosa, L. M. Lechuga, Opt. Express 19, 8336 (2011)

Ansprechpartner: Sebastian Kübler

Transport of magnetic particles in moving magnetic strayfield landscapes and its application in micromixers and biosensors

The development of efficient micro-total analysis systems, i.e. the functional integration of sensorial elements into microfluidic devices is an active field of research due to the increasing demands in biological, chemical, medical and pharmaceutical applications. In particular, the utilization of so called lab-on-a-chip devices for biomedical applications, e.g. drug delivery and artificial organs, offers an innovative approach for the development of novel point-of-care systems. The successful realization of a microfluidic total analysis system is related to the integration of efficient micro mixing units for the handling of smallest fluid volumes and the effective transport of provable biomolecules to the implemented sensor areas. The efficient mixing of the biomolecules is crucial for the functionality of the device since the mass transport in a microfluidic device is dominated by slow molecular diffusion and the laminar flow profile due to the adhesive interaction between the fluid and the surrounding channel walls, suppressing the formation of eddy currents that normally improve the mixing speed. Hence, a novel approach for active microfluidic mixing is investigated based on the domain wall movement transport of superparamagnetic bead rows on top of a magnetic parallel-stripe patterned Exchange-Bias layer system via specially tailored external magnetic fields [1]. The particles hereby act as active micro stirrers increasing the interface between the two fluids and hence, the rate of molecular diffusion, i.e. the mixing speed. In consequence, the interaction rate between the biomolecules and sensor units is enhanced, allowing for a highly-sensitive detection of disease relevant biomolecules [2]. This research project is financially funded by the Wirtschafts- und Infrastrukturbank Hessen titled „Hochempfindlicher Nachweis von Biomolekülen mit Hilfe von kontrolliert bewegter Mikro- und Nanopartikel“.

[1] A. Ehresmann, D. Lengemann, T. Weis, A. Albrecht, J. Langfahl-Klabes, F. Göllner, D. Engel, Adv. Mater. 2011 , 23, 5568
[2] D. Holzinger, D. Lengemann, F. Göllner, D. Engel and A. Ehresmann, Appl. Phys. Lett. 2012, 100, 153504

Contact: Dennis Holzinger

MaSC - Magnetic Security Coding

In this project, the concept of a magnetic safety code for protection against product piracy has to be examined. This concept bases on a surface having different magnetization directions in different areas. The pattern of the magnetizations should then serves as a carrier of information, which shows the authenticity of the products. Basis for this concept are exchange biased multilayer systems, whose magnetization direction are modeled by He ion bombardment.
The project aims to find patterns that can carry a sufficient amount of information and can also be read out quick and easy. Further, the system should be optimized with respect to its resistance to external influences such as temperature, external magnetic fields and mechanical stresses.
Another part of this project is to create and test a prototype of a readout device, which can read the produced structures.  This reader should be inexpensive to produce on the one hand and on the other ensure a quick and easy readout of the information on the security code.

Contact: Nicolas Müglich

Structural effects of ion bombardment on Exchange-Bias systems

The focus of this investigation is the characterization of structural modifications that occur in Exchange-Bias systems due to the bombardment with low energy Helium ions. Of main interest is the way how the generation and modification of magnetic properties are linked to structural effects invoked by ion bombardment and why this allows a magnetic pattering using IBMP. It is the focus of the research as there are mainly only phenomenological descriptions by now while the effect is widely used in experiments for several years.To perform those investigations multiple experimental approaches were developed to probe individual aspects of the material system. Thereby we want to isolate individual effects of the interaction between ions and magnetic layers to perform and correlate structural and magnetic characterizations.

Contact: Henning Huckfeldt

Molecular positioning by magnetic stray field landscapes

Thin film layer samples are used to produce a landscape of magnetic stray fields. These stray fields influence the magnetic behaviour of metallorganic molecules, guiding them to a controlled position on the sample surface. Thereby a control of the positioning of molecules by magnetic stray fields is achieved for the first time. Molecules with different magnetic properties (paramagnetic, diamagnetic) and special binding groups are synthesized in the group “Metallorganische Chemie” of Prof. Siemeling, university of Kassel. The sample is magnetically structured by the ion bombardment induced magnetic patterning process into (up to now) periodic stripe domains with head-to-head/tail-to-tail magnetization. This magnetization geometry leads to strong inhomogeneous magnetic fields emerging from the domain walls, propagating above the sample surface. Analogous to the macroscopic world diamagnetic materials are repelled from regions with high magnetic flux density, while paramagnetic species are attracted. Thereby the area of molecular monolayer growth can be chosen when using appropriate magnetic domain structures. 

Ansprechpartner: Florian Ahrend