Alexander Auge

Tunneling magnetoresistance sensors for high resolutive particle detection

Arrays of tunnel magnetoresistance sensors based on MgO as insulating layer are employed to detect magnetic microbeads. For single bead detection, elliptically shaped sensors of axis lengths of 400 and 100 nm are used. Due to high shape anisotropy a linear response of the sensor signal in a magnetic field range between −500 and 500 Oe can be reported. By performing static detection measurements of magnetic microbeads, a distinct signal shape correlated with the position of beads in respect to the sensor can be observed. The experimental data are compared to micromagnetic simulations carried out on a trilayer model.

Magnetic ratchet for biotechnological applications

Transport and separation of magnetic beads are important in “lab on a chip” environments for biotechnological applications. One possible solution for this is the on-off ratchet concept. An asymmetric magnetic potential and Brownian motion of magnetic beads are required for such a ratchet. The asymmetric magnetic potential is achieved by combining an external magnetic field with a spatially periodic array of conducting lines. In this work finite element method simulations are carried out to design this asymmetric potential and to evaluate transport rates. Furthermore, experiments are carried out so as to compare to the simulation results.

Inverse magnetocaloric effect of epitaxial Ni-Mn-Sn thin films

Epitaxial Ni-Mn-Sn thin films of 200 nm thickness were prepared by magnetron sputtering and deposited onto MgO(100) substrate. They reveal an inverse magnetocaloric effect with a martensitic phase transition around 260 K. The resulting magnetocaloric properties of these films have been determined performing magnetization measurements in the temperature range between 10 and 330 K applying different magnetic fields. The maximum values of entropy change and relative cooling power are 1.6 J kg−1 K−1 and 36.5 J kg−1 for cooling and 1.5 J kg−1 K−1 and 33.9 J kg−1 for heating in μ0ΔH=1 T, respectively. These data are comparable with bulk values of Ni-Mn-Sn Heusler alloys.

How to design magneto-based total analysis systems for biomedical applications

This article reviews recent developments on magnetoresistive detection of magnetic beads or nanoparticles by nanoscale sized sensors. Sensors are analyzed from an experimental and a numerical point of view in respect to their capability to either localize the position of a single magnetic particle or to detect the number of particles in a certain range. Guidelines are shown up on how to extend single sensors to sensor arrays with very high spatial resolution and how to modify the sensor shape in order to provide long distance measurements. Further, sensors in biological lab-on-a-chip environments are discussed. The magnetic ratchet and a gravitation based microfluidic component are reviewed as important tools to position and, therefore, detect biological components in continuous-flow devices.

A hydrodynamic switch: Microfluidic separation system for magnetic beads

In this work a device for separating small magnetic particles in continuous flow is introduced, consisting of two microfluidic channels that are connected by a junction channel. Applying two different flow rates, particles can be separated combining hydrodynamic and magnetophoretic effects. The two different flow rates introduce an additional degree of freedom that enables the microfluidic geometry to act as a hydrodynamic switch that can overcome diffusive contributions making the device applicable for particles of the size scale below 100 nm. Theoretical predictions based on finite element methods are compared to experimental observations.

Heusler nanoparticles for spintronics and ferromagnetic shape memory alloys

Heusler nanoparticles emerge as a new class of multifunctional materials. In this critical review, the latest progress in studies on Heusler nanoparticles is summarized. The authors discuss their structural and physical properties interesting for research fields such as spintronics and ferromagnetic shape memory alloys. As a young research field, the majority of studies on Heusler nanoparticles focus on their synthesis, structure, and magnetic characterizations. Important issues such as size dependent structure, phase transition, magnetic, and spin-related properties are still open. Further investigations are needed to verify the technical significance of Heusler nanoparticles for practical applications such as data storage, magnetic sensors, and microactuators.

Lab-on-a-Chip Magneto-Immunoassays: How to Ensure Contact between Superparamagnetic Beads and the Sensor Surface

Lab-on-a-chip immuno assays utilizing superparamagnetic beads as labels suffer from the fact that the majority of beads pass the sensing area without contacting the sensor surface. Different solutions, employing magnetic forces, ultrasonic standing waves, or hydrodynamic effects have been found over the past decades. The first category uses magnetic forces, created by on-chip conducting lines to attract beads towards the sensor surface. Modifications of the magnetic landscape allow for additional transport and separation of different bead species. The hydrodynamic approach uses changes in the channel geometry to enhance the capture volume. In acoustofluidics, ultrasonic standing waves force µm-sized particles onto a surface through radiation forces. As these approaches have their disadvantages, a new sensor concept that circumvents these problems is suggested. This concept is based on the granular giant magnetoresistance (GMR) effect that can be found in gels containing magnetic nanoparticles. The proposed design could be realized in the shape of paper-based test strips printed with gel-based GMR sensors.

Thickness dependent exchange bias in martensitic epitaxial Ni-Mn-Sn thin films

A thickness dependent exchange bias in the low temperature martensitic state of epitaxial Ni-Mn-Sn thin films is found. The effect can be retained down to very small thicknesses. For a Ni50Mn32Sn18 thin film, which does not undergo a martensitic transformation, no exchange bias is observed. Our results suggest that a significant interplay between ferromagnetic and antiferromagnetic regions, which is the origin for exchange bias, is only present in the martensite. The finding is supported by ab initio calculations showing that the antiferromagnetic order is stabilized in the phase.

Number sensitive detection and direct imaging of dipolar coupled magnetic nanoparticles by tunnel magnetoresistive sensors

The suitability of magnetic tunnel junctions for the detection of magnetic nanoparticles is related to their scalability onto the nanoscale size regime without a significant loss of sensitivity. Elliptically shaped MgO based tunnel magnetoresistance sensors are used to provide a sharp detection of 14 nm Co nanoparticles. The measured signal is related to the degree of coverage of the sensor area by a nanoparticle layer. Moreover, the nanoparticles magnetostatic interaction on the sensor surface is clearly distinguished by the presence of a coercitive field in the detected signal. Experimentally obtained results are compared to theoretical models.

A level set based approach for modeling oxidation processes of ligand stabilized metallic nanoparticles

The oxidation behavior of metallic nanoparticles is investigated in respect to ligand influences. The nanoparticle oxidation is modeled in a shell-core approach. The shell represents oxidation of surface atoms modeled by Johnson–Mehl–Avrami–Kolmogorov equations for isothermal growth. The oxidation of the nanoparticle core is described by a model introduced by Cabrera and Mott [Rep. Prog. Phys. 12, 163 (1949)]. In order to investigate the ligand influence one single parameter is introduced for both surface and bulk oxidation. The growth of the oxide layer is simulated in a level set framework via finite element methods. The theoretical results are compared to experimental findings of Kanninen et al. [J. Coll. Interf. Sci. 318, 88 (2008)].