Direct imaging of the structural change generated by dielectric breakdown in MgO based magnetic tunnel junctions

MgO based magnetic tunnel junctions are prepared to investigate the dielectric breakdown of the tunnel barrier. The breakdown is directly visualized by transmission electron microscopy measurements. The broken tunnel junctions are prepared for the microscopy measurements by focussed ion beam out of the junctions characterized by transport investigations. Consequently, a direct comparison of transport behavior and structure of the intact and broken junctions is obtained. Compared to earlier findings in Alumina based junctions, the MgO barrier shows much more microscopic pinholes after breakdown. This can be explained within a simple model assuming a relationship between the current density at the breakdown and the rate of pinhole formation

Branch Mode Selection during Early Lung Development

Many organs of higher organisms, such as the vascular system, lung, kidney, pancreas, liver and glands, are heavily branched structures. The branching process during lung development has been studied in great detail and is remarkably stereotyped. The branched tree is generated by the sequential, non-random use of three geometrically simple modes of branching (domain branching, planar and orthogonal bifurcation). While many regulatory components and local interactions have been defined an integrated understanding of the regulatory network that controls the branching process is lacking. We have developed a deterministic, spatio-temporal differential-equation based model of the core signaling network that governs lung branching morphogenesis. The model focuses on the two key signaling factors that have been identified in experiments, fibroblast growth factor (FGF10) and sonic hedgehog (SHH) as well as the SHH receptor patched (Ptc). We show that the reported biochemical interactions give rise to a Schnakenberg-type Turing patterning mechanisms that allows us to reproduce experimental observations in wildtype and mutant mice. The kinetic parameters as well as the domain shape are based on experimental data where available. The developed model is robust to small absolute and large relative changes in the parameter values. At the same time there is a strong regulatory potential in that the switching between branching modes can be achieved by targeted changes in the parameter values. We note that the sequence of different branching events may also be the result of different growth speeds: fast growth triggers lateral branching while slow growth favours bifurcations in our model. We conclude that the FGF10-SHH-Ptc1 module is sufficient to generate pattern that correspond to the observed branching modesComment: Initially published at PLoS Comput Bio

Hybrid FEM-BEM approach for two-and three-dimensional open boundary magnetostatic problems

ABSTRACT In order to eliminate additional degrees of freedom in the surrounding domain of a charged or polarized object, we implement an open boundary method based on a hybrid FEM-BEM approach which is tested for magnetostatic problems. The underlying functional dependency of domain and boundary variables entails a sparsity decrease of the system matrix with an increasing surface area to volume ratio. Such a case is commonly at hand if systems with a high aspect ratio are considered. Therefore, we also propose an effective way which allows for the treatment of such systems in a simplified two-dimensional form without neglecting the threedimensional characteristics of the external field. The approach is tested for the cases of a three-dimensional homogeneously magnetized sphere and a thin magnetic sheet

A hydrodynamic switch: Microfluidic separation system for magnetic beads

Weddemann A, Wittbracht F, Auge A, Hütten A. A hydrodynamic switch: Microfluidic separation system for magnetic beads. APPLIED PHYSICS LETTERS. 2009;94(17):173501.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

Magnetic Nanoparticles for Novel Granular Spintronic Devices -the gGMR sensor

ABSTRACT Superparamagnetic nanoparticles have a wide range of applications in modern electric devices. Recent developments have identified them as components for a new type of magnetoresistance sensor based on highly ordered monolayers of such nanocrystallites. In this work, we propose a model for the numeric evaluation of the sensor properties. Based on the solutions of the LandauLifshitz-Gilbert equation for a set of homogeneously magnetized spheres arranged in highly symmetric monolayers, we analyze how different device properties may be adjusted to specific demands by modifications of the microstructure. We characterize sensor properties and identify different measurement regimes which correspond to specific dominating energy contributions. In particular, we find a novel measuring mode where increased field sensitivity is bought at the cost of an inherent device noise

Magnetic ratchet for biotechnological applications

Auge A, Weddemann A, Wittbracht F, Hütten A. Magnetic ratchet for biotechnological applications. APPLIED PHYSICS LETTERS. 2009;94(18):183507.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

Uniform growth of clusters of magnetic nanoparticles in a rotating magnetic field

Regtmeier A-K, Wittbracht F, Rempel T, et al. Uniform growth of clusters of magnetic nanoparticles in a rotating magnetic field. Journal of Nanoparticle Research. 2012;14(8): 1061.It was recently shown that the exposure of magnetic microbeads to a homogeneous magnetic field, which rotates around the axis perpendicular to the field direction, generates highly ordered two-dimensional particle arrays. In this work, the impact of downscaling such systems is analyzed. Dilutions of cobalt nanoparticles with an average diameter of 6 nm were brought into a rotating homogeneous magnetic field. A strong localization of the number of particles within a certain cluster size can be observed if the rotation frequency is adjusted to a specific particle concentration. In particular, we obtain an increase of 85 % of the maximum of the cluster size distribution, when changing the rotation frequency of the magnetic field from 300 to 750 rpm for a cobalt concentration of 35.95 mmol/l. We propose a heuristic model to explain the observed frequency dependence

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

Albon C, Weddemann A, Auge A, et al. Number sensitive detection and direct imaging of dipolar coupled magnetic nanoparticles by tunnel magnetoresistive sensors. APPLIED PHYSICS LETTERS. 2009;95(16):163106.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. (C) 2009 American Institute of Physics. [doi:10.1063/1.3253410

Particle flow control by induced dipolar interaction of superparamagnetic microbeads

Weddemann A, Wittbracht F, Auge A, Huetten A. Particle flow control by induced dipolar interaction of superparamagnetic microbeads. Microfluidics and Nanofluidics. 2011;10(2):459-463.Magnetic particles diluted in liquid agglomerate in rod-like particle arrays if an external homogeneous magnetic field is applied. This work introduces a method to specifically exploit particle-particle interaction to obtain flow control of magnetic particles without changing the motion state of the carrier liquid. Experiments show the possibility to uncouple the particle flux from the motion state of liquid. We show how this method may be applied to design a microfluidic geometry in which the particle flow in a specific direction is either enabled or suppressed by the relative orientation of the fluid velocity and the external field

Influence of nanoparticular impurities on the magnetic anisotropy of self-assembled magnetic Co-nanoparticles

Regtmeier A-K, Meyer J, Mill N, et al. Influence of nanoparticular impurities on the magnetic anisotropy of self-assembled magnetic Co-nanoparticles. Journal Of Magnetism And Magnetic Materials. 2013;326:112-115.A suspension of monodisperse Au-particles of either 3 or 6 nm were mixed with a dilution of 6 nm Co-particles. The resulting mixture was employed for the formation of granular films and the transport properties of these assemblies were analyzed. An increased granular giant magnetoresistive response was observed for samples with a high content of Au-particles. The experimental data were compared to numeric solutions of the Landau-Lifshitz-Gilbert equation for discrete magnetic moments. The alteration of the magnetic properties can be related to the formation of a nanoparticular structure resulting from the minimization of the particle stray fields. (C) 2012 Elsevier B.V. All rights reserved