Observation of negative absolute resistance in a Josephson junction

We experimentally demonstrate the occurrence of negative absolute resistance (NAR) up to about $-1\Omega$ in response to an externally applied dc current for a shunted Nb-Al/AlO$_x$-Nb Josephson junction, exposed to a microwave current at frequencies in the GHz range. The realization (or not) of NAR depends crucially on the amplitude of the applied microwave current. Theoretically, the system is described by means of the resistively and capacitively shunted junction model in terms of a moderately damped, classical Brownian particle dynamics in a one-dimensional potential. We find excellent agreement of the experimental results with numerical simulations of the model.Comment: 4 pages, 3 figures, submitted to Physical Revie

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

Photolithographic fabrication of arbitrarily shaped SU-8 microparticles without sacrificial release layers

Bogunovic L, Anselmetti D, Regtmeier J. Photolithographic fabrication of arbitrarily shaped SU-8 microparticles without sacrificial release layers. Journal of Micromechanics and Microengineering. 2011;21(2): 027003.We report on an efficient high throughput method for the photolithographic fabrication of well-defined arbitrarily shaped SU-8 microparticles without a sacrificial release layer. The procedure eliminates the spincoating of a sacrificial layer otherwise needed for particle lift-off, thereby reducing processing time and costs. Statistical analysis of the size distribution revealed a standard deviation of less than 2.3% in size. The particles can be immediately released into aqueous solution. This allows for anisotropical functionalization of the particles with, for example, biological loads or elements of molecular recognition after the development of the SU-8 structures

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

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

From Magnetic Nanoparticles to Magnetoresistive Biosensors

Ennen I, Albon C, Weddemann A, et al. From Magnetic Nanoparticles to Magnetoresistive Biosensors. ACTA PHYSICA POLONICA A. 2012;121(2):420-425.This paper highlights recent advances in synthesis and magnetotransport properties of magnetic Co nanopartides. It is shown that magnetic Co nanoparticles self-assembled in nanoparticular monolayers revealing giant magnetoresistance similar to granular systems but with additional features resulting from dipolar interactions between small domains of nanoparticles. A spin-valve with one magnetic Co nanoparticular electrode is employed as a model to demonstrate that individual magnetic moments of Co nanoparticles can be coupled to a magnetic Co layer which in turn offers tailoring of the resulting giant magnetoresistance characteristics. In addition, it is demonstrated that combining a magnetic on-off ratchet with magnetic tunneling junctions integrated in the ratchet introduces a new biosensor concept enabling: (1) simultaneous transporting and separating biomolecules, (2) dynamical biomolecule detection when passing magnetic tunneling junctions in a 1D arrangement. It is projected that this biosensor concept could be applied for viruses as well as for bacteria

Paradoxical Brownian motion in a microfluidic device: Absolute negative mobility

Eichhorn R, Ros A, Regtmeier J, Duong TT, Reimann P, Anselmetti D. Paradoxical Brownian motion in a microfluidic device: Absolute negative mobility. European Physical Journal - Special Topics. 2007;143(1):159-164.We report on a paradoxical migration mechanism in a microstructured lab-on-a-chip environment. The phenomenon is based on a subtle interplay between Brownian motion (thermal noise), a periodic and symmetric microstructure, and a biased AC electric field. The resulting non-linear dynamics far from thermal equilibrium gives rise to absolute negative mobility, i.e. a migrational transport, which is - both for negative and positive bias - always opposite to the net acting force, in good agreement between experiment and theory

Pulsed-field separation of particles in a microfluidic device

Regtmeier J, Eichhorn R, Duong TT, Reimann P, Anselmetti D, Ros A. Pulsed-field separation of particles in a microfluidic device. European Physical Journal E. 2007;22(4):335-340.We demonstrate the proof-of-principle of a new separation concept for micrometer-sized particles in a structured microfluidic device. Under the action of externally applied, periodic voltage-pulses two different species of like-charged polystyrene beads are observed to simultaneously migrate into opposite directions. Based on a theoretical model of the particle motion in the microdevice that shows good agreement with the experimental measurements, the underlying separation mechanism is identified and explained. Potential biophysical applications, such as cell sorting, are briefly addressed

Analytics in Microfluidic Systems

Viefhues M. Analytics in Microfluidic Systems. In: Advances in Biochemical Engineering/Biotechnology. Berlin ; Heidelberg: Springer ; 2020.Microfluidic analysis proved to be very sufficient in supporting biotechnological studies. This is due to the wide range of new analysis methods that provide further insight into biotechnological questions but also to intrinsic advantages of the systems themselves. To name two of them, only very small sample amounts are needed, and the analytics are very fast. In this overview paper, microfluidic analysis methods are introduced with a special focus on electric analysis methods. The aim of this work is to shed light on the special advantages of miniaturized electrical analysis in microfluidics; the main theoretical aspects of the methods are given together with the potential scientific insight that can be gained by the respective methods