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Background: The application of superparamagnetic particles as biomolecular transporters in microfluidic systems for lab-on-a-chip applications crucially depends on the ability to control their motion. One approach for magnetic-particle... more
Background: The application of superparamagnetic particles as biomolecular transporters in microfluidic systems for lab-on-a-chip applications crucially depends on the ability to control their motion. One approach for magnetic-particle motion control is the superposition of static magnetic stray field landscapes (MFLs) with dynamically varying external fields. These MFLs may emerge from magnetic domains engineered both in shape and in their local anisotropies. Motion control of smaller beads does necessarily need smaller magnetic patterns, i.e., MFLs varying on smaller lateral scales. The achievable size limit of engineered magnetic domains depends on the magnetic patterning method and on the magnetic anisotropies of the material system. Smallest patterns are expected to be in the range of the domain wall width of the particular material system. To explore these limits a patterning technology is needed with a spatial resolution significantly smaller than the domain wall width.Result...
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Thermoelectric effects in magnetic tunnel junctions are promising to serve as the basis for logic devices or memories in a "green" information technology. However, up to now the readout contrast achieved with Seebeck effects was... more
Thermoelectric effects in magnetic tunnel junctions are promising to serve as the basis for logic devices or memories in a "green" information technology. However, up to now the readout contrast achieved with Seebeck effects was magnitudes smaller compared to the well-established tunnel magnetoresistance effect. Here, we resolve this problem by demonstrating that the tunnel magneto-Seebeck effect (TMS) in CoFeB/MgO/CoFeB tunnel junctions can be switched on to a logic "1" state and off to "0" by simply changing the magnetic state of the CoFeB electrodes. This new functionality is achieved by combining a thermal gradient and an electric field. Our results show that the signal crosses zero and can be adjusted by tuning a bias voltage that is applied between the electrodes of the junction; hence, the name of the effect is bias-enhanced tunnel magneto-Seebeck effect (bTMS). Via the spin- and energy-dependent transmission of electrons in the junction, the bTM...
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Extended abstract of a paper presented at Microscopy and Microanalysis 2010 in Portland, Oregon, USA, August 1 – August 5, 2010.
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In spin valve and magnetic tunnel junction systems, the soft sensing layer should be sensitive to small fields of a few hundred A/m and should not be influenced by the adjacent magnetic hard layer. In polycrystalline films, however,... more
In spin valve and magnetic tunnel junction systems, the soft sensing layer should be sensitive to small fields of a few hundred A/m and should not be influenced by the adjacent magnetic hard layer. In polycrystalline films, however, congruent interface roughness invokes a ferromagnetic coupling, which can be compensated by an antiferromagnetic magnetostatic dipole coupling in patterned elements. In this paper, we demonstrate the evolution of the magnetic interlayer interaction by annealing in tunnel junctions with Al2O3 barrier. Whereas the minor loop shifting field of the soft magnetic NiFe sense layer is constant or varies only slightly from 2 kA/m to 1.15 kA/m for 1–1.5 nm thick barriers, an eightfold increase of the coercivity is found after annealing above 350 °C. Microstructural investigations give evidence that small ferromagnetic clusters arise in the hard layer of the junctions (Cu/CoFe/Ru/CoFe) by diffusion of Cu, Co, and Fe. The density and the stray field of these clusters are determined by ma...
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Research Interests: Engineering, Physics, Materials Science, Applied Physics, Magnetic thin film, and 15 moreCrystal structure, Alumina, Mathematical Sciences, Physical sciences, Magnetic thin films, Magnetic Properties, Magnetic Susceptibility, Metal, Electronic properties, Magnetoresistance, Magnetic Circular Dichroism, Magnetic Moments, Ferromagnetism, Manganese Compounds, and Absorption Spectra
Research Interests: Engineering, Physics, Condensed Matter Physics, Materials Science, Applied Physics, and 13 moreMagnesium, Mathematical Sciences, Magnetization Reversal, Physical sciences, Magnetic Properties, Iron, Current Density, Magnetization dynamics, Exchange coupling, Magnetic multilayer, Quantum Tunnelling, Magnetization, and Magnetic Anisotropy
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Research Interests: Engineering, Physics, Image Processing, Microscopy, Mesoscopic Physics, and 13 moreSurface Roughness, Scanning tunneling microscopy, Transducers, Deconvolution, Physical sciences, Surface topography, Surface Reconstruction, Quantum Tunnelling, Correlations, Roughness, DDC, Surface Finish, and Rough Surface
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We identify and investigate thermal spin transport phenomena in sputter-deposited Pt/NiFe_{2}O_{x} (4≥x≥0) bilayers. We separate the voltage generated by the spin Seebeck effect from the anomalous Nernst effect (ANE) contributions and... more
We identify and investigate thermal spin transport phenomena in sputter-deposited Pt/NiFe_{2}O_{x} (4≥x≥0) bilayers. We separate the voltage generated by the spin Seebeck effect from the anomalous Nernst effect (ANE) contributions and even disentangle the ANE in the ferromagnet (FM) from the ANE produced by the Pt that is spin polarized due to its proximity to the FM. Further, we probe the dependence of these effects on the electrical conductivity and the band gap energy of the FM film varying from nearly insulating NiFe_{2}O_{4} to metallic Ni_{33}Fe_{67}. A proximity-induced ANE could only be identified in the metallic Pt/Ni_{33}Fe_{67} bilayer in contrast to Pt/NiFe_{2}O_{x} (x>0) samples. This is verified by the investigation of static magnetic proximity effects via x-ray resonant magnetic reflectivity.
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Spin caloritronics studies the interplay between charge-, heat- and spin-currents, which are initiated by temperature gradients in magnetic nanostructures. A plethora of new phenomena has been discovered that promises, e.g., to make... more
Spin caloritronics studies the interplay between charge-, heat- and spin-currents, which are initiated by temperature gradients in magnetic nanostructures. A plethora of new phenomena has been discovered that promises, e.g., to make wasted heat in electronic devices useable or to provide new read-out mechanisms for information. However, only few materials have been studied so far with Seebeck voltages of only some microvolt, which hampers applications. Here, we demonstrate that half-metallic Heusler compounds are hot candidates for enhancing spin-dependent thermoelectric effects. This becomes evident when considering the asymmetry of the spin-split density of electronic states around the Fermi level that determines the spin-dependent thermoelectric transport in magnetic tunnel junctions. We identify Co2FeAl and Co2FeSi Heusler compounds as ideal due to their energy gaps in the minority density of states, and demonstrate devices with substantially larger Seebeck voltages and tunnel m...
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We describe an atomic force microscope (AFM) for the characterization of self-sensing tunneling magnetoresistive (TMR) cantilevers. Furthermore, we achieve a large scan-range with a nested scanner design of two independent piezo scanners:... more
We describe an atomic force microscope (AFM) for the characterization of self-sensing tunneling magnetoresistive (TMR) cantilevers. Furthermore, we achieve a large scan-range with a nested scanner design of two independent piezo scanners: a small high resolution scanner with a scan range of 5 × 5 × 5 μm3 is mounted on a large-area scanner with a scan range of 800 × 800 × 35 μm3. In order to characterize TMR sensors on AFM cantilevers as deflection sensors, the AFM is equipped with a laser beam deflection setup to measure the deflection of the cantilevers independently. The instrument is based on a commercial AFM controller and capable to perform large-area scanning directly without stitching of images. Images obtained on different samples such as calibration standard, optical grating, EPROM chip, self-assembled monolayers and atomic step-edges of gold demonstrate the high stability of the nested scanner design and the performance of self-sensing TMR cantilevers.
Page 1. 5 Analysis of the Disturbing Influence of Stray Fields in Very Small MRAM Cells by Computer Simulation ... The devices show too an asymmetrical behavior in the magnetization reversal. The simulation calculations confirm both... more
Page 1. 5 Analysis of the Disturbing Influence of Stray Fields in Very Small MRAM Cells by Computer Simulation ... The devices show too an asymmetrical behavior in the magnetization reversal. The simulation calculations confirm both experimental results. 5.1 Introduction ...
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Page 1. 6 Magnetic Tunnel Junctions Günter Reiss1, Jan Schmalhorst1, Andre Thomas1, Andreas Hütten1, and Shinji Yuasa2 1 Thin Films and Physics of Nanostructures, Department of Physics, Bielefeld University, Bielefeld ...
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We present a new design and the application of a near-field magneto-optical microscope, which combines the operation in the collection (PSTM) or illumination mode (NSOM) and the simultaneous application of an external magnetic field.... more
We present a new design and the application of a near-field magneto-optical microscope, which combines the operation in the collection (PSTM) or illumination mode (NSOM) and the simultaneous application of an external magnetic field. Magnetic information is obtained via polarisation contrast showing the relative orientation of the magnetisation in magnetic thin films and microstructures due to the Faraday effect in
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MgO-based magnetic tunnel junctions with up to 230% tunnel magnetoresistance ratio at room temperature and up to 345% at 13 K are prepared. The lower electrode is either exchange-biased or free, while the top electrode is free or an... more
MgO-based magnetic tunnel junctions with up to 230% tunnel magnetoresistance ratio at room temperature and up to 345% at 13 K are prepared. The lower electrode is either exchange-biased or free, while the top electrode is free or an exchanged-biased artificial ferrimagnet, ...
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ABSTRACT The best way to achieve a virgin state of a magnetized sample is to heat it up to a temperature high enough to destroy every magnetic order and thereafter cooling it down in a zero magnetic field. This procedure is often not... more
ABSTRACT The best way to achieve a virgin state of a magnetized sample is to heat it up to a temperature high enough to destroy every magnetic order and thereafter cooling it down in a zero magnetic field. This procedure is often not possible because high temperatures may cause irreversible changes in the sample's constitution e. g. by crystallization in amorphous systems.Because of magnetic history and memory effects it is difficult to measure the same initial magnetization curves after different demagnetizing procedures for materials consisting of nanoparticles with high Curie temperatur TC. Very often the demagnetization by alternating decreasing magnetic fields (AC demagnetization) is used instead.Ferrofluids offer the possibility to demagnetize the sample by heating it above the melting point of the liquid in zero external field. The brownian motion destroys any “frozen in” magnetic order and relieves single domain particles in the fluid. After zero field cooling the virgin state may be reproducibly achieved. We present an experimental study showing the influence of different demagnetization sequences on the initial magnetization curves in frozen ferrofluids. In this contribution we show the deviation between thermally demagnetized and AC demagnetized samples by substracting both initial curves from each other. We detect a “frozen in“ pattern which shows the “steps” of the AC demagnetization sequences. In a further step we focus on the experimental determination of the switching field distribution (SFD) of the particles. We compare the SFD with a plot proposed by Thamm and Hesse (1996).
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... NEWS & VIEWS nature materials | VOL 4 | OCTOBER 2005 | www.nature.com/naturematerials 725 GÜNTER REISS AND ANDREAS HÜTTEN are in the Department of Physics, University of Bielefeld, Bielefeld 33615, Germany. ... First name.... more
... NEWS & VIEWS nature materials | VOL 4 | OCTOBER 2005 | www.nature.com/naturematerials 725 GÜNTER REISS AND ANDREAS HÜTTEN are in the Department of Physics, University of Bielefeld, Bielefeld 33615, Germany. ... First name. Last name. E-mail address. ...