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ABSTRACT: Spin-dependent transport of photogenerated electrons across Fe/GaAs(0 0 1) interfaces is calculated using a one-dimensional electron transport model. Creation of spin-polarized electrons by photoexcitation in this model is defined by the band structure of GaAs along the [0 0 1] direction and the optical selection rules. The tunnel probability across the interface is obtained from Chang's model and first principles calculations are employed to calculate the spin polarization of Fe for electrons propagating along the [0 0 1] direction. By combining the above ingredients, the spin-filtering current, ISF, was calculated for different parameter values, including Schottky barrier height and photon energy. The model is used to fit with experimental results of the photoexcitation technique, yielding qualitative agreement especially for the observed sign switching of the spin-filtering current.
Journal of Physics D Applied Physics 07/2010; 43(30):305001. · 2.54 Impact Factor
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T J Hayward,
B Hong,
K N Vyas,
J J Palfreyman,
J F K Cooper,
Z Jiang,
J R Jeong,
J Llandro,
T Mitrelias, J A C Bland,
C H W Barnes
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ABSTRACT: We present proof-of-principle experiments and simulations that demonstrate a new biological assay technology in which microscopic tags carrying multi-bit magnetic codes are used to label probe biomolecules. It is demonstrated that these 'micro-barcode tags' can be encoded, transported using micro-fluidics and are compatible with surface chemistry. We also present simulations and experimental results which suggest the feasibility of decoding the micro-barcode tags using magnetoresistive sensors. Together, these results demonstrate substantial progress towards meeting the critical requirements of a magnetically encoded, high-throughput and portable biological assay platform. We also show that an extension of our technology could potentially be used to label libraries consisting of similar to 10(4) distinct probe molecules, and could therefore have a strong impact on mainstream medical diagnostics.
Journal of Physics D-applied Physics. 05/2010; 43(17):175001.
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ABSTRACT: Spin-dependent transport for photoexcited electrons in an epitaxial Fe/GaAs interface was characterized from 5 to 300 K. The presence of spin-dependent transport was confirmed at all the measured temperatures and the spin polarization across the interface is found to increase with decreasing temperature. A time-of-flight-type model based on the Dyakonov–Perel (DP) spin relaxation mechanism was employed to explain the temperature dependence, providing that the estimated spin relaxation time in GaAs is 62 ps at 5 K. This short spin relaxation time can be explained by the stronger efficiency of the DP mechanism for hot-electrons.
Applied Physics Letters 01/2010; 96(2):022505-022505-3. · 3.84 Impact Factor
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T. Mitrelias,
T. Trypiniotis,
J. J. Palfreyman,
B. Hong,
K. Vyas,
T. J. Hayward,
J. Llandro,
K. P. Kopper, J. A. C. Bland,
P. A. Robertson,
C. H. W. Barnes
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ABSTRACT: Microarrays and suspension (or bead)-based technologies have attracted significant interest for their broad applications in high throughput molecular biology. However, the throughput of microarrays will always be limited by the array density and the slow diffusion of molecules to their binding sites. Suspension-based technologies, in which all the reactions take place directly on the surface of microcarriers functionalized with molecular probes, could offer true multiplexing due to the possibility of extending their detection capability by a straightforward expansion of the size of the chemical library of probes. To fully exploit their potential, the microcarriers must be tagged, but the number of distinct codes available from spectrometric/graphical/physical encoding methods is currently fairly limited. A digital magnetic tagging method based on magnetic microtags, which have been anisotropy engineered to provide stable magnetization directions which correspond to digital codes, is reported. The tags can be suspended in solution and functionalized with a variety of biological molecular probes. Magnetic tagging offers several benefits compared to the traditional optical encoding techniques currently employed. It offers minimal background signals, potential for a large number of distinct codes, miniaturization of devices, and the ability to write a code in situ. Experimental data showing the reading of individual magnetic microbars from samples comprising 50×20 μ m <sup>2</sup> Ni elements, as well as micromagnetic simulations that show the feasibility of stray field detection, are presented. The stray fields of the magnetic microbars spanning a range of 60 mOe were detected by a microfabricated fluxgate sensor scanned in a raster fashion over the sample that was placed about 70 μ m away. Free floating tags have also been fabricated for use in microfluidic systems. A -
magnetic lab-on-a-chip device could be used for tagging biomolecular probes for applications in genome sequencing, immunoassays, clinical diagnostics, drug discovery, and general pathogen detection and screening.
Journal of Applied Physics 05/2009; · 2.17 Impact Factor
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ABSTRACT: The structural and magnetic properties of thin Ni films grown on Cu/Fe/MgO(001) and Cu/MgO(001) buffer layers are investigated and compared to those grown on Cu/Si(001). The use of an Fe seed layer a few monolayers thick leads to the epitaxial growth of high surface quality Cu(001) buffer layers on MgO(001), while Cu growth on the bare MgO(001) substrate results in polycrystalline films. Magneto-optic Kerr effect magnetometry shows that Ni films grown on Cu/Fe/MgO(001) exhibit dominant perpendicular magnetic anisotropy up to ∼90 Å, which is similar to that of Ni films grown on Cu/Si(001). The polycrystalline Ni films also exhibit perpendicular magnetic remanence, but with a dominant in-plane magnetization component.
Journal of Physics Condensed Matter 04/2009; 21(15):156002. · 2.55 Impact Factor
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ABSTRACT: A design of a biological molecule carrier is presented for the application of high throughput multiplexing biological assays. This carrier contains a bit addressable “magnetic barcode” made of either Permalloy or cobalt thin films, sandwiched between two planar SU8 protective layers. We describe how the design of the magnetic carriers is optimized by engineering the coercivity of each barcode element, allowing the number of available signatures to be increased. Fully encapsulated digital magnetic carriers which carry a 5 bit addressable barcode were also fabricated and are presented. Writing and reading of digital carriers were both performed after releasing in dried solution.
Journal of Applied Physics 03/2009; · 2.17 Impact Factor
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ABSTRACT: We relate the energy dissipation processes at the femtosecond (electron-spin relaxation time tau el-sp) and nanosecond time scale (Gilbert relaxation taualpha) to the microscopic model proposed by Koopmans [Phys. Rev. Lett. 95, 267207 (2005)]. At both time scales, Elliot-Yafet scattering is proposed as the dominant contribution. We controllably manipulate the energy dissipation by transition metal doping (Pd) and rare earth doping (Dy) of a Permalloy film. While a change in taualpha of more than a factor of 2 is observed, tau el-sp remains constant. We explain the discrepancies as due to relaxation channels not considered in the model.
Physical Review Letters 01/2009; 101(23):237401. · 7.37 Impact Factor
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ABSTRACT: We have investigated a new magnetic labelling technology for high-throughput biomolecular identification and DNA sequencing. Planar multi-bit magnetic tags comprising a magnetic barcode formed by an ensemble of micron-sized thin film ferromagnetic Co bars and a 15 x 15 micron Au square for immobilization of probe molecules have been designed and fabricated. We show that by using a globally applied magnetic field and magneto-optical Kerr microscopy the magnetic elements in the multi-bit magnetic tags can be addressed individually and encoded/decoded remotely. The power of the approach is the read/write technique, which allows modest globally applied magnetic fields to write almost unlimited numbers of codes to populations of tags rather than individuals. The magnetic nature of the technology also lends itself naturally to fast, remote decoding and the ability to rewrite tags if needed. We demonstrate the critical steps needed to show the feasibility of this technology, including fabrication, remote writing and reading, and successful functionalization of the tags as verified by fluorescence detection. This approach is ideal for encoding information on tags in microfluidic flow or suspension, in order to label oligonucleotides during split-and-mix synthesis, and for combinatorial library-based high-throughput multiplexed bioassays.
Lab on a Chip 12/2008; 8(11):1883-7. · 5.67 Impact Factor
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ABSTRACT: Spin polarization of the tunnel conductivity has been studied for Fe/GaAs junctions with Schottky barriers. It is shown that band matching of resonant interface states within the Schottky barrier defines the sign of spin polarization of electrons transported through the barrier. The results account very well for experimental results including the tunneling of photo-excited electrons, and suggest that the spin polarization (from -100% to 100%) is dependent on the Schottky barrier height. They also suggest that the sign of the spin polarization can be controlled with a bias voltage. Comment: 5 pages, 4 figures
09/2008;
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ABSTRACT: The effect of MgO tunnel barriers on the magnetic moment of ultrathin magnetic Fe and Co layers is studied by polarized neutron reflectivity. The MgO barriers were grown by molecular beam epitaxy using either direct evaporation from a source crystal or evaporation of Mg in an oxygen background atmosphere. A decrease of the magnetic moment is observed for both Fe and Co, and atomic intermixing of 1.5–3.5 ML at interfaces between the Fe, Co, and MgO is determined. Evaporation from a MgO source crystal yields a tunnel layer which displays better stoichiometry and epitaxy than is obtained using the other growth method.
Applied Physics Letters 07/2008; 93(1):012505-012505-3. · 3.84 Impact Factor
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ABSTRACT: Magnetic nanowires are considered as an alternative to magnetic or coloured beads for the labeling of biological entities. Single metal and multisegment magnetic nanowires of lengths between 5–20 μm have been grown, magnetically characterized and released, and selective functionalization of the gold layer with fluorescently labeled DNA is demonstrated. The high magnetic moment of these nanowires, which is scaleable with their length, makes these nanowires a powerful alternative to bead‐based labeling techniques.
AIP Conference Proceedings. 06/2008; 1025(1):34-43.
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ABSTRACT: Research into lab‐on‐a‐chip multiplexed bioassays has focused on libraries of biochemical probes, indexed by optically encoded micron‐sized labels. However, few current methods have reconciled large multiplexing capability with a rapid detection system amenable to miniaturization. Magnetic identification of labels provides a strong candidate solution to this problem, yet no proposed single‐label magnetic detection system can both read and encode magnetic labels. We present a magnetic multiplexed assay in lab‐on‐a‐chip format which identifies target biomolecules from the hybridization results by reading encoded magnetic beads. We show that a microfabricated magnetoresistive ring‐shaped sensor can read the magnetic moments of individual commercially available paramagnetic beads using an active digital technique. This work provides proof of principle for a new approach to magnetic labeling of biomolecules for high‐throughput bioassays.
AIP Conference Proceedings. 06/2008; 1025(1):176-185.
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ABSTRACT: In this study micromagnetic simulations are used to evaluate two novel approaches of magnetically tagging biomolecules in high‐throughput biological assays. Comparisons are made between a simple magnetic moment‐based tagging system, where the total magnetic moment of each microscopic tag encodes the identity of an attached biomolecule, and a multibit tagging system, where each tag is comprised of multiple magnetic binary bits. We show that although detection of the tags using magnetoresistive sensors is feasible in both cases, the multibit technology offers over a thousand times more distinct tags than the simple moment encoded approach. The advantages of using multibit magnetic tags to label biomolecules, rather than existing optical tagging techniques, are also discussed.
AIP Conference Proceedings. 06/2008; 1025(1):111-124.
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ABSTRACT: By means of time-resolved Kerr spectroscopy experiments we relate the energy dissipation processes on the femtosecond (electron-spin relaxation time $\tau_{el-sp}$) and nanosecond timescale (Gilbert relaxation $\tau_{\alpha}$) and compare the results to the first microscopic model, which was proposed by Koopmans. For both energy dissipation processes, Elliot-Yafet scattering is proposed as the dominant contributor. We controllably manipulate the energy dissipation processes by transition metal doping (Pd) and rare earth doping (Dy) of a Permalloy film and find that while a change of $\tau_{\alpha}$ of more than a factor two is observed, \tau_{el-sp}$ remains constant, contrary to the predictions of the model. We explain the discrepancies by relaxation channels not considered in the original microscopic model and identify thereby the applicability of the model and possible necessary extensions to the model.
05/2008;
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ABSTRACT: We report on the effects of O <sub>2</sub> and H <sub>2</sub> dosing on the magnetism of Co <sub>75</sub> Fe <sub>25</sub> films, highlighting the extreme sensitivity of the magnetic behavior of ultrathin films to specific chemical interactions at the surface. We have performed a systematic in situ study of the magnetic behavior as a function of O <sub>2</sub> and H <sub>2</sub> dosage on ultrathin CoFe and Co films grown on Cu(110) by means of the magneto-optical Kerr effect and low energy electron diffraction. Oxygen and hydrogen dosing are found to have distinctly different influences on the evolution of the coercive field H<sub>c</sub> of the CoFe and Co films. Significant differences become also apparent for the evolution of the M-H loop squareness upon gas adsorption. Oxygen dosing leads to a rapid decrease in squareness above 10 L (1 L ≡10<sup>-6</sup> Torr s ) for CoFe films. For Co films, however, the squareness increases initially and then stabilizes at a higher dosage level. Furthermore, a striking sensitivity of the easy axis magnetization in the CoFe films was observed; 50 L of O <sub>2</sub> caused a rotation of the easy axis by 60° within the plane. The observed differences between the different gases and between the different ferromagnetic films are briefly discussed in terms of d band occupation and details of chemical bonding.
Journal of Applied Physics 05/2008; · 2.17 Impact Factor
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ABSTRACT: In this paper, we review some of the key concepts in ultrathin film magnetism which underpin nanomagnetism. We survey the results of recent experimental and theoretical studies of well characterized epitaxial structures based on Fe, Co and Ni to illustrate how intrinsic fundamental properties such as the magnetic exchange interactions, magnetic moment and magnetic anisotropies change markedly in ultrathin films as compared with their bulk counterparts, and to emphasize the role of atomic scale structure, strain and crystallinity in determining the magnetic properties. After introducing the key length scales in magnetism, we describe the 2D magnetic phase transition and survey studies of the thickness dependent Curie temperature and the critical exponents which characterize the paramagnetic–ferromagnetic phase transition. We next discuss recent experimental and theoretical results on the determination of the exchange constant, followed by an overview of measurements of the magnetic moment in the elemental 3d transition metal thin films in the various crystal phases that have been successfully stabilized, thereby illustrating the sensitivity of the magnetic moment to the local symmetry and to the atomic environment. Finally, we discuss briefly the magnetic anisotropies of Fe, Co and Ni in the fcc crystalline phase, to emphasize the role of structure and the details of the interface in influencing the magnetic properties. The dramatic effect that adsorbates can have on the magnetic anisotropies of thin magnetic films is also discussed. Our survey demonstrates that the fundamental properties, namely, the magnetic moment and magnetic anisotropies of ultrathin films have dramatically different behaviour compared with those of the bulk while the comparable size of the structural and magnetic contributions to the total energy of ultrathin structures results in an exquisitely sensitive dependence of the magnetic properties on the film structure.
Reports on Progress in Physics 04/2008; 71(5):056501. · 14.72 Impact Factor
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ABSTRACT: We show that by obtaining a crystalline structure in a Co / Mg O / Fe tunnel junction, we are able to control the magnetic anisotropies in the spin valve and therefore, induce independent switching of the two magnetic layers. In situ and ex situ structural characterization confirms that the multilayer is fully epitaxial, with smooth interfaces throughout. In confirmation of the high quality of the insulating barrier, we also present a layer-selective measurement of the magnetization of the top electrode using current-in-plane transport measurements.
Applied Physics Letters 03/2008; · 3.84 Impact Factor
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M. Laufenberg,
M. Kläui,
D. Backes,
W. Bührer,
H. Ehrke,
D. Bedau,
U. Rüdiger,
F. Nolting,
L. J. Heyderman,
S. Cherifi,
A. Locatelli,
R. Belkhou,
S. Heun,
C. A. F. Vaz, J. A. C. Bland,
T. Kasama,
R. E. Dunin-Borkowski,
A. Pavlovska,
E. Bauer,
Rolf Haug
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ABSTRACT: In this article, a comprehensive study of head-to-head domain wall spin structures in Ni80Fe20 and Co nanostructures is presented.
Quantitative domain wall type phase diagrams for NiFe and Co are obtained and compared with available theoretical predictions
and micromagnetic simulations. Differences to the experiment are explained taking into account thermal excitations. Thermally
induced domain wall type transformations are observed from which a vortex core nucleation barrier height is obtained. The
stray field of a domain wall is mapped directly with sub-10nm resolution using off-axis electron holography, and the field
intensity is found to decrease as 1/r with distance. The magnetic dipolar coupling of domain walls in NiFe and Co elements is studied using X-ray magnetic circular
dicroism photoemission electron microscopy. We observe that the spin structures of interacting domain walls change from vortex
to transverse walls, when the distance between the walls is reduced. Using the measured stray field values, the energy barrier
height distribution for the nucleation of a vortex core is obtained.
12/2007: pages 281-293;
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ABSTRACT: The dependence of the exchange bias field and coercivity enhancement on ferromagnetic (FM) and antiferromagnetic (AF) layer thickness in exchange biased bilayers has been systematically investigated in CoFe/FeMn and CoFe/PdMn bilayers for digital encoding applications in biotechnology. A magnetic multilayer structure can be used as a digitally encoded tag if each (bi)layer has two magnetic states, positive and negative saturation, available at remanence and if each layer can be uniquely identified by its coercivity. We will demonstrate that by adjusting the AF and FM layer thickness in an AF/FM bilayer, both the bias field and the coercivity of the bilayer can be controlled. By contrasting CoFe/FeMn bilayers with CoFe/PdMn bilayers, it becomes apparent that the relative magnitudes of the coercivity enhancement and bias field depend on the particular AF material, although the qualitative behavior remains unchanged. In order to create a multilayer that can retain one of many magnetic states at remanence, a large coercivity enhancement but absent or small bias field are preferred. Analysis of the bilayers suggest that PdMn is a better choice of AF layer for this purpose and results on some multilayer films are shown which validate this claim.
Journal of Applied Physics 11/2007; 102(10):103908-103908-7. · 2.17 Impact Factor
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ABSTRACT: We present a magnetic multiplexed assay technology which encodes the identities of target biomolecules according to the moment of magnetic beads to which they are attached. An active digital technique based on a microfabricated magnetoresistive ring-shaped sensor is demonstrated, which can distinguish the magnetic moments of micron-sized superparamagnetic beads. We propose that this development is key to combining nonvolatile magnetic labeling with biochemical libraries for high-throughput bioassays and rapid multiplexed detection.
Applied Physics Letters 11/2007; 91(20):203904-203904-3. · 3.84 Impact Factor
