D. Atkinson

Durham University, Durham, England, United Kingdom

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Publications (90)299.08 Total impact

  • [Show abstract] [Hide abstract]
    ABSTRACT: Printed nanocomposites are of significant applications potential in numerous technologies, such as touch-sensitive sensors and surfaces. Here, temperature dependent electrical transport measurements were undertaken on a recently developed screen-printed, multi-component, nanocomposite ink to develop a detailed understanding of the electrical transport mechanisms. A theoretical model combining contributions from linear percolative conduction, and non-linear conduction attributed to field-assisted quantum tunneling, successfully describes the temperature dependent conduction observed.
    ACS Applied Materials & Interfaces 07/2014; · 5.90 Impact Factor
  • D M Burn, T P A Hase, D Atkinson
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    ABSTRACT: Modification of the magnetic properties in a thin-film ferromagnetic/non-magnetic bilayer system by low-dose focused ion-beam (FIB) induced intermixing is demonstrated. The highly localized capability of FIB may be used to locally control magnetic behaviour at the nanoscale. The magnetic, electronic and structural properties of NiFe/Au bilayers were investigated as a function of the interfacial structure that was actively modified using focused Ga(+) ion irradiation. Experimental work used MOKE, SQUID, XMCD as well as magnetoresistance measurements to determine the magnetic behavior and grazing incidence x-ray reflectivity to elucidate the interfacial structure. Interfacial intermixing, induced by low-dose irradiation, is shown to lead to complex changes in the magnetic behavior that are associated with monotonic structural evolution of the interface. This behavior may be explained by changes in the local atomic environment within the interface region resulting in a combination of processes including the loss of moment on Ni and Fe, an induced moment on Au and modifications to the spin-orbit coupling between Au and NiFe.
    Journal of Physics Condensed Matter 05/2014; 26(23):236002. · 2.22 Impact Factor
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    ABSTRACT: The influence of interfacial intermixing on the picosecond magnetization dynamics of ferromagnetic/non-magnetic thin-film bilayers was studied. Low-dose focused-ion-beam irradiation was used to induce intermixing across the interface between a 10 nm Ni81Fe19 layer and a 2–3 nm capping layer of either Au or Cr. Time-resolved magneto-optical Kerr effect was used to study magnetization dynamics as a function of ion-beam dose. With an Au cap, the damping of the un-irradiated bilayer was comparable with native Ni81Fe19 and increased with increasing ion dose. In contrast, for Ni81Fe19/Cr the damping was higher than that for native Ni81Fe19, but the damping decreased with increasing dose.
    Applied Physics Letters 01/2014; 104(24):242410-242410-5. · 3.79 Impact Factor
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    ABSTRACT: 1/f noise spectroscopy is used to investigate charge conduction networks within polymer blend space-charge-limited diodes (SCLDs) fabricated from regioregular poly(3-hexylthiophene) (P3HT) and either isotactic-polystyrene (i-PS) or amorphous-polystyrene (a-PS). Conducting AFM measurements showed that i-PS blends have heterogeneous conduction characterised by current 'hotspots', whereas a-PS blends showed homogeneous conduction. The difference in conducting networks between blends was clearly revealed when examining the noise spectra for the range of blend devices. Furthermore, the shape of the noise spectra suggested that as the blend composition changed, the charges sampled differing breadths of the density of states. These data suggest that noise measurements can be used as an informative technique to electrically characterise the effects of blend morphology and its effects within polymer electronic devices.
    Journal of Materials Chemistry 01/2014; 2(9):1742-1748. · 5.97 Impact Factor
  • David Burn, Erhan Arac, Del Atkinson
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    ABSTRACT: The magnetization reversal processes in ferromagnetic nanowires with sinusoidally modulated edges were investigated as a function of modulation amplitude and wavelength. The reversal processes were studied in two regimes: nucleation controlled reversal and magnetization reversal mediated by domain-wall propagation. In the latter case, domain walls were introduced using both nucleation-pad structures and local pulsed-field injection techniques. The reversal behavior shows that competing effects govern the switching fields in these structures, giving a minimum as a function of modulation wavelength, showing promising results for improved control of domain-wall propagation behavior. The experimental results were interpreted with detailed micromagnetic simulations and an analytical model, based on the demagnetization effects of the modulation upon the spin structure of the wire. The analysis highlights consistent trends in the reversal behavior resulting from modulation, and, significantly, the switching behavior is found to be scalable in relation to the amplitude and wavelength.
    Physical Review B 09/2013; 88(10):104422. · 3.66 Impact Factor
  • David Burn, Del Atkinson
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    ABSTRACT: The control of individual magnetic domain walls has potential for future spintronic memory and data processing applications. The speed and reliability of such devices are determined by the dynamic properties of the domain walls. Typically, spin precession limitations lead to Walker breakdown, limiting wall velocity resulting in low mobility. Here, we show the suppression of Walker breakdown by the careful design of small amplitude periodic nanowire structuring to match the periodicity of domain wall spin structure transformations. This opens up a channel for energy dissipation via spin wave emission, allowing a domain wall to maintain its spin structure during propagation.
    Applied Physics Letters 06/2013; 102(24):242414. · 3.79 Impact Factor
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    ABSTRACT: Printable electronics is an innovative area of technology with great commercial potential. Here, a screen-printed functional ink, comprising a combination of semiconducting acicular particles, electrically insulating nanoparticles and a base polymer ink, is described that exhibits pronounced pressure sensitive electrical properties for applications in sensing and touch sensitive surfaces. The combination of these components in the as-printed ink yield a complex structure and a large and reproducible touch pressure sensitive resistance range. In contrast to the case for some composite systems, the resistance changes occur down to applied pressures of 13 Pa. Current-voltage measurements at fixed pressures show monotonic non-linear behaviour, which becomes more Ohmic at higher pressures and in all cases shows some hysteresis. The physical basis for conduction, particularly in the low pressure regime, can be described in terms of field assisted quantum mechanical tunnelling.
    Nanotechnology 03/2013; 24(16):165501. · 3.84 Impact Factor
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    ABSTRACT: A detailed investigation of magnetic field alignment of template released ferromagnetic nanowires has been undertaken. The distributions of magnetic field induced angular alignments of Ni0.8Fe0.2, Co, and Ni nanowires grown by electro-deposition and deposited onto substrates from a dilute suspension have been investigated as a function of magnetic field strengths up to ∼1 kOe. The nominal diameter of the nanowires investigated is either ∼200 nm (Ni0.8Fe0.2) or ∼300 nm (Co and Ni). The percentage of nanowires aligned within 0°–10° and 0°–20° of the applied field axis is observed to increase rapidly with increasing field strength up to ∼200 Oe, followed by a slower increase in alignment for the Ni0.8Fe0.2 and Ni wires and little improvement in alignment for the Co wires at higher fields. The proportion of aligned wires within 0°–20° is found to reach ∼82% for Ni0.8Fe0.2, ∼71% for Ni and only 53% for the Co nanowires using a magnetic field of 1 kOe. The influence of wire length upon the efficacy of magnetic alignment is investigated using Ni0.8Fe0.2 and Ni nanowires; this showed that the fractional alignment improved for longer nanowires.
    Journal of Applied Physics 07/2012; 112(1). · 2.21 Impact Factor
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    ABSTRACT: A detailed analysis of the structural and compositional changes in NiFe/Au bilayers induced by a focused ion beam (FIB) is presented. NiFe/Au bilayers with different thickness were irradiated with a focused 30 keV Ga+ ion beam, and the evaluation of the individual layers and interfaces were investigated systematically as a function of a broad range of irradiation fluence using grazing incidence x ray reflectivity (GIXRR) and angular dependent x ray fluorescence (ADXRF) techniques carried out at synchrotron radiation sources. Experimental data were collected from 1.3 mm x 4.5 mm structures, and irradiation of such a broad areas with a 100-nm-wide focused ion beam is a challenging task. Two irradiation regimes were identified: For Ga+ fluences < 15.6 x 1014 ion/cm2 (low dose regime), the main influence of the focused ion beam is on the interface and, beyond this dose (high dose regime), sputtering effects and ion implantation becomes significant, eventually causing amorphization of the bilayer system. The broadening of the NiFe/Au interface occurs even at the lowest dose, and above a critical fluence (Φ = 1.56 x 1014 ion/cm2) can be represented by an interfacial-intermixed layer (NixFeyAu(1-x-y); x = 0.5-0.6, y = 0.1-0.15) formed between the NiFe and Au layers. The thickness of this layer increases with irradiation fluence in the low dose regime. A linear relationship is found between the squared intermixing length and irradiation fluence, indicating that FIB-induced mixing is diffusion controlled. The ballistic model fails to describe FIB-induced intermixing, indicating that thermodynamical factors, which might be originated from FIB specific features, should be taken into account. Despite the complexity of the chemical and structural formation, good agreement between the experiment and theory highlights the functionality of the combined GIXRR and ADXRF techniques for studying intermixing in high resolution.
    Journal of Applied Physics 03/2012; 2012(111):044324. · 2.21 Impact Factor
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    ABSTRACT: Designs for two novel multilevel magnetic random access memory (MRAM) concepts are presented in this paper along with their associated control circuit architectures. Both the ChiralMEM and 3-D-MRAM concepts contain eight states with distinct electrical resistances, giving a 3 bit-per-cell capacity. Operation of the two memory concepts are presented along with designs for the circuitry in particular focusing on the conversion of three conventional binary bits to octal encoded data and the required sequence for writing eight states per cell using current-driven magnetic fields. Discrimination and subsequent conversion of the eight readout resistance levels back to three conventional binary bits are discussed along with the write sequence for controlling arrays of multibit memory cells.
    IEEE Transactions on Nanotechnology 01/2012; 11(1):63-70. · 1.80 Impact Factor
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    ABSTRACT: Chirality-dependent pinning of magnetic domain walls (DWs) has been observed in a planar Ni81Fe19 nanowire with two spatially separated notches on opposite edges of the wire. Using focused magneto-optical Kerr effect measurements of field-driven magnetization reversal we show that several different metastable remanent magnetization states can be formed reproducibly, and that the application of a component of magnetic field transverse to the wire axis maintains the chirality of DW as it interacts with both notches and overcomes the problems of chirality loss due to Walker breakdown.
    Journal of Applied Physics 01/2011; 109(1):013903-013903-5. · 2.21 Impact Factor
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    ABSTRACT: We report magnetoresistance, focused Kerr effect, and Lorentz microscopy experiments performed on a nanoscale Permalloy bridge connecting microscale pads. These pads can be switched from a parallel to antiparallel state through the application of small fields, causing a detectable magnetoresistance. We show that this switching field Hsw is modified by the application of a high current density (Jdc) through spin-transfer torque effects, caused by the spin-current interacting with the magnetization gradients generated by the device geometry, yielding an estimate for the spin-transfer torque efficiency ξ = dHsw/dJdc = 0.027±0.001 Oe/MA cm−2.
    Applied Physics Letters 11/2010; 97(20):202505-202505-3. · 3.79 Impact Factor
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    ABSTRACT: Using the surface, interface, and element specificity of x-ray resonant magnetic scattering in combination with x-ray magnetic circular dichroism, we have spatially resolved the magnetic spin polarization, and the associated interface proximity effect, in a Mn-based high-susceptibility material close to a ferromagnetic Co layer. We have measured the magnetic polarization of Mn and Cu3d electrons in paramagnetic CuMn alloy layers in [Co/Cu(x)/CuMn/Cu(x)]{sub 20} multilayer samples with varying copper layer thicknesses from x=0 to 25 {angstrom}. The size of the Mn and CuL{sub 2,3} edge dichroism shows a decrease in the Mn-induced polarization for increasing copper thickness indicating the dominant interfacial nature of the Cu and Mn spin polarization. The Mn polarization is much higher than that of Cu. Evidently, the Mn moment is a useful probe of the local spin density. Mn atoms appear to be coupled antiferromagnetically with the Co layer below x = 10 {angstrom} and ferromagnetically coupled above. In contrast, the interfacial Cu atoms remain ferromagnetically aligned to the Co layer for all thicknesses studied.
    Physical Review B 11/2010; 82(18). · 3.66 Impact Factor
  • David Burn, Del Atkinson
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    ABSTRACT: The magnetization behavior of a Permalloy thin-film (nominally Ni81Fe19) was investigated as a function of combined quasistatic and pulsed magnetic fields measured using magneto-optic Kerr effect magnetometry. We observed complex field dependent switching behavior that depends on the relative contributions to the total field of the quasistatic and pulsed fields. As the pulsed field amplitude was increased, complex switching behavior occurs for total fields in excess of the coercive field. A simple phenomenological domain wall propagation model suggests a qualitative understanding of this complex behavior based on Walker breakdown of the domain wall motion occurring in the Permalloy thin-film.
    Journal of Applied Physics 10/2010; 108(7):073926. · 2.21 Impact Factor
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    D S Eastwood, L K Bogart, D Atkinson
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    ABSTRACT: Micromagnetic modelling is used to establish the dimensional scaling dependence of the magnetic fields required for domain wall injection and chirality dependent pinning in planar permalloy nanowires with asymmetric structural notches. The wire width, thickness and notch constriction width are systematically varied whilst the axial magnetization is reversed under a globally applied magnetic field. A component of magnetic field transverse to the direction of propagation is applied throughout the reversal to control the domain wall chirality, and therefore determine the strength of pinning at the notch. We deduce that thicker wires with a narrower width cause larger depinning fields and stronger chirality dependent pinning, an observation relevant for domain wall memory applications, and particularly those exploiting chirality dependent pinning in nanowires.
    08/2010; 118(81).
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    ABSTRACT: We have investigated the threshold current density required for depinning a domain wall from constrictions in NiFe nanowires, which give rise to pinning potentials of fixed amplitude but variable profile. We observed it to vary linearly with the angle of the triangular constriction. These results are reproduced using micromagnetic simulations including the adiabatic and nonadiabatic spin-torque terms. By curve-fitting the calculated variations to the experimental results, we obtain the nonadiabaticity parameter beta=0.04(+/-0.005) and current spin polarization P=0.51(+/-0.02).
    Physical Review Letters 04/2009; 102(12):127203. · 7.73 Impact Factor
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    ABSTRACT: We report on domain wall pinning behavior and the potential-energy landscapes created by notches of two different geometries in planar Permalloy nanowires. Domain wall depinning was probed experimentally using spatially resolved magneto-optical Kerr effect measurements. The spin structure of pinned domain walls was determined using Lorentz microscopy, and domain wall pinning behavior was also analyzed using micromagnetic simulations, which are in good qualitative agreement with experimental results. All notch structures have dimensions that are comparable with the domain wall length scales. For the notch structures investigated, the depinning field experienced by a domain wall is found to be relatively insensitive to notch geometry although the pinning behavior is highly sensitive to both the wall type and the wall chirality spin structure. Energetically, the notches present both potential barriers and/or potential wells depending on the micromagnetic structure of the domain wall, and we find that the chirality of the domain wall is a key determinant of the pinning potential landscape. The pinning behavior of domain walls is discussed in detail, and direct quantitative measurements of the width and depth of the potential wells and/or barriers responsible for domain wall pinning are given for vortex walls pinned in triangular and rectangular notches.
    Physical Review B 02/2009; · 3.66 Impact Factor
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    L. K. Bogart, D. Atkinson
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    ABSTRACT: The anisotropic magnetoresistance (AMR) of individual magnetic domain walls in planar nanowires has been systematically investigated as a function of the micromagnetic wall structure, width, and thickness of the nanowire. Experimentally derived thickness dependence of both the thin film resistivity and the anisotropic magnetoresistance were incorporated into the calculations. We found that the AMR value can be used to identify the wall structure and that the wall resistance is sensitive to wire width for widths less than 300 nm. Furthermore, in comparison with the detailed analysis here, a previous simplified domain wall model significantly underestimates domain wall resistance in narrower wires.
    Applied Physics Letters 02/2009; · 3.79 Impact Factor
  • Nuclear Instruments & Methods in Physics Research Section B-beam Interactions With Materials and Atoms - NUCL INSTRUM METH PHYS RES B. 01/2009;
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    ABSTRACT: Using the surface, interface, and element specificity of x-ray resonant magnetic scattering in combination with x-ray magnetic circular dichroism, we have spatially resolved the polarization, and hence the spin accumulation in Mn high susceptibility material in close proximity to a ferromagnetic layer. The magnetic polarization of Mn and Cu 3d electrons in paramagnetic CuMn layers is detected in a Co/Cu(x)/CuMn structure for varying copper layer thicknesses (x). The size of the Mn and Cu L2-3-edge dichroism shows a decrease in the polarization for increasing copper thickness indicating the dominant interfacial nature of the Cu and Mn spin polarization. The Mn polarization appears to be much higher than that of Cu.
    Journal of Applied Physics 01/2009; 105. · 2.21 Impact Factor

Publication Stats

2k Citations
299.08 Total Impact Points

Institutions

  • 2001–2014
    • Durham University
      • Department of Physics
      Durham, England, United Kingdom
  • 2009
    • University of Leeds
      • School of Physics and Astronomy
      Leeds, ENG, United Kingdom
  • 2005
    • The University of Sheffield
      • Department of Materials Science and Engineering
      Sheffield, ENG, United Kingdom
    • Imperial College London
      • Department of Physics
      Londinium, England, United Kingdom
  • 2000
    • Universidad del País Vasco / Euskal Herriko Unibertsitatea
      • Departamento de Electricidad y Electrónica
      Leioa, Basque Country, Spain
  • 1993–2000
    • University of Bath
      • Department of Physics
      Bath, ENG, United Kingdom