Andrew Kent - Academia.edu (original) (raw)
Papers by Andrew Kent
physica status solidi (a), 2007
Journal of Applied Physics, 2021
Solitonic magnetic excitations such as domain walls and, specifically, skyrmionics enable the pos... more Solitonic magnetic excitations such as domain walls and, specifically, skyrmionics enable the possibility of compact, high density, ultrafast, allelectronic, low-energy devices, which is the basis for the emerging area of skyrmionics. The topological winding of skyrmion spins affects their overall lifetime, energetics, and dynamical behavior. In this Perspective, we discuss skyrmionics in the context of the present-day solid-state memory landscape and show how their size, stability, and mobility can be controlled by material engineering, as well as how they can be nucleated and detected. Ferrimagnets near their compensation points are promising candidates for this application, leading to a detailed exploration of amorphous CoGd as well as the study of emergent materials such as Mn 4 N and inverse Heusler alloys. Along with material properties, geometrical parameters such as film thickness, defect density, and notches can be used to tune skyrmion properties, such as their size and stability. Topology, however, can be a double-edged sword, especially for isolated metastable skyrmions, as it brings stability at the cost of additional damping and deflective Magnus forces compared to domain walls. Skyrmion deformation in response to forces also makes them intrinsically slower than domain walls. We explore potential analog applications of skyrmions, including temporal memory at low density-one skyrmion per racetrack-that capitalizes on their near ballistic current-velocity relation to map temporal data to spatial data and decorrelators for stochastic computing at a higher density that capitalizes on their interactions. We summarize the main challenges of achieving a skyrmionics technology, including maintaining positional stability with very high accuracy and electrical readout, especially for small ferrimagnetic skyrmions, deterministic nucleation, and annihilation and overall integration with digital circuits with the associated circuit overhead.
Journal of Physics D: Applied Physics, 2017
Building upon the success and relevance of the 2014 Magnetism Roadmap, this 2017 Magnetism Roadma... more Building upon the success and relevance of the 2014 Magnetism Roadmap, this 2017 Magnetism Roadmap edition follows a similar general layout, even if its focus is naturally shifted, and a different group of experts and, thus, viewpoints are being collected and presented. More importantly, key developments have changed the research landscape in very relevant ways, so that a novel view onto some of the most crucial developments is warranted, and thus, this 2017 Magnetism Roadmap article is a timely endeavour. The change in landscape is hereby not exclusively scientific, but also reflects the magnetism related industrial application portfolio. Specifically, Hard Disk Drive technology, which still dominates digital storage and will continue to do so for many years, if not decades, has now limited its footprint in the scientific Topical Review Original content from this work may be used under the terms of the Creative Commons Attribution 3.0 licence. Any further distribution of this work must maintain attribution to the author(s) and the title of the work, journal citation and DOI.
Physical Review B, 2006
Spin transfer in asymmetric Co/Cu/Co bilayer magnetic nanopillars junctions has been studied at l... more Spin transfer in asymmetric Co/Cu/Co bilayer magnetic nanopillars junctions has been studied at low temperature as a function of free-layer thickness. The phase diagram for current-induced magnetic excitations has been determined for magnetic fields up to 7.5 T applied perpendicular to the junction surface and free-layers thicknesses from 2 to 5 nm. The junction magnetoresistance is independent of thickness. The critical current for magnetic excitations decreases linearly with decreasing free-layer thickness, but extrapolates to a finite critical current in the limit of zero thickness. The limiting current is in quantitative agreement with that expected due to a spinpumping contribution to the magnetization damping. It may also be indicative of a decrease in the spin-transfer torque efficiency in ultrathin magnetic layers.
Physical Review Letters, 2003
Current-induced magnetization dynamics in Co/Cu/Co trilayer nanopillars (∼100 nm in diameter) hav... more Current-induced magnetization dynamics in Co/Cu/Co trilayer nanopillars (∼100 nm in diameter) have been studied experimentally at low temperatures for large applied fields perpendicular to the layers. At 4.2 K an abrupt and hysteretic increase in resistance is observed at high current densities for one polarity of the current, comparable to the giant magnetoresistance (GMR) effect observed at low fields. A micromagnetic model, that includes a spin-transfer torque, suggests that the current induces a complete reversal of the thin Co layer to alignment antiparallel to the applied field-that is, to a state of maximum magnetic energy.
Physical Review B, 2005
Current-induced magnetic excitations in Co/Cu/Co bilayer nanopillars (∼50 nm in diameter) have be... more Current-induced magnetic excitations in Co/Cu/Co bilayer nanopillars (∼50 nm in diameter) have been studied experimentally at low temperatures for large applied fields perpendicular to the layers. At sufficiently high current densities excitations, which lead to a decrease in differential resistance, are observed for both current polarities. Such bipolar excitations are not expected in a single domain model of spin-transfer. We propose that at high current densities strong asymmetries in the longitudinal spin accumulation cause spin-wave instabilities transverse to the current direction in bilayer samples, similar to those we have reported for single magnetic layer junctions.
Journal of Magnetism and Magnetic Materials, 2014
We present a systematic experimental study of the spin-torque-induced magnetic switching statisti... more We present a systematic experimental study of the spin-torque-induced magnetic switching statistics at room temperature, using all-perpendicularly magnetized spin-valves as a model system. Three physical regimes are distinguished: a short-time ballistic limit below a few nanoseconds, where spin-torque dominates the reversal dynamics from a thermal distribution of initial conditions; a long time limit, where the magnetization reversal probability is determined by spin-torque-amplified thermal activation; and a cross-over regime, where the spin-torque and thermal agitation both contribute. For a basic quantitative understanding of the physical processes involved, an analytical macrospin model is presented which contains both spin-torque dynamics and finite temperature effects. The latter was treated rigorously using a Fokker-Plank formalism, and solved numerically for specific sets of parameters relevant to the experiments to determine the switching probability behavior in the short-time and cross-over regimes. This analysis shows that thermal fluctuations during magnetization reversal greatly affect the switching probability over all the time scales studied, even in the short-time limit.
Physical Review B, 2006
A unique type of ferromagnetic microelement is explored. Unlike conventional ferromagnetic elemen... more A unique type of ferromagnetic microelement is explored. Unlike conventional ferromagnetic elements, the entities studied here are not separated topographically from each other, but embedded into a surrounding, continuous film. Fabrication of such elements is achieved by local irradiation of antiferromagnetically coupled Fe/ Cr/ Fe trilayers with 30 keV Ga + ions, which cause a local destruction of the Cr interlayer in these systems. As a result, a transition to ferromagnetic properties is induced within micron-sized irradiated areas, which act as ferromagnetic elements. Since the surrounding area of these elements consists of magnetic material, i.e., two Fe layers which are still antiferromagnetically coupled, interesting coupling phenomena in lateral direction can be observed. In particular, the magnetic configuration within such systems leads to the formation of complex domain walls at the boundary between irradiated and nonirradiated areas exhibiting different types of fine structure. In addition, it is found that the capability of storing information in the form of magnetic single domain states in remanence depends on the geometry of the patterned elements. The fabrication method presented here is an efficient way to create magnetic model systems on the micron scale of different geometries and sizes for comparative studies of micromagnetics and magnetization reversal processes.
Journal of Applied Physics, 2015
We present the results of zero temperature macrospin and micromagnetic simulations of spin transf... more We present the results of zero temperature macrospin and micromagnetic simulations of spin transfer switching of thin film nanomagnets in the shape of an ellipse with a spin-polarization tilted out of the layer plane. The perpendicular component of the spin-polarization is shown to increase the reversal speed, leading to a lower current for switching in a given time. However, for tilt angles larger than a critical angle, the layer magnetization starts to precess about an out-of-plane axis, which leads to a final magnetization state that is very sensitive to simulation conditions. As the ellipse lateral size increases, this out-of-plane precession is suppressed, due to the excitation of spatially non-uniform magnetization modes. V
We investigate the effect of eddy currents on ferromagnetic resonance (FMR) in ferromagnetnormal ... more We investigate the effect of eddy currents on ferromagnetic resonance (FMR) in ferromagnetnormal metal (FM/NM) bilayer structures. Eddy-current effects are usually neglected for NM layer thicknesses below the microwave (MW) skin depth (800 nm for Au at 10 GHz). However, we show that in much thinner NM layers (10-100 nm of Au or Cu) they induce a phase shift in the FMR excitation when the MW driving field has a component perpendicular to the sample plane. This results in a strong asymmetry of the measured absorption lines. In contrast to typical eddy-current effects, the asymmetry is larger for thinner NM layers and is tunable through changing the sample geometry and the NM layer thickness.
We consider magnetization reversal due to thermal fluctuations in thin, submicron-scale rings. Th... more We consider magnetization reversal due to thermal fluctuations in thin, submicron-scale rings. These mesoscopic ferromagnetic particles are of particular interest as potential information storage components in magnetoelectronic devices, because their lack of sharp ends result in a magnetization density that is significantly more stable against reversal than in thin needles and other geometries. Their two-dimensional nature and rotational symmetry allow
Spintronics VII, 2014
In this work we report on time resolved magnetization reversal driven by spin transfer torque in ... more In this work we report on time resolved magnetization reversal driven by spin transfer torque in an orthogonal spin transfer (OST) magnetic tunnel junction device. We focus on the transitions from parallel (P) to antiparallel (AP) states and the reverse transitions (AP to P) under the influence of 10 ns voltage pulses. The electrical response is monitored with a fast real-time oscilloscope and thus timing information of the reversal process is obtained. The P to AP transition switching time decreases with increasing current and shows only direct switching behavior. The AP to P transition shows similar behavior, but has a broader distribution of switching times at high currents. The rare AP to P switching events that occur at later times are due to the occurrence of a pre-oscillation, which could be identified in time resolve voltage traces. A possible origin of these pre-oscillations is seen in micromagnetic simulations, where they are associated with the breakdown of the uniform precession of the magnetization, and lead to reversal of the magnetization at later times.
Journal of Physics: Conference Series, 2009
GIS-based models have been developed to map ambient PM10 and PM25 mass concentrations across the ... more GIS-based models have been developed to map ambient PM10 and PM25 mass concentrations across the UK. The resulting maps are used for the assessments of air quality required by the EU ambient air quality directives, health impact assessment and the development of UK air quality policy. Maps are presented for 2006 along with projections to 2020. The largest single contribution to the UK population-weighted mean annual mean background concentrations of PM10 in 2006 is estimated to be from secondary PM (43%), followed by the contribution from primary PM (24%). Concentrations are predicted to decline by 15% for PM10 and 13% for PM25 over the period from 2006 to 2020. The extent of exceedence of the 24-hour limit value is predicted to decline from 1.9% to 0.1% of urban major roads over the same period. The potential health benefits of reductions in ambient PM are large. A reduction in concentration of 0.93 mug m-3 as a result of a possible package of measures has been estimated within the UK Air Quality Strategy to result in a reduction in life years lost of approximately 2 - 4 million over a period of 100 years.
Journal of Materials Chemistry, 2011
A series of S ¼ ½ organic radicals-TEMPO (TEMPO ¼ 2,2,6,6-tetramethylpiperidine-1-oxyl), 4methoxy... more A series of S ¼ ½ organic radicals-TEMPO (TEMPO ¼ 2,2,6,6-tetramethylpiperidine-1-oxyl), 4methoxy-TEMPO, and 4-oxo-TEMPO-have been included in the cavities of porous lamellar host frameworks constructed from guanidinium (G) cations and organodisulfonate (DS) anions, which assemble into hydrogen-bonded guanidinium sulfonate (GS) sheets connected by organodisulfonate pillars that create cavities between the sheets. The pillars can project above or below the GS sheet in different configurations, affording various architectural framework isomers whose selectivity is determined by the relative sizes of the guests and the pillars. Each framework isomer is endowed with uniquely sized and shaped cavities, which influence the molecular arrangement of the radical guests included within the host lattice. Notably, TEMPO guests in the 1-D channels of simple brick G 2 BPDS host (BPDS ¼ biphenyl-4,4 0-disulfonate) arrange as a two-leg ladder (a chain of dimers), whereas TEMPO guests in the cavities of zigzag brick G 2 NDS host (NDS ¼ naphthalene-2,6-disulfonate) form dimers that are distributed into a 2-D square-planar lattice. The high-temperature magnetic susceptibility, c mol , conformed with Curie-Weiss behavior, with negative Weiss constants, consistent with bulk antiferromagnetic ordering. The N eel temperatures (T N) for G 2 BPDS$2(TEMPO) and G 2 NDS$2(TEMPO) were found to be 3.83 K and 4.10 K, respectively, as indicated by the presence of a broad maximum in c mol near these temperatures. In contrast, T N for host-free crystalline TEMPO is lower at 2.3 K, suggesting that enforced ordering can alter the magnetic properties of the TEMPO guests. The low-temperature c mol data for G 2 BPDS$2(TEMPO) agreed well with the Troyer two-leg ladder spin model, with J/k B ¼ À7.29 K along the rungs of the ladder (intra-dimer) and J/k B ¼ À0.45 K along the legs of the ladder (inter-dimer). The c mol data for G 2 NDS$2(TEMPO) agreed best with Bleaney-Bowers behavior for isolated dimers with J/k B ¼ À3.31 K for the encapsulated TEMPO dimer. These results demonstrate that magnetic behavior, including dimensionality, can be manipulated through judicious selection of host and magnetic guests.
Journal of Applied Physics, 1997
Journal of Applied Physics, 1998
The effect of probe geometry on the classical Hall response to a weak perpendicular inhomogeneous... more The effect of probe geometry on the classical Hall response to a weak perpendicular inhomogeneous magnetic field is studied numerically. An electric potential equation based on a classical model of the two-dimensional Hall effect is solved numerically for a generalized flux distribution to find the Hall response function. We find that the magnitude and shape of this response function is strongly affected by probe geometry. Asymmetric cross-shaped Hall probes, with one narrow voltage lead, have a strongly peaked response more localized than in symmetric probe arrangements. This suggests novel lithographic patterns that may improve the spatial resolution of Hall magnetometry and scanning Hall probe microscopy.
Journal of applied …, 2006
Current-induced excitations in bilayer magnetic nanopillars have been studied with large magnetic... more Current-induced excitations in bilayer magnetic nanopillars have been studied with large magnetic fields applied perpendicular to the layers at low temperature. Junctions investigated all have Cu/ Co/ Cu/ Co/ Cu as core layer stacks. Two types of such junctions are compared, one with the core stack sandwiched between Pt layers ͑type A͒, the other with Pt only on one side of the stack ͑type B͒. Transport measurements show that these two types of junctions have similar magnetoresistance and slope of critical current with respect to field, while A samples have higher resistance. The high-field bipolar excitation, as was previously reported ͓Özyilmaz et al., Phys. Rev. B 71, 140403͑R͒ ͑2005͔͒, is present in B samples only. This illustrates the importance of contact layers to spin-current-induced phenomena. This also confirms a recent prediction on such spin-wave excitations in bilayers.
Journal of applied …, 2005
It has been shown within density-functional theory that in Mn 12 acetate there are effects due to... more It has been shown within density-functional theory that in Mn 12 acetate there are effects due to disorder by solvent molecules and a coupling between vibrational and electronic degrees of freedom. We calculate the in-plane principal axes of the second-order anisotropy caused by the second effect and compare them with those of the fourth-order anisotropy due to the first effect. We find that the two types of the principal axes are not commensurate with each other, which results in a complete quenching of the tunnel-splitting oscillation as a function of an applied transverse field.
… IEEE Transactions on, 2001
Studies of the magnetic properties of Fe microfabricated wires with a mechanically stable 40 to 1... more Studies of the magnetic properties of Fe microfabricated wires with a mechanically stable 40 to 120 nm-wide constriction produced with focused ion beam are reported. The 2 to 20 m-wide wires are fabricated by photolithography from 3 nm and 25 nm-thick epitaxial (110) bcc Fe films. This fabrication method appears to preserve the magnetic properties of the constriction. The structures are studied by MFM and by magneto-optic Kerr effect (MOKE) measurements. The ratio of the wire linewidth to its thickness determines the demagnetization factor and, therefore, influences the coercivity of the wire. Both a step in the width of the wire and a nano-constriction in the middle of the wire are found to be domain wall (DW) pinning centers.
Journal of Applied Physics, 2011
Spin-wave excitations due to spin-momentum transfer in ferromagnetic thin films will enable new t... more Spin-wave excitations due to spin-momentum transfer in ferromagnetic thin films will enable new types of information processing and memory storage. Here, we show how arrays of spin-torque nano-oscillators (STNOs) can be used to create anisotropic spin-wave interference patterns, which can be used for information processing. We consider STNO arrays contacting a thin ferromagnetic film. Contacts to the film (including the
physica status solidi (a), 2007
Journal of Applied Physics, 2021
Solitonic magnetic excitations such as domain walls and, specifically, skyrmionics enable the pos... more Solitonic magnetic excitations such as domain walls and, specifically, skyrmionics enable the possibility of compact, high density, ultrafast, allelectronic, low-energy devices, which is the basis for the emerging area of skyrmionics. The topological winding of skyrmion spins affects their overall lifetime, energetics, and dynamical behavior. In this Perspective, we discuss skyrmionics in the context of the present-day solid-state memory landscape and show how their size, stability, and mobility can be controlled by material engineering, as well as how they can be nucleated and detected. Ferrimagnets near their compensation points are promising candidates for this application, leading to a detailed exploration of amorphous CoGd as well as the study of emergent materials such as Mn 4 N and inverse Heusler alloys. Along with material properties, geometrical parameters such as film thickness, defect density, and notches can be used to tune skyrmion properties, such as their size and stability. Topology, however, can be a double-edged sword, especially for isolated metastable skyrmions, as it brings stability at the cost of additional damping and deflective Magnus forces compared to domain walls. Skyrmion deformation in response to forces also makes them intrinsically slower than domain walls. We explore potential analog applications of skyrmions, including temporal memory at low density-one skyrmion per racetrack-that capitalizes on their near ballistic current-velocity relation to map temporal data to spatial data and decorrelators for stochastic computing at a higher density that capitalizes on their interactions. We summarize the main challenges of achieving a skyrmionics technology, including maintaining positional stability with very high accuracy and electrical readout, especially for small ferrimagnetic skyrmions, deterministic nucleation, and annihilation and overall integration with digital circuits with the associated circuit overhead.
Journal of Physics D: Applied Physics, 2017
Building upon the success and relevance of the 2014 Magnetism Roadmap, this 2017 Magnetism Roadma... more Building upon the success and relevance of the 2014 Magnetism Roadmap, this 2017 Magnetism Roadmap edition follows a similar general layout, even if its focus is naturally shifted, and a different group of experts and, thus, viewpoints are being collected and presented. More importantly, key developments have changed the research landscape in very relevant ways, so that a novel view onto some of the most crucial developments is warranted, and thus, this 2017 Magnetism Roadmap article is a timely endeavour. The change in landscape is hereby not exclusively scientific, but also reflects the magnetism related industrial application portfolio. Specifically, Hard Disk Drive technology, which still dominates digital storage and will continue to do so for many years, if not decades, has now limited its footprint in the scientific Topical Review Original content from this work may be used under the terms of the Creative Commons Attribution 3.0 licence. Any further distribution of this work must maintain attribution to the author(s) and the title of the work, journal citation and DOI.
Physical Review B, 2006
Spin transfer in asymmetric Co/Cu/Co bilayer magnetic nanopillars junctions has been studied at l... more Spin transfer in asymmetric Co/Cu/Co bilayer magnetic nanopillars junctions has been studied at low temperature as a function of free-layer thickness. The phase diagram for current-induced magnetic excitations has been determined for magnetic fields up to 7.5 T applied perpendicular to the junction surface and free-layers thicknesses from 2 to 5 nm. The junction magnetoresistance is independent of thickness. The critical current for magnetic excitations decreases linearly with decreasing free-layer thickness, but extrapolates to a finite critical current in the limit of zero thickness. The limiting current is in quantitative agreement with that expected due to a spinpumping contribution to the magnetization damping. It may also be indicative of a decrease in the spin-transfer torque efficiency in ultrathin magnetic layers.
Physical Review Letters, 2003
Current-induced magnetization dynamics in Co/Cu/Co trilayer nanopillars (∼100 nm in diameter) hav... more Current-induced magnetization dynamics in Co/Cu/Co trilayer nanopillars (∼100 nm in diameter) have been studied experimentally at low temperatures for large applied fields perpendicular to the layers. At 4.2 K an abrupt and hysteretic increase in resistance is observed at high current densities for one polarity of the current, comparable to the giant magnetoresistance (GMR) effect observed at low fields. A micromagnetic model, that includes a spin-transfer torque, suggests that the current induces a complete reversal of the thin Co layer to alignment antiparallel to the applied field-that is, to a state of maximum magnetic energy.
Physical Review B, 2005
Current-induced magnetic excitations in Co/Cu/Co bilayer nanopillars (∼50 nm in diameter) have be... more Current-induced magnetic excitations in Co/Cu/Co bilayer nanopillars (∼50 nm in diameter) have been studied experimentally at low temperatures for large applied fields perpendicular to the layers. At sufficiently high current densities excitations, which lead to a decrease in differential resistance, are observed for both current polarities. Such bipolar excitations are not expected in a single domain model of spin-transfer. We propose that at high current densities strong asymmetries in the longitudinal spin accumulation cause spin-wave instabilities transverse to the current direction in bilayer samples, similar to those we have reported for single magnetic layer junctions.
Journal of Magnetism and Magnetic Materials, 2014
We present a systematic experimental study of the spin-torque-induced magnetic switching statisti... more We present a systematic experimental study of the spin-torque-induced magnetic switching statistics at room temperature, using all-perpendicularly magnetized spin-valves as a model system. Three physical regimes are distinguished: a short-time ballistic limit below a few nanoseconds, where spin-torque dominates the reversal dynamics from a thermal distribution of initial conditions; a long time limit, where the magnetization reversal probability is determined by spin-torque-amplified thermal activation; and a cross-over regime, where the spin-torque and thermal agitation both contribute. For a basic quantitative understanding of the physical processes involved, an analytical macrospin model is presented which contains both spin-torque dynamics and finite temperature effects. The latter was treated rigorously using a Fokker-Plank formalism, and solved numerically for specific sets of parameters relevant to the experiments to determine the switching probability behavior in the short-time and cross-over regimes. This analysis shows that thermal fluctuations during magnetization reversal greatly affect the switching probability over all the time scales studied, even in the short-time limit.
Physical Review B, 2006
A unique type of ferromagnetic microelement is explored. Unlike conventional ferromagnetic elemen... more A unique type of ferromagnetic microelement is explored. Unlike conventional ferromagnetic elements, the entities studied here are not separated topographically from each other, but embedded into a surrounding, continuous film. Fabrication of such elements is achieved by local irradiation of antiferromagnetically coupled Fe/ Cr/ Fe trilayers with 30 keV Ga + ions, which cause a local destruction of the Cr interlayer in these systems. As a result, a transition to ferromagnetic properties is induced within micron-sized irradiated areas, which act as ferromagnetic elements. Since the surrounding area of these elements consists of magnetic material, i.e., two Fe layers which are still antiferromagnetically coupled, interesting coupling phenomena in lateral direction can be observed. In particular, the magnetic configuration within such systems leads to the formation of complex domain walls at the boundary between irradiated and nonirradiated areas exhibiting different types of fine structure. In addition, it is found that the capability of storing information in the form of magnetic single domain states in remanence depends on the geometry of the patterned elements. The fabrication method presented here is an efficient way to create magnetic model systems on the micron scale of different geometries and sizes for comparative studies of micromagnetics and magnetization reversal processes.
Journal of Applied Physics, 2015
We present the results of zero temperature macrospin and micromagnetic simulations of spin transf... more We present the results of zero temperature macrospin and micromagnetic simulations of spin transfer switching of thin film nanomagnets in the shape of an ellipse with a spin-polarization tilted out of the layer plane. The perpendicular component of the spin-polarization is shown to increase the reversal speed, leading to a lower current for switching in a given time. However, for tilt angles larger than a critical angle, the layer magnetization starts to precess about an out-of-plane axis, which leads to a final magnetization state that is very sensitive to simulation conditions. As the ellipse lateral size increases, this out-of-plane precession is suppressed, due to the excitation of spatially non-uniform magnetization modes. V
We investigate the effect of eddy currents on ferromagnetic resonance (FMR) in ferromagnetnormal ... more We investigate the effect of eddy currents on ferromagnetic resonance (FMR) in ferromagnetnormal metal (FM/NM) bilayer structures. Eddy-current effects are usually neglected for NM layer thicknesses below the microwave (MW) skin depth (800 nm for Au at 10 GHz). However, we show that in much thinner NM layers (10-100 nm of Au or Cu) they induce a phase shift in the FMR excitation when the MW driving field has a component perpendicular to the sample plane. This results in a strong asymmetry of the measured absorption lines. In contrast to typical eddy-current effects, the asymmetry is larger for thinner NM layers and is tunable through changing the sample geometry and the NM layer thickness.
We consider magnetization reversal due to thermal fluctuations in thin, submicron-scale rings. Th... more We consider magnetization reversal due to thermal fluctuations in thin, submicron-scale rings. These mesoscopic ferromagnetic particles are of particular interest as potential information storage components in magnetoelectronic devices, because their lack of sharp ends result in a magnetization density that is significantly more stable against reversal than in thin needles and other geometries. Their two-dimensional nature and rotational symmetry allow
Spintronics VII, 2014
In this work we report on time resolved magnetization reversal driven by spin transfer torque in ... more In this work we report on time resolved magnetization reversal driven by spin transfer torque in an orthogonal spin transfer (OST) magnetic tunnel junction device. We focus on the transitions from parallel (P) to antiparallel (AP) states and the reverse transitions (AP to P) under the influence of 10 ns voltage pulses. The electrical response is monitored with a fast real-time oscilloscope and thus timing information of the reversal process is obtained. The P to AP transition switching time decreases with increasing current and shows only direct switching behavior. The AP to P transition shows similar behavior, but has a broader distribution of switching times at high currents. The rare AP to P switching events that occur at later times are due to the occurrence of a pre-oscillation, which could be identified in time resolve voltage traces. A possible origin of these pre-oscillations is seen in micromagnetic simulations, where they are associated with the breakdown of the uniform precession of the magnetization, and lead to reversal of the magnetization at later times.
Journal of Physics: Conference Series, 2009
GIS-based models have been developed to map ambient PM10 and PM25 mass concentrations across the ... more GIS-based models have been developed to map ambient PM10 and PM25 mass concentrations across the UK. The resulting maps are used for the assessments of air quality required by the EU ambient air quality directives, health impact assessment and the development of UK air quality policy. Maps are presented for 2006 along with projections to 2020. The largest single contribution to the UK population-weighted mean annual mean background concentrations of PM10 in 2006 is estimated to be from secondary PM (43%), followed by the contribution from primary PM (24%). Concentrations are predicted to decline by 15% for PM10 and 13% for PM25 over the period from 2006 to 2020. The extent of exceedence of the 24-hour limit value is predicted to decline from 1.9% to 0.1% of urban major roads over the same period. The potential health benefits of reductions in ambient PM are large. A reduction in concentration of 0.93 mug m-3 as a result of a possible package of measures has been estimated within the UK Air Quality Strategy to result in a reduction in life years lost of approximately 2 - 4 million over a period of 100 years.
Journal of Materials Chemistry, 2011
A series of S ¼ ½ organic radicals-TEMPO (TEMPO ¼ 2,2,6,6-tetramethylpiperidine-1-oxyl), 4methoxy... more A series of S ¼ ½ organic radicals-TEMPO (TEMPO ¼ 2,2,6,6-tetramethylpiperidine-1-oxyl), 4methoxy-TEMPO, and 4-oxo-TEMPO-have been included in the cavities of porous lamellar host frameworks constructed from guanidinium (G) cations and organodisulfonate (DS) anions, which assemble into hydrogen-bonded guanidinium sulfonate (GS) sheets connected by organodisulfonate pillars that create cavities between the sheets. The pillars can project above or below the GS sheet in different configurations, affording various architectural framework isomers whose selectivity is determined by the relative sizes of the guests and the pillars. Each framework isomer is endowed with uniquely sized and shaped cavities, which influence the molecular arrangement of the radical guests included within the host lattice. Notably, TEMPO guests in the 1-D channels of simple brick G 2 BPDS host (BPDS ¼ biphenyl-4,4 0-disulfonate) arrange as a two-leg ladder (a chain of dimers), whereas TEMPO guests in the cavities of zigzag brick G 2 NDS host (NDS ¼ naphthalene-2,6-disulfonate) form dimers that are distributed into a 2-D square-planar lattice. The high-temperature magnetic susceptibility, c mol , conformed with Curie-Weiss behavior, with negative Weiss constants, consistent with bulk antiferromagnetic ordering. The N eel temperatures (T N) for G 2 BPDS$2(TEMPO) and G 2 NDS$2(TEMPO) were found to be 3.83 K and 4.10 K, respectively, as indicated by the presence of a broad maximum in c mol near these temperatures. In contrast, T N for host-free crystalline TEMPO is lower at 2.3 K, suggesting that enforced ordering can alter the magnetic properties of the TEMPO guests. The low-temperature c mol data for G 2 BPDS$2(TEMPO) agreed well with the Troyer two-leg ladder spin model, with J/k B ¼ À7.29 K along the rungs of the ladder (intra-dimer) and J/k B ¼ À0.45 K along the legs of the ladder (inter-dimer). The c mol data for G 2 NDS$2(TEMPO) agreed best with Bleaney-Bowers behavior for isolated dimers with J/k B ¼ À3.31 K for the encapsulated TEMPO dimer. These results demonstrate that magnetic behavior, including dimensionality, can be manipulated through judicious selection of host and magnetic guests.
Journal of Applied Physics, 1997
Journal of Applied Physics, 1998
The effect of probe geometry on the classical Hall response to a weak perpendicular inhomogeneous... more The effect of probe geometry on the classical Hall response to a weak perpendicular inhomogeneous magnetic field is studied numerically. An electric potential equation based on a classical model of the two-dimensional Hall effect is solved numerically for a generalized flux distribution to find the Hall response function. We find that the magnitude and shape of this response function is strongly affected by probe geometry. Asymmetric cross-shaped Hall probes, with one narrow voltage lead, have a strongly peaked response more localized than in symmetric probe arrangements. This suggests novel lithographic patterns that may improve the spatial resolution of Hall magnetometry and scanning Hall probe microscopy.
Journal of applied …, 2006
Current-induced excitations in bilayer magnetic nanopillars have been studied with large magnetic... more Current-induced excitations in bilayer magnetic nanopillars have been studied with large magnetic fields applied perpendicular to the layers at low temperature. Junctions investigated all have Cu/ Co/ Cu/ Co/ Cu as core layer stacks. Two types of such junctions are compared, one with the core stack sandwiched between Pt layers ͑type A͒, the other with Pt only on one side of the stack ͑type B͒. Transport measurements show that these two types of junctions have similar magnetoresistance and slope of critical current with respect to field, while A samples have higher resistance. The high-field bipolar excitation, as was previously reported ͓Özyilmaz et al., Phys. Rev. B 71, 140403͑R͒ ͑2005͔͒, is present in B samples only. This illustrates the importance of contact layers to spin-current-induced phenomena. This also confirms a recent prediction on such spin-wave excitations in bilayers.
Journal of applied …, 2005
It has been shown within density-functional theory that in Mn 12 acetate there are effects due to... more It has been shown within density-functional theory that in Mn 12 acetate there are effects due to disorder by solvent molecules and a coupling between vibrational and electronic degrees of freedom. We calculate the in-plane principal axes of the second-order anisotropy caused by the second effect and compare them with those of the fourth-order anisotropy due to the first effect. We find that the two types of the principal axes are not commensurate with each other, which results in a complete quenching of the tunnel-splitting oscillation as a function of an applied transverse field.
… IEEE Transactions on, 2001
Studies of the magnetic properties of Fe microfabricated wires with a mechanically stable 40 to 1... more Studies of the magnetic properties of Fe microfabricated wires with a mechanically stable 40 to 120 nm-wide constriction produced with focused ion beam are reported. The 2 to 20 m-wide wires are fabricated by photolithography from 3 nm and 25 nm-thick epitaxial (110) bcc Fe films. This fabrication method appears to preserve the magnetic properties of the constriction. The structures are studied by MFM and by magneto-optic Kerr effect (MOKE) measurements. The ratio of the wire linewidth to its thickness determines the demagnetization factor and, therefore, influences the coercivity of the wire. Both a step in the width of the wire and a nano-constriction in the middle of the wire are found to be domain wall (DW) pinning centers.
Journal of Applied Physics, 2011
Spin-wave excitations due to spin-momentum transfer in ferromagnetic thin films will enable new t... more Spin-wave excitations due to spin-momentum transfer in ferromagnetic thin films will enable new types of information processing and memory storage. Here, we show how arrays of spin-torque nano-oscillators (STNOs) can be used to create anisotropic spin-wave interference patterns, which can be used for information processing. We consider STNO arrays contacting a thin ferromagnetic film. Contacts to the film (including the