We have used high-resolution scanning Hall probe microscopy to directly image the dynamics of vor... more We have used high-resolution scanning Hall probe microscopy to directly image the dynamics of vortex depinning as a transport critical current Jc is exceeded. Our microscope has 1--2 mum spatial resolution and ~30 mG field sensitivity. The samples used were high-quality niobium thin films patterned into 100-mum-wide strips with low critical currents. At very low fields (< 1 G) individual singly-quantized vortices can be clearly resolved. At higher fields (up to several kG), although individual vortices can no longer be resolved, their presence is very clear from the strong fluctuations in the local field. For J < J_c, a slope begins to appear in the magnetic profile due to the flow of the current itself. When Jc is first exceeded, vortices in certain regions of the sample appear to vanish. This is presumably due to the fast-moving vortices blurring under the low ( ~100 Hz) bandwidth of our probe. Vortices in other regions remain pinned and visible. As the current is further increased, these vortices also appear to vanish. Thus we have direct spatial images of plastic flow and sequential depinning of vortices. When the current is reduced back to zero, there are regions where the vortices have been permanently removed from the sample; newly created antivortices are also visible.
Journal of Micromechanics and Microengineering, 2003
In this paper we report the development of a new III-V cantilever-based atomic force sensor with ... more In this paper we report the development of a new III-V cantilever-based atomic force sensor with piezoresistive detection and an integrated Hall probe for scanning Hall probe microscopy. We give detailed descriptions of the fabrication process and characterization of the new integrated sensor, which will allow the investigation of magnetic samples with no sample preparation at both room and cryogenic temperatures. We also introduce a novel piezoresistive material based on the ternary alloy n+-Al0.4Ga0.6As which allows us to achieve a cantilever deflection sensitivity DeltaR/(R Deltaz) = 2 × 10-6 Å-1 at room temperature.
We demonstrate controllable dual-bath electrodeposition of nickel on architecture-tunable three-d... more We demonstrate controllable dual-bath electrodeposition of nickel on architecture-tunable three-dimensional (3D) silver microcrystals. Magnetic hysteresis loops of individual highly faceted Ag-Ni core-shell elements reveal magnetization reversal that comprises multiple sharp steps corresponding to different stable magnetic states. Finite-element micromagnetic simulations on smaller systems show several jumps during magnetization reversal which correspond to transitions between different magnetic vortex states. Structures of this type could be realizations of an advanced magnetic data storage architecture whereby each element represents one multibit, storing a combination of several conventional bits depending on the overall number of possible magnetic states associated with the 3D core-shell shape.
Scanning Hall probe and local Hall magnetometry measurements have been used to investigate flux d... more Scanning Hall probe and local Hall magnetometry measurements have been used to investigate flux distributions in large mesoscopic superconducting disks with sizes that lie near the crossover between the bulk and mesoscopic vortex regimes. Results obtained by directly mapping the magnetic induction profiles of the disks at different applied fields can be quite successfully fitted to analytic models which assume a continuous distribution of flux in the sample. At low fields, however, we do observe clear signatures of the underlying discrete vortex structure and can resolve the characteristic mesoscopic compression of vortex clusters in increasing magnetic fields. Even at higher fields, where single vortex resolution is lost, we are still able to track configurational changes in the vortex patterns, since competing vortex orders impose unmistakable signatures on "local" magnetisation curves as a function of the applied field. Our observations are in excellent agreement with molecular dynamics numerical simulations which lead us to a natural definition of the lengthscale for the crossover between discrete and continuum behaviours in our system.
A scanning Hall probe microscope is used to study flux pinning in a thin superconducting Pb film ... more A scanning Hall probe microscope is used to study flux pinning in a thin superconducting Pb film covering a square array of single-domain Co dots with in-plane magnetization. We show that single flux quanta of opposite sign thread the superconducting film below Tc at the opposite poles of these dipoles. Depending on the polarity of the applied field, flux lines are attracted to a specific pole of the dipoles, due to the direct interaction with the vortexlike structures induced by the local stray field.
Scanning Hall Probe Microscopy (SHPM)[1] is a quantitative and non-invasive technique to image ma... more Scanning Hall Probe Microscopy (SHPM)[1] is a quantitative and non-invasive technique to image magnetic samples with high spatial and magnetic field resolution: ~ 120nm & 60mG/Hz^1/2 at room temperature. A nano-Hall probe is scanned over the sample surface to measure the surface magnetic fields using conventional scanning tunneling microscopy-positioning techniques. We have developed new down to ~120x120nm size Bi and InSb Hall probes machined FIB milling. 120nm Bi sensors[2] have a sensitivity of 3.3x10-4 Ω/G and a noise level of 7.2 G/Hz^1/2 . The new InSb sensors have a sensitivity of 0.03 Ω/G and a noise level of 8 mG/Hz^1/2 at room temperature. This corresponds to ×8 better noise performance compared to conventional GaAs, based sensors used in RT-SHPM. We used these new sensors to study magnetic domain structures of crystalline garnet films and Ni_80Fe_20 rectangular permalloy microstructures microfabricated by lift-off technique. Bi and InSb nano-Hall probes are shown to be high spatial resolution, high sensitivity and low noise alternatives to GaAs sensors for RT-SHPM. There seems to be more room for improving the spatial resolution down to <50nm and the noise of Hall probes to 1mG/Hz^1/2 at room temperature. [1] A. Oral et. al. Appl. Phys. Lett., 69, 1324 (1996), A. Sandhu et. al., Jpn. J. Appl. Phys. 40(5B), L524 (2001) [2] A. Sandhu, H. Masuda, K. Kurosawa K, A. Oral and S.J. Bending, Electronics Letters 37 (22), 1335-1336 (2001).
Electrochemistry offers highly flexible routes to fabrication of a wide variety of mesostructures... more Electrochemistry offers highly flexible routes to fabrication of a wide variety of mesostructures, including three-dimensional (3D) crystallites, thin films and nanowires. Using this method we have grown various 3D superconducting Pb mesostructures with vastly different morphologies. We present here results on a truncated(half)-icosahedron with a hexagonal base and a tripod structure with a triangular base. Using Hall probe magnetometry we have obtained magnetisation curves for these structures at several temperatures and see evidence of geometry-driven flux entry and exit as well as flux trapping caused by specific sample geometries. We also observe behaviour that we interpret in terms of the formation of giant vortices, bearing in mind that bulk Pb is a type-I superconducting material.
We have imaged interacting crossing pancake vortex (PV) and Josephson vortex (JV) lattices in hig... more We have imaged interacting crossing pancake vortex (PV) and Josephson vortex (JV) lattices in highly anisotropic Bi2Sr2CaCu2O8+δ single crystals under tilted magnetic fields. The dependence of vortex structures on in-plane field is in good quantitative agreement with theoretical predictions, yielding an almost temperature-independent anisotropy parameter of γ=640+/-25. We directly confirm that the PV/JV attraction arises from small PV displacements in the presence of JV supercurrents and demonstrate how the existence of quenched disorder leads to indirect JV pinning and dynamic vortex fragmentation.
We report on the temperature dependence of the magnetoresistance in a twodimensional electron gas... more We report on the temperature dependence of the magnetoresistance in a twodimensional electron gas subjected to an abrupt magnetic field gradient. The gradient leads to the formation of snake states and cycloid states that drift perpendicular to it. By studying changes in the magnetoresistance in the temperature range 1-40K we show significantly different behaviour for these two types of magnetic orbit. Snake states have a robust temperature dependence because of their vanishingly small period whereas cycloid states have a similar period to closed cyclotron orbits. This interpretation is supported by a drift diffusion model comparing the elastic scattering time to the period of both orbits. The Shubnikov-de Haas oscillations are also studied and exhibit a decline over the same temperature range as the cycloid states whilst the snake states remain largely unaffected.
ABSTRACT Hybrid structures composed of superconducting films that are magnetically coupled to arr... more ABSTRACT Hybrid structures composed of superconducting films that are magnetically coupled to arrays of nanoscale ferromagnetic dots have attracted enormous interest in recent years. Broadly speaking, such systems fall into one of two distinct regimes. Ferromagnetic dots with weak moments pin free vortices, leading to enhanced superconducting critical currents, particularly when the conditions for commensurability are satisfied. Dots with strong moments spontaneously generate one or more vortex–antivortex (V–AV) pairs which lead to a rich variety of pinning, anti-pinning and annihilation phenomena. We describe high resolution Hall probe microscopy of flux structures in various hybrid samples composed of superconducting Pb films deposited on arrays of ferromagnetic Co or Co/Pt dots with both weak and strong moments. We show directly that dots with very weak perpendicular magnetic moments do not induce vortex–antivortex pairs, but still act as strong polarity-dependent vortex pinning centres for free vortices. In contrast, we have directly observed spontaneous V–AV pairs induced by large moment dots with both in-plane and perpendicular magnetic anisotropy, and studied the rich physical phenomena that arise when they interact with added “free” (anti)fluxons in an applied magnetic field. The interpretation of our imaging results is supported by bulk magnetometry measurements and state-of-the-art Ginzburg–Landau and London theory calculations.
Fabrication, Materials, Characterization and Applications, 2010
... uk Atif Aziz Department of Materials Science and Metallurgy University of Cambridge, Cambridg... more ... uk Atif Aziz Department of Materials Science and Metallurgy University of Cambridge, Cambridge CB2 3QZ, UK We describe how artificial ... GMR devices, and related structures based on tunnelling magnetoresistance, are now found in the read heads of nearly all magnetic hard ...
The present intense drive to develop current-switched magnetic storage media has lead to a renewe... more The present intense drive to develop current-switched magnetic storage media has lead to a renewed interest in ferrimagnetic garnet films which, for several decades, were the focus of devices exploiting manipulation of magnetic `bubbles'. In such uniaxial materials, the appearance of Bloch lines in structured domain walls strongly influences their dynamic properties in an applied magnetic field. Here we show
Rhenium diselenide (ReSe 2 ) is a layered indirect gap semiconductor for which micromechanical cl... more Rhenium diselenide (ReSe 2 ) is a layered indirect gap semiconductor for which micromechanical cleavage can produce monolayers consisting of a plane of rhenium atoms with selenium atoms above and below. ReSe 2 is unusual among the transition-metal dichalcogenides in having a low symmetry; it is triclinic, with four formula units per unit cell, and has the bulk space group P1.
Part I. The goal of Part I of this thesis is to gain an understanding of &amp;amp;quot;real&a... more Part I. The goal of Part I of this thesis is to gain an understanding of &amp;amp;quot;real&amp;amp;quot; potential tunnel barriers in metal/insulator/metal tunnel junctions beyond the conventional &amp;amp;#39;average potential barrier&amp;amp;#39; approximation which is used to describe them. In particular we hope to understand the role of localized states in the tunnel barrier and their contribution to the conductance of the
The operation of a tunneling hot-electron-transfer amplifier has been simulated by treating it as... more The operation of a tunneling hot-electron-transfer amplifier has been simulated by treating it as a double-barrier diode where the field across each barrier can be varied independently. Current flow is evaluated by calculating the transmission coefficient of the entire structure starting from a coherent-transport framework and then by phenomenologically introducing both elastic and inelastic scattering of the electrons in the
We have used high-resolution scanning Hall probe microscopy to directly image the dynamics of vor... more We have used high-resolution scanning Hall probe microscopy to directly image the dynamics of vortex depinning as a transport critical current Jc is exceeded. Our microscope has 1--2 mum spatial resolution and ~30 mG field sensitivity. The samples used were high-quality niobium thin films patterned into 100-mum-wide strips with low critical currents. At very low fields (< 1 G) individual singly-quantized vortices can be clearly resolved. At higher fields (up to several kG), although individual vortices can no longer be resolved, their presence is very clear from the strong fluctuations in the local field. For J < J_c, a slope begins to appear in the magnetic profile due to the flow of the current itself. When Jc is first exceeded, vortices in certain regions of the sample appear to vanish. This is presumably due to the fast-moving vortices blurring under the low ( ~100 Hz) bandwidth of our probe. Vortices in other regions remain pinned and visible. As the current is further increased, these vortices also appear to vanish. Thus we have direct spatial images of plastic flow and sequential depinning of vortices. When the current is reduced back to zero, there are regions where the vortices have been permanently removed from the sample; newly created antivortices are also visible.
Journal of Micromechanics and Microengineering, 2003
In this paper we report the development of a new III-V cantilever-based atomic force sensor with ... more In this paper we report the development of a new III-V cantilever-based atomic force sensor with piezoresistive detection and an integrated Hall probe for scanning Hall probe microscopy. We give detailed descriptions of the fabrication process and characterization of the new integrated sensor, which will allow the investigation of magnetic samples with no sample preparation at both room and cryogenic temperatures. We also introduce a novel piezoresistive material based on the ternary alloy n+-Al0.4Ga0.6As which allows us to achieve a cantilever deflection sensitivity DeltaR/(R Deltaz) = 2 × 10-6 Å-1 at room temperature.
We demonstrate controllable dual-bath electrodeposition of nickel on architecture-tunable three-d... more We demonstrate controllable dual-bath electrodeposition of nickel on architecture-tunable three-dimensional (3D) silver microcrystals. Magnetic hysteresis loops of individual highly faceted Ag-Ni core-shell elements reveal magnetization reversal that comprises multiple sharp steps corresponding to different stable magnetic states. Finite-element micromagnetic simulations on smaller systems show several jumps during magnetization reversal which correspond to transitions between different magnetic vortex states. Structures of this type could be realizations of an advanced magnetic data storage architecture whereby each element represents one multibit, storing a combination of several conventional bits depending on the overall number of possible magnetic states associated with the 3D core-shell shape.
Scanning Hall probe and local Hall magnetometry measurements have been used to investigate flux d... more Scanning Hall probe and local Hall magnetometry measurements have been used to investigate flux distributions in large mesoscopic superconducting disks with sizes that lie near the crossover between the bulk and mesoscopic vortex regimes. Results obtained by directly mapping the magnetic induction profiles of the disks at different applied fields can be quite successfully fitted to analytic models which assume a continuous distribution of flux in the sample. At low fields, however, we do observe clear signatures of the underlying discrete vortex structure and can resolve the characteristic mesoscopic compression of vortex clusters in increasing magnetic fields. Even at higher fields, where single vortex resolution is lost, we are still able to track configurational changes in the vortex patterns, since competing vortex orders impose unmistakable signatures on "local" magnetisation curves as a function of the applied field. Our observations are in excellent agreement with molecular dynamics numerical simulations which lead us to a natural definition of the lengthscale for the crossover between discrete and continuum behaviours in our system.
A scanning Hall probe microscope is used to study flux pinning in a thin superconducting Pb film ... more A scanning Hall probe microscope is used to study flux pinning in a thin superconducting Pb film covering a square array of single-domain Co dots with in-plane magnetization. We show that single flux quanta of opposite sign thread the superconducting film below Tc at the opposite poles of these dipoles. Depending on the polarity of the applied field, flux lines are attracted to a specific pole of the dipoles, due to the direct interaction with the vortexlike structures induced by the local stray field.
Scanning Hall Probe Microscopy (SHPM)[1] is a quantitative and non-invasive technique to image ma... more Scanning Hall Probe Microscopy (SHPM)[1] is a quantitative and non-invasive technique to image magnetic samples with high spatial and magnetic field resolution: ~ 120nm & 60mG/Hz^1/2 at room temperature. A nano-Hall probe is scanned over the sample surface to measure the surface magnetic fields using conventional scanning tunneling microscopy-positioning techniques. We have developed new down to ~120x120nm size Bi and InSb Hall probes machined FIB milling. 120nm Bi sensors[2] have a sensitivity of 3.3x10-4 Ω/G and a noise level of 7.2 G/Hz^1/2 . The new InSb sensors have a sensitivity of 0.03 Ω/G and a noise level of 8 mG/Hz^1/2 at room temperature. This corresponds to ×8 better noise performance compared to conventional GaAs, based sensors used in RT-SHPM. We used these new sensors to study magnetic domain structures of crystalline garnet films and Ni_80Fe_20 rectangular permalloy microstructures microfabricated by lift-off technique. Bi and InSb nano-Hall probes are shown to be high spatial resolution, high sensitivity and low noise alternatives to GaAs sensors for RT-SHPM. There seems to be more room for improving the spatial resolution down to <50nm and the noise of Hall probes to 1mG/Hz^1/2 at room temperature. [1] A. Oral et. al. Appl. Phys. Lett., 69, 1324 (1996), A. Sandhu et. al., Jpn. J. Appl. Phys. 40(5B), L524 (2001) [2] A. Sandhu, H. Masuda, K. Kurosawa K, A. Oral and S.J. Bending, Electronics Letters 37 (22), 1335-1336 (2001).
Electrochemistry offers highly flexible routes to fabrication of a wide variety of mesostructures... more Electrochemistry offers highly flexible routes to fabrication of a wide variety of mesostructures, including three-dimensional (3D) crystallites, thin films and nanowires. Using this method we have grown various 3D superconducting Pb mesostructures with vastly different morphologies. We present here results on a truncated(half)-icosahedron with a hexagonal base and a tripod structure with a triangular base. Using Hall probe magnetometry we have obtained magnetisation curves for these structures at several temperatures and see evidence of geometry-driven flux entry and exit as well as flux trapping caused by specific sample geometries. We also observe behaviour that we interpret in terms of the formation of giant vortices, bearing in mind that bulk Pb is a type-I superconducting material.
We have imaged interacting crossing pancake vortex (PV) and Josephson vortex (JV) lattices in hig... more We have imaged interacting crossing pancake vortex (PV) and Josephson vortex (JV) lattices in highly anisotropic Bi2Sr2CaCu2O8+δ single crystals under tilted magnetic fields. The dependence of vortex structures on in-plane field is in good quantitative agreement with theoretical predictions, yielding an almost temperature-independent anisotropy parameter of γ=640+/-25. We directly confirm that the PV/JV attraction arises from small PV displacements in the presence of JV supercurrents and demonstrate how the existence of quenched disorder leads to indirect JV pinning and dynamic vortex fragmentation.
We report on the temperature dependence of the magnetoresistance in a twodimensional electron gas... more We report on the temperature dependence of the magnetoresistance in a twodimensional electron gas subjected to an abrupt magnetic field gradient. The gradient leads to the formation of snake states and cycloid states that drift perpendicular to it. By studying changes in the magnetoresistance in the temperature range 1-40K we show significantly different behaviour for these two types of magnetic orbit. Snake states have a robust temperature dependence because of their vanishingly small period whereas cycloid states have a similar period to closed cyclotron orbits. This interpretation is supported by a drift diffusion model comparing the elastic scattering time to the period of both orbits. The Shubnikov-de Haas oscillations are also studied and exhibit a decline over the same temperature range as the cycloid states whilst the snake states remain largely unaffected.
ABSTRACT Hybrid structures composed of superconducting films that are magnetically coupled to arr... more ABSTRACT Hybrid structures composed of superconducting films that are magnetically coupled to arrays of nanoscale ferromagnetic dots have attracted enormous interest in recent years. Broadly speaking, such systems fall into one of two distinct regimes. Ferromagnetic dots with weak moments pin free vortices, leading to enhanced superconducting critical currents, particularly when the conditions for commensurability are satisfied. Dots with strong moments spontaneously generate one or more vortex–antivortex (V–AV) pairs which lead to a rich variety of pinning, anti-pinning and annihilation phenomena. We describe high resolution Hall probe microscopy of flux structures in various hybrid samples composed of superconducting Pb films deposited on arrays of ferromagnetic Co or Co/Pt dots with both weak and strong moments. We show directly that dots with very weak perpendicular magnetic moments do not induce vortex–antivortex pairs, but still act as strong polarity-dependent vortex pinning centres for free vortices. In contrast, we have directly observed spontaneous V–AV pairs induced by large moment dots with both in-plane and perpendicular magnetic anisotropy, and studied the rich physical phenomena that arise when they interact with added “free” (anti)fluxons in an applied magnetic field. The interpretation of our imaging results is supported by bulk magnetometry measurements and state-of-the-art Ginzburg–Landau and London theory calculations.
Fabrication, Materials, Characterization and Applications, 2010
... uk Atif Aziz Department of Materials Science and Metallurgy University of Cambridge, Cambridg... more ... uk Atif Aziz Department of Materials Science and Metallurgy University of Cambridge, Cambridge CB2 3QZ, UK We describe how artificial ... GMR devices, and related structures based on tunnelling magnetoresistance, are now found in the read heads of nearly all magnetic hard ...
The present intense drive to develop current-switched magnetic storage media has lead to a renewe... more The present intense drive to develop current-switched magnetic storage media has lead to a renewed interest in ferrimagnetic garnet films which, for several decades, were the focus of devices exploiting manipulation of magnetic `bubbles'. In such uniaxial materials, the appearance of Bloch lines in structured domain walls strongly influences their dynamic properties in an applied magnetic field. Here we show
Rhenium diselenide (ReSe 2 ) is a layered indirect gap semiconductor for which micromechanical cl... more Rhenium diselenide (ReSe 2 ) is a layered indirect gap semiconductor for which micromechanical cleavage can produce monolayers consisting of a plane of rhenium atoms with selenium atoms above and below. ReSe 2 is unusual among the transition-metal dichalcogenides in having a low symmetry; it is triclinic, with four formula units per unit cell, and has the bulk space group P1.
Part I. The goal of Part I of this thesis is to gain an understanding of &amp;amp;quot;real&a... more Part I. The goal of Part I of this thesis is to gain an understanding of &amp;amp;quot;real&amp;amp;quot; potential tunnel barriers in metal/insulator/metal tunnel junctions beyond the conventional &amp;amp;#39;average potential barrier&amp;amp;#39; approximation which is used to describe them. In particular we hope to understand the role of localized states in the tunnel barrier and their contribution to the conductance of the
The operation of a tunneling hot-electron-transfer amplifier has been simulated by treating it as... more The operation of a tunneling hot-electron-transfer amplifier has been simulated by treating it as a double-barrier diode where the field across each barrier can be varied independently. Current flow is evaluated by calculating the transmission coefficient of the entire structure starting from a coherent-transport framework and then by phenomenologically introducing both elastic and inelastic scattering of the electrons in the
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Papers by Simon Bending