Stuart B. Field

Chaotic dynamics of falling disks

The study of the motion of flat bodies falling in a viscous mediumdates back at least to Newton and Maxwell, and is relevant to problems in meteorology, sedimentology, aerospace engineering and chemical engineering. More recent theoretical studies have emphasized the role played by deterministic chaos, although many experimental studies,,, were performed before the development of such ideas. Here we report experimental observations of the dynamics of disks falling in water/glycerol mixtures. We find four distinct types of motion, which are mapped out in a ‘phase diagram’. The apparently complex behaviour can be reduced to a series of one-dimensional maps, which display a discontinuity at the crossover from periodic to chaotic motion. This discontinuity leads to an unusual intermittency transition, not previously observed experimentally, between the two behaviours.

Scanning Hall probe microscope images of field penetration into niobium films

Abstract A high resolution scanning Hall probe microscope has been used to study the penetration of magnetic flux into thin strips of superconducting niobium as the applied field is slowly ramped. The strips, with widths w =100 μm, and thicknesses d ≈1 μm, are thick enough such that vortices are truly three dimensional ( d ≫ λ ). However, the small ratio d / w implies very strong demagnetization effects, and the relative smallness of d emphasizes the importance of the long-range force between vortex ends over the short-range force between their bulk core currents. The microscope has 1–2 μm spatial resolution and around 30 mG field sensitivity, allowing high-resolution imaging of flux features over its approx. 150×150 μm 2 scan range. At low fields of a few tens of gauss, we observe Meissner screening of the external field. As the field is increased towards several kilogauss, flux begins to enter the sample in the form of small (≈10 μm wide) dendritic fingers. These fingers persist over all temperatures investigated, from 0.3 to 0.95 T c . They appear to grow in such a way as to maximize their separation from neighbouring fingers. This suggests a growth mechanism of the flux front mediated by a competition between long-range repulsive interactions between mesoscopic flux-containing regions, and the strong pinning that maintains the stability of the flux front.

Imaging and magnetotransport in superconductor/magnetic dot arrays

Magnetoresistance and scanning Hall probe microscopy studies of Nb-film/Ni-dot structures are reported. The dots act as pinning sites for superconducting vortices. The transport measurements focus on the effects of introducing disorder into the positions of the pinning lattice near the superconducting critical temperature Tc in structures with 250 nm diameter Ni dots randomized about an ideal square lattice with lattice constant a=560 nm. Features observable in the ordered arrays at higher multiples of the matching field H0=Φ0/a2 are washed out in the disordered arrays, but those at H0 remain. Scanning Hall probe microscope images were taken of the vortex configurations at fields up to 1.2H0 in ordered arrays of 1-μm-diameter dots on a 5.2 μm×4 μm rectangular lattice. These show that despite the relatively weak pinning of the magnetic dots, ordering commensurate with the dot lattice occurs even for fields below H0. Both transport and imaging studies point to the importance of interstitial vortices in dete...

Integrated Electromagnetic Second Stage Microactuator for a Hard Disk Recording Head

A slider with an integrated microactuator (SLIM) allows actuating a read-write element of a hard disk drive (HDD) in both the vertical direction allowing a flying height adjustment as well as in the lateral direction allowing a second stage actuation. The microactuator system consists of a pair of electromagnetic variable reluctance (VR) micro actuators. The microactuator system is fabricated using thin-film technology. Each actuator has a permalloy C-core carrying a two-layer spiral Cu coil with a total of 16 turns. The insulation materials are SU-8 (in the lateral direction) and Si3N4 (in the vertical direction). The total size of one magnetic VR microactuator is 460 mum times 300 mum times 61 mum. This paper discusses design considerations, presents the FEM simulation conducted, describes the fabrication technology, and provides experimental results.

Si metal–oxide semiconductor field effect transistor with 70‐nm slotted gates for study of quasi‐one‐dimensional quantum transport

We have fabricated dual‐gate Si metal–oxide semiconductor field effect transistor devices in which the lower gate is slotted and the upper gate, separated by 45 nm of SiO2, is planar. By appropriate adjustment of the potentials on the two gates, the field lines from the upper gate are pinched, creating an inversion layer about one‐half the slot width. The resist patterning was done by x‐ray lithography, using a mask that combined crystallographic‐template/sidewall‐shadowing techniques with UV lithography. The slotted lower gate was produced by lift‐off. The metallization was a sandwich structure of Cr/W/Cr which permitted high‐temperature annealing. Very high mobilities, ∼15 000 cm2 /V s at 4.2 K, were achieved as a result. The combination of high mobility and extremely narrow inversion channel (∼30 nm) yields very clear structure in the conductance as a function of gate voltage, which cannot be accounted for by either localization effects or universal conductance fluctuations. Although these oscillations...

Capacitive position sensor with simultaneous, linear X–Y readout

A design is presented for a compact capacitive position sensor suitable for operation at cryogenic temperatures and high magnetic fields. The four quadrants of a base electrode are excited with phases differing by 90°; from its in- and out-of-phase components, the current injected into the movable electrode yields simultaneous information about the x and y sensor positions. This current is also linear in the position of the sensor. The effects of possible sensor imperfections are analyzed in detail.

Disorder and correlations in extended superconducting nanostructures

Abstract Experiments are presented on the magnetic properties of two types of extended superconducting nanostructures where disorder can be introduced in a controlled way. Magnetotransport measurements on Nb films overlaying arrays of 250-nm diameter Ni dots show that the superstructure observed at higher multiples of the matching field H 0 = Φ 0 / a 2 , where a =560 nm is the dot lattice constant, are systematically suppressed as disorder is introduced into the dot arrays. In arrays of superconducting rings in external fields corresponding to half-integral numbers of flux quanta per ring, flux quanta trapped in individual rings repel each other due to the magnetic coupling between rings, and the system is analogous to an Ising antiferromagnet. Disorder enters through small, random variations in ring sizes, and plays the role of a random field in the Ising model. SQUID magnetometry and scanning Hall microscopy (SHM) were used to probe the dynamics and specific magnetic configuration of square, honeycomb, kagome, and triangular lattice arrays containing up to 10 6 micron-size Al rings. The dynamics are dominated by a temperature-dependent energy barrier E B and hysteresis in the flux state of the ring populations. This population hysteresis is directly observed in ∂ M /∂ T measurements. SHM measurements at Φ 0 /2 per ring show antiferromagnetic correlations that can be suppressed by going to higher flux fractions due to increases in the effective random field.

Evaluation of an electromagnetic microactuator using scanning Hall probe microscopy measurements

Scanning Hall probe microscopy (SHPM) was used to evaluate integrated magnetic microactuators designed for hard disk drives. The Hall probe measurements of the generated magnetic field strength as a function of the applied current or distance were compared to results of a finite element method simulation. The SHPM measurements and simulation results are in good agreement, confirming that simulations supported by SHPM measurements can be used successfully to predict the performance of microactuators.

A Multi-Sensor System for Measuring Bovine Embryo Metabolism

This paper presents the development of a multi-sensor platform capable of simultaneous measurement of dissolved oxygen (DO) concentration, glucose and lactate concentrations in a micro-chamber for real-time evaluation of metabolic flux in bovine embryos. A micro-chamber containing all three sensors (DO, glucose, and lactate) was made to evaluate metabolic flux of single oocytes or embryos at different stages of development in ≤ 120 µL of respiration buffer. The ability of the sensor to detect a metabolic shift from oxidative phosphorylation (OXPHOS) to glycolysis was demonstrated in embryos by an ablation of oxygen consumption and an increase in lactate production following addition of oligomycin, an inhibitor of mitochondrial adenosine triphosphate (ATP) synthesis. An increased reliance upon glycolysis relative to OXPHOS was demonstrated in embryos as they developed from morula to hatched blastocysts by a progressive increase in the lactate/oxygen flux ratio, consistent with isolated metabolic assessments reported previously. These studies highlight the utility of a metabolic multi-sensor for integrative real-time monitoring of aerobic and anaerobic energy metabolism in bovine embryos, with potential applications in the study of metabolic processes in oocyte and early embryonic development.