A. M. Blackburn

Vortex beam production and contrast enhancement from a magnetic spiral phase plate

Electron vortex beam probes offer the possibility of mapping magnetic moments with atomic resolution. In this work we consider using the stray magnetic field produced from a narrow ferromagnetic rod magnetised along its long axis to produce a vortex beam probe, as an alternative to the currently used holographic apertures or gratings. We show through numerical modelling, electron holography observations and direct imaging of the electron probe, that a long narrow ferromagnetic rod induces a phase shift in the wave-function of passing electrons that approximately describes a helix in the regions near its ends. Directing this rod towards the optical axis of a charged-particle beam probe forming system at a limiting aperture position, with the free-end sufficiently close to the axis, is shown to offer a point spread function composed of vortex modes, with evidence of this appearing in observations of the electron probe formed from inserting a micro-fabricated CoFe rod into the beam path of a 300 keV transmission electron microscope (TEM). If the rod is arranged to contain the magnetic flux of h/e, thus producing a maximum phase shift of 2π, it produces a simple spiral-like phase contrast transfer function for weak phase objects. In this arrangement the ferromagnetic rod can be used as a phase plate, positioned at the objective aperture position of a TEM, yielding enhanced image contrast which is simulated to be intermediate between comparable Zernike and Hilbert phase plates. Though this aspect of the phase plate performance is not demonstrated here, agreement between our observations and models for the probe formed from an example rod containing a magnetic flux of ~2.35h/e, indicate this phase plate arrangement could be a simple means of enhancing contrast and gaining additional information from TEM imaged weak phase samples, while also offering the capability to produce vortex beam probes. However, steps still need to be taken to either remove or improve the support membrane for the rod in our experiments to reduce any effects from charging in the phase plate.

Characterisation of ferromagnetic rings for Zernike phase plates using the Aharonov-Bohm effect

Holographic measurements on magnetised thin-film cobalt rings have demonstrated both onion and vortex states of magnetisation. For a ring in the vortex state, the difference between phases of electron paths that pass through the ring and those that travel outside it was found to agree very well with Aharonov-Bohm theory within measurement error. Thus the magnetic flux in thin-film rings of ferromagnetic material can provide the phase shift required for phase plates in transmission electron microscopy. When a ring of this type is used as a phase plate, scattered electrons will be intercepted over a radial range similar to the ring width. A cobalt ring of thickness 20 nm can produce a phase difference of π/2 from a width of just under 30 nm, suggesting that the range of radial interception for this type of phase plate can be correspondingly small.

Singlet–triplet physics and shell filling in carbon nanotube double quantum dots

Carbon nanotube double quantum dots, whose shell-like electronic structure is reminiscent of that of a simple molecule, provide a useful system to study the interaction of just a few electrons at a time.

Nucleation and propagation of domains walls in a Co∕Pt multilayer wire

The domain wall processes in Hall bar devices patterned from Co∕Pt multilayers with perpendicular magnetic anisotropy have been studied by Kerr microscopy and extraordinary Hall effect measurements. The samples are extremely thin (<2nm) so that they show full remanence and a square hysteresis loop with a coercive field of ∼25Oe. The Kerr microscope observations of the as-patterned Hall bars have shown an uncontrolled domain wall nucleation followed by rapid propagation, without significant pinning. This shows that the nucleation field exceeds any propagation and pinning fields in these samples. Controlled domain wall nucleation by irradiation of a selected area of the Co∕Pt multilayer structure with different doses of Ga+ ions on the multilayers has been studied proving the decrease of coercivity in such irradiated areas with respect to the rest of the sample. This method can be used to lower the nucleation field below any pinning fields that exist in the sample, enabling controlled wall positioning withi...

Tungsten pedestal structure for nanotriode devices

The nanotriode is a vacuum nanoelectronic device with a turn-on voltage for a field emission of ∼8 V, in which electrons are emitted from a metal nanotip with a radius ∼1 nm in an integrated vacuum chamber, over a distance of ∼100 nm. However, surface breakdown of the gate-cathode dielectric within the device chamber occurs ∼2 V outside of its operating range, limiting the reliable range of field-emission observation. To increase this surface breakdown voltage, while maintaining the operating voltage range, a pedestal structure has been developed that can be incorporated into the nanotriode. This structure has a thicker dielectric layer, while maintaining a gate-cathode separation similar to that used previously. Measurements on vacuum-encapsulated two-terminal devices show field emission in the same voltage range and with similar characteristics to those previously observed; in consequence of the pedestal, higher surface breakdown voltages are achieved and the significance of the leakage current is reduced.

Spin and orbital splitting in ferromagnetic contacted single wall carbon nanotube devices

We observed the coulomb blockade phenomena in ferromagnetic contacting single wall semiconducting carbon nanotube devices. No obvious Coulomb peaks shift was observed with existing only the Zeeman splitting at 4 K. Combining with other effects, the ferromagnetic leads prevent the orbital spin states splitting with magnetic field up to 2 T at 4 K. With increasing magnetic field further, both positive or negative coulomb peaks shift slopes are observed associating with clockwise and anticlockwise orbital state splitting. The strongly suppressed/enhanced of the conductance has been observed associating with the magnetic field induced orbital states splitting/converging.

In-vacuum resonant tunneling in the nanopentode

Nanopentode device fabrication process and predicted characteristics are presented. The fabrication process involves a combination of reaction ion etching and wet etching. The fabricated device structure makes it possible to simultaneously create an in-vacuum potential-energy barrier and well, creating a gaseous-state device similar in some respects to the solid-state resonant tunneling diode. The calculated transmission probability for electrons through the entire cathode-anode gap, in a one-dimensional approximation, shows resonances at certain gate-voltage arrangement. This strengthens the possibility of observing quantum interference effects in multiple-gate vacuum microelectronic devices.

Incorporation of a tungsten pedestal structure into the nanotriode device

In this paper, the nanotriode fabrication process has been modified to incorporate a pedestal for the nanopillars, which increases the dielectric surface leakage gap, whilst maintaining the extractor-nanopillar gap.