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.

► Thin-film rings of cobalt were magnetised and examined holographically. ► Both onion and vortex states of magnetisation were obtained, depending on ring width. ► Vortex phase shift agreed with Aharonov–Bohm theory within measurement error. ► For a ring used as phase plate, the radial range of interception may be as small as 30 nm.