  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 pi/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.
