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Magnetic phase plate for transmission electron microscopy

We seek to use a thin-film ring located in a diffraction plane of a transmission electron microscope (TEM) to produce a phase shift between direct and scattered electrons, using the Aharonov-Bohm effect. The aim is to enhance imaging of weak phase objects, to extend the imaging capability of the TEM to much lower spatial frequencies than are feasible at present and to reduce the electron dose in cryo- and other biological microscopy.

In the vortex state of magnetization, flux is azimuthal and continuous around the ring, so the response is not directional. This state is obtained, in a small ring, during reversal of in-plane magnetization and is then stable against substantial changes of in-plane field. Earlier work will be extended by using material with high coercivity, so that the azimuthal flux will be maintained also in a strong axial field, allowing the phase plate to be used with the objective lens of the TEM energised.

The flux needed for the desired phase shift is so small that a cobalt ring designed for a phase shift of π/2 might have a width of only 45 nm. Other multiples of this phase shift and width are also possible. A ring with width of this order will then intercept a far smaller range of information-bearing electrons than most other designs of phase plate. Further, if the width is appreciably greater than the thickness, then shape anisotropy causes the vortex state to be retained in axial field of the order of 1T.


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