  Flux vortices in superconductors can be imaged using transmission electron
microscopy because the electron beam is deflected by the magnetic flux
associated with the vortices. This technique has a better spatial and temporal
resolution than many other imaging techniques and is sensitive to the magnetic
flux density within each vortex not simply the fields at the sample surface.
Despite these advantages, only two groups have successfully employed the
technique using specially adapted instruments. Here we demonstrate that
vortices can be imaged with a modern, commercial transmission electron
microscope operating at 300 kV equipped with a field emission gun, Lorentz lens
and a liquid helium cooled sample holder. We introduce superconductivity for
non-specialists and discuss techniques for simulating and optimising images of
flux vortices. Sample preparation is discussed in detail as the requirement for
samples with very large (>10um), flat areas so that the image is not dominated
by diffraction contrast is the main difficulty with the technique. We have
imaged vortices in superconducting Bi2Sr2CaCu2O8+d and use correlation
functions to investigate the ordered arrangements they adopt as a function of
applied magnetic field.
