  Electron microscopy stands out as electron waves providing higher spatial
resolving power compared to their optical counterpart. Here we investigate
theoretically the interaction of twisted electrons generated in transmission
electron microscope (TEM) and magnetic impurity, in which the magnetic dipole
moment is taken as a demonstration element. In addition to the usual optical
phase, the inhomogeneous vector potential generated by the magnetic dipole
moment makes additional contribution to the intrinsic orbital angular momentum
of electrons, resulting in a Gouy phase shift. By interfering the outgoing
twisted electron beam with a reference Gaussian-cylindrical wave, one can
determine the magnitude and orientation of magnetic dipole directly via the
rotational and deformed interference pattern. The obtained results demonstrate
the usefulness of twisted electron beams for probing the atomic- and nanoscale
magnetism of impurity by TEM and the proposed model provides the conceptual
basis for future developments of the TEM method.
