  The interactions between vortices in a thin superconducting film and one
magnetic dipole in the presence of a magnetic field applied parallel to the
film surfaces are studied theoretically in the London limit. The dipole
magnetic moment is assumed to have constant magnitude and freedom to rotate.
The pinning potential for an arbitrary vortex configuration is calculated
exactly. It is found that, due to the dipole freedom to rotate, the pinning
potential differs significantly from that for a permanent dipole. In
particular, its dependence on the applied field is non-trivial and allows for
tuning of the pinning potential by the applied field. The critical current for
one vortex pinned by the dipole is obtained numerically as a function of the
applied field and found to depend strongly on the field. Order of magnitude
changes in the critical current resulting from changes in the direction and
magnitude of the applied field are reported, with discontinuous changes taking
place in some cases. The effect of vortex pinning by random material defects on
the critical current is investigated using a simple model. It is found that if
random pinning is weak the critical current remains strongly dependent on the
applied field. Possible applications to vortices pinned by arrays of magnetic
dots are briefly considered.
