  We report an interplay between magnetism and charge transport in the
ferromagnetic semiconductor GdN, pointing to the formation of magnetic polarons
centred on nitrogen vacancies. The scenario goes some way to resolving a
long-standing disagreement between the measured and predicted Curie temperature
in GdN. It further constitutes an extension of concepts that relate closely to
the behaviour of ferromagnetic semiconductors generally, and EuO in particular.
