  Films of ZnO doped with magnetic ions, Mn and Co and, in some cases, with Al
have been fabricated with a very wide range of carrier densities. Ferromagnetic
behaviour is observed in both insulating and metallic films, but not when the
carrier density is intermediate. Insulating films exhibit variable range
hopping at low temperatures and are ferromagnetic at room temperature due to
the interaction of the localised spins with static localised states. The
magnetism is quenched when carriers in the localised states become mobile. In
the metallic (degenerate semiconductor) range, robust ferromagnetism reappears
together with very strong magneto-optic signals and room temperature anomalous
Hall data. This demonstrates the polarisation of the conduction bands and
indicates that, when ZnO is doped into the metallic regime, it behaves as a
genuine magnetic semiconductor.
