  We investigate the role of magnetic fields in the fragmentation of
self-gravitating discs using 3D global ideal magnetohydrodynamic simulations
performed with the "phantom" smoothed particle hydrodynamics code.
  For initially toroidal fields, we find two regimes. In the first, where the
cooling time is greater than five times the dynamical time, magnetic fields
reduce spiral density wave amplitudes, which in turn suppresses fragmentation.
This is the case even if the magnetic pressure is only a tenth of the thermal
pressure. The second regime occurs when the cooling time is sufficiently short
that magnetic fields cannot halt fragmentation.
  We find that magnetised discs produce more massive fragments, due to both the
additional pressure exerted by the magnetic field, and the additional angular
momentum transport induced by Maxwell stresses. The fragments are confined to a
narrower range of initial semimajor axes than those in unmagnetised discs. The
orbital eccentricity and inclination distributions of unmagnetised and
magnetised disc fragments are similar. Our results suggest the fragmentation
boundary could be at cooling times a factor of two lower than predicted by
purely hydrodynamical models.
