  Recent theoretical and experimental evidence indicates that many of the
materials that are thought to exist in the mantles of terrestrial exoplanets
will metallize and become good conductors of electricity at mantle pressures.
This allows for strong electromagnetic coupling of the core and the mantle in
these planets. We use a numerical dynamo model to study the effect of a
metallized lower mantle on the dynamos of terrestrial exoplanets using several
inner core sizes and mantle conductivities.
  We find that the addition of an electrically conducting mantle results in
stronger core-mantle boundary fields because of the increase in magnetic field
stretching. We also find that a metallized mantle destabilizes the dynamo
resulting in less dipolar, less axisymmetric poloidal magnetic fields at the
core-mantle boundary. The conducting mantle efficiently screens these fields to
produce weaker surface fields. We conclude that a conducting mantle will make
the detection of extrasolar terrestrial magnetic fields more difficult, while
making the magnetic fields in the dynamo region stronger.
