  The stability of three static and spherically symmetric black hole solutions
with nonlinear electromagnetism as a source is investigated in three different
ways. We show that the specific heat of all the solutions displays an infinite
discontinuity with a change of sign, but the turning point method indicates
that the solutions are thermodynamically stable (much in the same way as in the
case of the Reissner-Nordstrom geometry). We also show that the black holes
analyzed here are dynamically stable, thus suggesting that there may be a
relation between thermodynamical and dynamical stability for nonvacuum black
holes.
