  The equation of state of nuclear matter is strongly affected by the presence
of a magnetic field. Here we study the equilibrium configuration of asymmetric
nuclear matter for a wide range of densities, isospin composition, temperatures
and magnetic fields. Special attention is paid to the low density and low
temperature domain, where a thermodynamical instability exists. Neglecting
fluctuations of the Coulomb force, a coexistence of phases is found under such
conditions, even for extreme magnetic intensities. We describe the nuclear
interaction by using the non--relativistic Skyrme potential model within a
Hartree--Fock approach. We found that the coexistence of phases modifies the
equilibrium configuration, masking most of the manifestations of the spin
polarized matter. However, the compressibility and the magnetic susceptibility
show clear signals of this fact. Thermal effects are significative for both
quantities, mainly out of the coexistence region.
