  It is shown that the electric current of massive fermions along the external
magnetic field can be excited in the case when particles possess anomalous
magnetic moments and electroweakly interact with background matter. This
current is calculated on the basis of the exact solution of the Dirac equation
in the external fields. It is shown that the magnetic field becomes unstable if
this current is taken into account in the Maxwell equations. Considering a
particular case of a degenerate electron gas, which can be found in a neutron
star, it is revealed that the seed magnetic field can be significantly
enhanced. The application of the results to astrophysics is also discussed.
