  A phase transition from paramagnetism to ferromagnetism in neutron star
interior is explored. Since there is $^3$P$_2$ neutron superfluid in neutron
star interior, it can be treated as a system of magnetic dipoles. Under the
presence of background magnetic field, the magnetic dipoles tend to align in
the same direction. Below a critical temperature, there is a phase transition
from paramagnetism to ferromagnetism. And this gives a convenient explanation
of the strong magnetic field of magnetars. In our point of view, there is an
upper limit for the magnetic field strength of magnetars. The maximum field
strength of magnetars is about $(3.0-4.0)\times 10^{15}$ G. This can be tested
directly by further investigations.
  Magnetars are instable due to the ultra high Fermi energy of electrons. The
Landau column becomes a very long cylinder along the magnetic field, but it is
very narrow and the Fermi energy of electron gas is given as
$E_F(e)\approx40(B/B_{cr})^{1/4}$ when $B\gg B_{cr}$. $E_F(e)\approx90MeV$ When
$B\sim10^{15}$ G. Hence, the electron capture process $e^-+p\to n+\nu_e$ will
be happen rapidly. Thus the $^3$P$_2$ Cooper pairs will be destroyed quickly by
the outgoing neutrons with high energy. It will cause the isotropic superfluid
disappear and then the magnetic field induced by the $^3$P$_2$ Cooper pairs
will be also disappear. These energy will immediately be transmitted into
thermal energy and then transformed into the radiation energy with X-ray - soft
$\gamma$-ray. We may get a conclusion that the activity of magnetars originates
from instability caused by the high Fermi energy of electrons in extra strong
magnetic field.
