The physics of strong magnetic fields and activity of magnetars
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 $e^-+p\to n+ν_e$ 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 $γ$-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.