Is a Field-Induced Ferromagnetic Phase Transition in the Magnetar Core   Actually Possible?
  Spin polarized states in dense neutron matter with BSk20 Skyrme force are considered in magnetic fields up to $10^{20}$ G. It is shown that the appearance of the longitudinal instability in a strong magnetic field prevents the formation of a fully spin polarized state in neutron matter, and only the states with moderate spin polarization can be developed. 
A. A. Isayev, J. Yang
astro-ph.HE