  Ferromagnetic spin ordering can take place in neutron stars. This phase
transition alters the neutron star equation of state. Here, applying the
scalar-tensor theories of gravity, we investigate the structure of neutron
stars which are in the ferromagnetic phase. Considering the equation of state
of ferromagnetic neutron matter with Skyrme-type interactions at zero
temperature and using the scalar-tensor theories of gravity with sufficiently
negative coupling constant, we explore the spontaneous scalarization in
ferromagnetic neutron stars. In this regard, the mass versus the central
density, the profiles of scalar field and mass and density, the central scalar
field for different central densities, and the mass-radius relation of
ferromagnetic neutron stars are presented. Moreover, we investigate the
influences of the coupling constant on the scalarization of ferromagnetic
neutron stars. The effects of the coupling constant on the critical densities
of scalarization and the scalar charge of ferromagnetic neutron stars are also
calculated. In addition, we study the maximum value of the coupling constant at
which the spontaneous scalarization takes place presenting the influence of the
neutron matter equation of state on this critical value.
