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Neutron matter under strong magnetic fields: a comparison of models

The equation of state of neutron matter is affected by the presence of a magnetic field due to the intrinsic magnetic moment of the neutron. Here we study the equilibrium configuration of this system for a wide range of densities, temperatures and magnetic fields. Special attention is paid to the behavior of the isothermal compressibility and the magnetic susceptibility. Our calculation is performed using both microscopic and phenomenological approaches of the neutron matter equation of state, namely the Brueckner--Hartree--Fock (BHF) approach using the Argonne V18 nucleon-nucleon potential supplemented with the Urbana IX three-nucleon force, the effective Skyrme model in a Hartree--Fock description, and the Quantum Hadrodynamic formulation with a mean field approximation. All these approaches predict a change from completely spin polarized to partially polarized matter that leads to a continuous equation of state. The compressibility and the magnetic susceptibility show characteristic behaviors, which reflect that fact. Thermal effects tend to smear out the sharpness found for these quantities at T=0. In most cases a thermal increase of 10 MeV is enough to hide the signals of the change of polarization. The set of densities and magnetic field intensities for which the system changes it spin polarization is different for each model. However, there is an overall agreement between the three theoretical descriptions.