Ferromagnetic instabilities in quarkyonic matter

Abstract: We investigate the magnetic properties of quarkyonic matter, which naturally bridges nuclear and quark matter at intermediate densities relevant to neutron star cores. We extend the quarkyonic model to include spin polarization, where nucleons near the Fermi surface can be polarized while quarks in the deep Fermi sea remain unpolarized due to strong Pauli blocking. After including neutron interactions with spin-dependent terms, we find that quarkyonic matter can develop ferromagnetic instabilities at low densities, characterized by negative magnetic susceptibility. This ferromagnetic behavior occurs in pure neutron matter, independent of proton contributions, and results from the competition between attractive spin-dependent interactions and kinetic energy costs. The system returns to paramagnetic behavior at higher densities when Pauli pressure dominates. Our results demonstrate that the splitting of Fermi momenta of quarkyonic matter produces fundamentally different magnetic responses compared to conventional nuclear matter, with important implications for neutron star magnetism and magnetar physics.