Ferromagnetism in quark matter

We investigate the magnetic property of the cold quark matter employing the magnetic moment $\vec{\mu}'$ as the order parameter. Through analysis of the effective potential $V (\vec{\mu}^{' 2})$, we find that, at relatively high densities $\rho > \rho_c$, the quark matter is in the normal phase. At $\rho = \rho_c$, the magnetic phase transition takes place and, in the low-density region $\rho < \rho_c$, the quark matter is in the ferromagnetic phase. For up, down and strange quarks, $\rho_c = 1.62 \rho_0$, $1.47 \rho_0$ and $2.20 \rho_0$ ($\rho_0$ the nuclear density), respectively. We also find that the leptons ($e$ and $\mu$) inside the quark matter are in the normal phase.