We show a possibility that a strong “magnetic field” $\sim {10}^{15}\mathrm{G}$ is produced by color ferromagnetic quark matter in neutron stars. In the quark matter a color magnetic field is generated spontaneously owing to the Savvidy mechanism and a gluon condensate arises for the stabilization of the field. Since the quark matter is electrically charged in the neutron stars, the rotation of the quarks around the color magnetic field produces the strong “magnetic field.”

• Received 23 August 2005

DOI:https://doi.org/10.1103/PhysRevD.72.114003