  Properties in magnetic ordered states of graphene nanoribbons with zigzag
shaped edges are investigated by applying mean-field approximation to the
Hubbard model with on-site repulsion $U$. We observe that magnetic moments and
critical temperature show anomalous power-law dependences as a function of $U$;
the actual values of the power are determined by only the width of ribbons.
Such singular behaviours are found to be due to localized nature of the
electronic states close to Fermi energy.
