Two ferromagnetic phases in spin-Fermion systems

We consider spin-Fermion systems which obtain their magnetic properties from a system of localized magnetic moments being coupled to conducting electrons. The dynamical degrees of freedom are spin-s operators of localized spins and spin-1∕2 Fermi operators of itinerant electrons. We develop a modified spin-wave theory and obtain that the system has two ferromagnetic phases. At the characteristic temperature T^*, the magnetization of itinerant electrons becomes zero, and the high temperature ferromagnetic phase (T^*<T<T_C) is a phase where only localized electrons give contribution to the magnetization of the system. An anomalous increasing of magnetization below T^* is obtained in good agreement with experimental measurements of the ferromagnetic phase of UGe₂.