Kondo screening coexisting with ferromagnetic order as a possible ground state for Kondo lattice systems

We consider the competition between the Kondo screening effect and ferromagnetic long-range order (FLRO) within a mean-field theory of the Kondo lattice model for low conduction electron densities n(c). Depending on the parameter values, several types of FLRO ground states are found. When n(c) < 0.16, a polarized FLRO phase is dominant in the large Kondo coupling limit. For 0.16 < n(c) < 0.82, a non-polarized FLRO phase appears in the weak Kondo coupling region; while in the intermediate coupling region the ground state corresponds to the polarized and nonpolarized FLRO phases, respectively, coexisting with the Kondo screening. For a strong Kondo coupling, the product of pure Kondo singlets is the ground state. Moreover, we also find that a weak magnetic field makes the pure Kondo singlet phase vanish, while the nonpolarized FLRO state with the Kondo screening spans a large area in the phase diagram.