  We show that the presence of free carriers in a substance can generate the
multiferroic behavior. Namely, if the substance has mixed-valence ions, which
can supply free carriers and have electric dipole and spin moments, all three
types of long-range order (ferromagnetic, ferroelectric and magnetoelectric
(ME)) can occur at low temperature. The physical origin of the effect is that
charge carriers can mediate the multiferroic behavior via spin - spin (RKKY),
dipole-dipole and dipole - spin interactions. Our estimate of the interaction
magnitude shows that there exist an optimal carrier concentration, at which the
strength of ME interaction is maximal and comparable to that of spin-spin RKKY
interaction. This permits to conclude that in substances, where RKKY
interaction between local spins is not small, a substantial value of free
carriers mediated ME interaction can occur. Our analysis shows that disorder in
the above substances does not suppress multiferroic effects.
