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A theoretical model is proposed to explain the magnetic properties of uncompensated CaGe:YIG with excess calcium. The model proposes that the excess uncompensated charge is localized at low temperatures on the tetrahedral site to produce an Fe4+ ion. At high temperatures, the charge is assumed to be delocalized and the magnetic behavior is similar to that of the charge‐compensated YIG. Because of the strong ferromagnetic superexchange between the Fe3+ ion on the octahedral site and the Fe4+ on the tetrahedral site, the ground state of the spin on the Fe4+ site is proposed to be parallel to the octahedral site. Therefore, the theoretical model assumes the presence of three Ising‐like magnetic sublattices: (a) a tetrahedral site Fe4+, (b) a tetrahedral site Fe3+, and (c) a octahedral site Fe3+. The magnetization, calculated in the molecular‐field theory, shows a compensation point in accordance with the observation. The cubic anisotropy energies are calculated by considering the fourth‐order terms in the sp...