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Y-type hexaferrites with tunable conical magnetic structures are promising single-phase multiferroics that exhibit large magnetoelectric effects. We have investigated the influence of Co substitution on the magnetoelectric proper-ties in the Y-type hexaferrites Ba0.3Sr1.7CoxMg2?xFe12O22 (x=0.0, 0.4, 1.0, 1.6). The spin-induced electric polar-ization can be reversed by applying a low magnetic field for all the samples. The magnetoelectric phase diagrams of Ba0.3Sr1.7CoxMg2?xFe12O22 are obtained based on the measurements of magnetic field dependence of dielectric constant at selected temperatures. It is found that the substitution of Co ions can preserve the ferroelectric phase up to a higher temperature, and thus is beneficial for achieving single-phase multiferroics at room temperature.