Enhanced magnetoelectric coupling and mechanism transition in Ba_1.5Sr_1.5-based Z-type hexaferrites via Jahn-teller active Cu²⁺ substitution

Current exploration of magnetoelectric Z-type hexaferrites is predominantly confined to Sr₃Co₂Fe₂₄O₄₁ (Sr₃-based) compositions. Expanding the compositional space to higher Ba content (e.g., Ba_1.5Sr_1.5-based) has been hindered by the suppression of the ferroelectric transverse conical phase below 200 K. To overcome this limitation, a series of Cu-substituted Ba_1.5Sr_1.5Co_{2- x} Cu _x Fe₂₄O₄₁ ceramics were synthesized and systematically investigated. Appropriate Cu substitution enhances the electrical resistivity by suppressing oxygen vacancies, and more importantly, introducing Jahn-Teller (J-T) active Cu²⁺ ions to induce J-T distortions that modulate the coordination environment of magnetic ions, thereby tuning the magnetoelectric coupling properties. Our results identify x = 0.75 as a critical composition that effectively breaks this thermal ceiling, extending the magnetoelectric phase up to 250 K while achieving an enhanced polarization (3.2 μC/m²) and dynamic magnetoelectric responses ( α = 9 mV/(cm·Oe), β = −27 μV/(cm·Oe²)). More significantly, the observed changes in the magnetoelectric coupling behavior resulting from Cu substitution are consistent with a potential transition of the dominant mechanism from p-d hybridization to the spin-current model at high Cu concentrations, which is plausibly associated with the breaking of crystal symmetry elements induced by J-T distortions. This work demonstrates the feasibility and potential mechanism of utilizing Jahn-Teller active Cu²⁺ ion doping to modulate magnetoelectric coupling, offering a new strategy for designing high-performance multiferroics.