Selectively doped barium ferrite ceramics with giant permittivity and high tunability under extremely low electric bias

Tunable materials have been extensively studied due to their potential applications in many electrical devices. High tunability has been practically achieved in a number of ferroelectric materials such as perovskite phases under usually high DC electric field of 10–100 kV/cm. In this work, single phased M-type barium ferrite ceramics with colossal permittivity accompanied by defect pair dipoles and giant tunability under super low DC bias were successfully prepared by the sol-gel method. Results show that Zr4+ ions substituted for Fe3+ in the spinel phase of ferrites. The concentration of Fe2+ increased from 37.23% to 43.22% and subsequently decreased to 36.72% with increasing Zr4+ ions from 0 to 0.1 and then continuously to 0.3, respectively. The highest content of Fe2+ was ∼43.22% and thus the maximum concentration of Fe2+/Fe3+ pair dipoles formed between Fe2+ generated and Fe3+ nearby appeared in the ferrites with Zr4+ doping of 0.1. Not only in Zr4+ doped ferrites but also in the ferrites with doping other high valent ions, Fe2+/Fe3+ pair dipoles formed and controlled permittivity. Giant permittivity of above 30 k appeared in the ferrites with Zr4+ content of 0.1–0.3 and was controlled by external bias to form tunability. The activation energy of modulation of defect pair dipoles was only ∼0.182 eV, which is 85% lower than 1.2 eV of traditional perovskite BaTiO3. High dielectric tunability of more than 65% with only a low DC electric field of <25 V/cm was obtained in BaFe11.9Zr0.1O19 ferrites, which was in high contrast to conventional ferroelectrics where a high DC bias of dozens of kV/cm was required. Similarly, dielectric tunability of ∼40% with a low electric field of <40 V/cm was exhibited in Nb5+ or Ti4+ doped barium ferrites. Such a high tunability controlled by an extremely low bias field in barium ferrite ceramics doped by the target ions might be promised for novel applications in tunable devices.