Abstract

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.

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