Interstitial Ba2+ ions doped (00 l) oriented barium ferrite thin film with high dielectric tunability under ultra-low bias for microwave applications

The barium ferrites (BaFeO₁₂) thin films with barium ions of over stoichiometry ratio of Ba/Fe = x/12 (x = 1.0–3.0) in sol precursor were successfully prepared on silicon substrates by sol–gel and spin-coating method. Results show that the rod-like ferrite grains of 100 ~ 200 nm in size were formed densely in the thin film. The (00 l) orientation of the ferrite phase was controlled to form in the thin film. Excess Ba²⁺ existed in cell lattice and doped in the interstitial sites of triangular biconical or octahedral interstices. Fe²⁺/Fe³⁺ electron pair dipoles were generated by doping Ba²⁺, increasing initially and decreasing finally. The highest content of Fe²⁺/Fe³⁺ dipoles appeared in the 4-layered thin film prepared with over stoichiometry ratio of Ba:Fe = 2:12 in sol precursor, contributing a highest dielectric tunability of 69%, low dielectric loss of 0.03 and FOM of 21.6 at 10 kHz under super low DC bias of ~ 200 V/cm. Such high tunability and low loss under super low electric field in the barium ferrite thin films show great potential for novel applications in dielectric tunable devices.