Zr4+ doping-controlled permittivity and permeability of BaFe12−xZrxO19 and the extraordinary EM absorption power in the millimeter wavelength frequency range

Abstract

M-type barium ferrite powders doped with Zr⁴⁺ ions (BaFe_12−xZr_xO₁₉x = 0–0.4), possessing high permittivity and multi-resonant permeability, were synthesized by the sol–gel process. The single-phase hexagonal plate-like barium ferrites were formed with Zr⁴⁺ substituting Fe³⁺ at the 4f₁ sites initially and then at the 2b sites afterwards with increasing doping content. The H_a of the ferrites decreased from 15.75 kOe to 8.13 kOe with increasing Zr⁴⁺ from x = 0 to 0.4. The ε′ and ε′′ of the ferrite with Zr⁴⁺ doping were increased to 7.7–6 and 3.4–1.5 over 18–40 GHz from 5.8–5 and 0.7–0.4 without doping, respectively. Contributed mainly by the enhanced permittivity, the matching thicknesses of the Zr⁴⁺ ions doped ferrites were as low as only ∼1 mm or below, i.e. 1.5–3 times thinner than those reported recently. The complex permittivity was still close to the complex permeability in the doped ferrites. Hence, a strong reflection loss of ∼−50 dB could be attained and the absorptivity of electromagnetic (EM) power per unit thickness reached as high as 0.156% μm⁻¹, i.e. 2–5 times higher than those of novel absorbing materials. The multi-resonance permeability peaks originated from Fe³⁺ ions, and exchange couplings among Fe³⁺, Fe²⁺ and O⁻ contributed multiple reflection loss peaks and thus, a broad bandwidth of ∼12 GHz (RL < −10 dB), which was 2–4 times broader than those normally reported. Consequently, the Zr⁴⁺-doped barium ferrites are promising for use as excellent absorbing materials in the millimeter wavelength frequency range.