Complex permittivity, permeability, magnetic and microwave absorbing properties of Ni2+ substituted mechanically milled U-type hexaferrites

The polycrystalline samples of Ni2+ substituted U-type hexaferrite; (Ba0.7Bi0.2)4(Co1−xNix)2Fe36O60 (0.25 ≤ x ≤ 1.00, in steps of 0.25) were prepared through solid state reaction method. XRD patterns, fitted with Le Bail method, revealed the formation of U-type hexaferrite phase in as-prepared samples. Scanning electron micrograph (SEM) indicates the presence of plate-like hexagonal particles with average sizes of ∼1-4 μm. In order to reduce the grain sizes down to nanoscale, two stage high energy mechanical jet milling was performed on x = 0.75 composition sample. The jet milled hexaferrite exhibits relatively smaller grain sizes (∼20-50 nm), lower saturation magnetisation (Ms∼38.6 emu/g) and higher coercivity (Hc∼590.2 Oe) in comparison to unmilled sample of same composition (x = 0.75). The jet milled sample, with nanoscale hexaferrite particles, exhibits excellent reflection loss (RL) of −43.8 dB (99.99% MW absorption) at 11.3 GHz frequency. In addition, the jet milled sample displays a broad frequency bandwidth (∼8.6 GHz) for RL ≤ −10 dB (90% MW absorption), extending over ∼9.1-17.7 GHz frequency range.