Microwave properties of BaFe11Mg2+0.25X2+0.25Ti4+0.25O19 (X2+=Cu, Mn, Zn, Ni and Co) nanoparticles in 0–26.5 GHz range

The structural, magnetic and microwave properties of cation substituted barium hexaferrite nanoparticles were investigated. Samples having composition of BaFe11Mg0.252+X0.252+Ti0.54+O19 (X2+=Cu, Mn, Zn, Ni and Co) were synthesized with solid state reaction route and 1 wt% boron was added as a catalyst to initialize the crystal growth. XRD analysis showed that hard hexaferrite phase was successfully obtained. The lattice parameters “a” and “c” are almost the same in all substituted ions except Cu2+, in which elongation in c-axis was detected. The highest saturation magnetization occurred in Ni2+ and the highest coercivity was observed in Cu2+ substituted samples. This was explained by site preferences of cations which has direct influence on strength of Fe–O–Fe superexchange interactions. The both ac (σac) and dc (σdc) conductivities have temperature dependency and vary with substitution of Cu2+, Zn2+, Co2+, Mn2+ and Ni2+ ions. The maximum ac conductivity was observed as 2.39×10−6 S cm−1 in Ni2+ substituted sample at 120 °C and at 10 Hz. The increase in dielectric constants (ε′, ε″) with variation of substituted divalent ions was explained by reduction of electron hopping between Fe2+ and Fe3+ ions. The absorbtion of incident microwave occurs at two different frequencies, around 10 and 20 GHz. The reflection loss close to 10 GHz was observed in all samples, whereas Mn, Zn and Ni substituted samples have the secondary reflection minima. The mechanism of reflection loss at low frequency is quarter wave cancellation at matching thickness. And, at high frequency, it is due to the natural resonance and dipole relaxation of fillers. The best absorbtion capability was obtained in Mn2+ substituted sample, which has reflection losses of −30 and −29 dB at 9.5 and 19.2 GHz.