Increased magnetic coercivity and enhanced microwave absorption in Cr²⁺–Al³⁺ doped BaSr ferrites and the composites with multiwall carbon nanotube

Abstract The Cr 3+ and Al 3+ ions in Ba x Sr 1− x Fe 12− y (Cr 0.5 Al 0.5 ) y O 19 decrease the saturation magnetization, and increase the coercive field. The Cr 3+ and Al 3+ doping linearly increases the magnetocrystalline anisotropic constant K . The increased dipole polarizations due to the doping ions contribute to the dielectric permittivity in the frequency range of 1–18 GHz. As a result of the increased dielectric loss, both the attenuation factor and the impedance matching are improved. While Ba 0.48 Sr 0.52 Fe 12 O 19 has no effective microwave absorption ( RL < −10 dB), the doped Ba 0.41 Sr 0.59 Fe 11.7 Cr 0.08 Al 0.21 O 19 has a bandwidth of 7.7 GHz in the frequency range of 5.6–12.3 GHz under 4 mm, and Ba 0.36 Sr 0.64 Fe 11.67 Cr 0.11 Al 0.22 O 19 with more Cr 3+ and Al 3+ ions has a bandwidth of 7.7 GHz in 6–13.7 GHz under 4 mm. The composites of Ba x Sr 1− x Fe 12− y (Cr 0.5 Al 0.5 ) y O 19 /multiwall carbon nanotubes (MWCNTs) have obviously higher dielectric loss, attenuation factor, and improved impedance matching than the corresponding compounds. The microwave absorption bandwidths of the composites are significantly widened. This is due to the fact that the composites are composed of the highly conductive MWCNTs and the dielectric Ba x Sr 1− x Fe 12− y (Cr 0.5 Al 0.5 ) y O 19 . All the composites have a bandwidth of 13.7 GHz from 4.3 GHz to 18 GHz under 4 mm. The Ba 0.48 Sr 0.52 Fe 12 O 19 /MWCNTs composite has a minimum reflection loss RL = −56 dB at a thin thickness of 2.6 mm. This work presents a promising method of designing microwave absorbing materials.