The tunable magnetic and microwave absorption properties of the Nb5+-Ni2+ co-doped M-type barium ferrite

M-type barium ferrites with Nb5+-Ni2+ ions substituting for Fe3+ ions (BaFe12-2xNbxNixO19, BFNNO, x = 0-0.8) were prepared using the sol-gel method. The results show that barium ferrite with hexagonal plate-like grains is obtained in the samples with x = 0-0.8 sintered at 1350 degrees C for 3 h. The concentration of Fe2+ ions increases by similar to 24% from 15.93% with x = 0 to 19.70% with x = 0.8. The anisotropic field H-a and coercive field H-c are adjusted freely from 15.75 kOe and 1.00 kOe to 6.04 kOe and 80 Oe with increasing x from 0 to 0.7-0.8, respectively. Meanwhile, the saturation magnetization, M-s, still maintains high values of >50 emu g(-1). Multiple magnetic resonance peaks and two reflection loss (RL) peaks appear in BFNNO, which shift from >40 GHz to <18 GHz with increasing Nb5+-Ni2+ ions. The broad bandwidth (BW) of 4.5 to 9.3 GHz, low RLmin of -17.4 to -31.8 dB and tunable effective absorption frequency range (EAFR, RL < -10 dB) from 440 GHz to <18 GHz are achieved in BFNNO under a matching thickness (d(mp)) of 0.85-1.45 mm with x = 0-0.8. This indicates that BFNNO is a competitive absorbing candidate which can operate over wide frequency ranges.