The co-precipitation and solid state methods were used in the synthesis of barium hexaferrite (BaM). Phase pure BaM was obtained with 1, 2, 3, 5, 10, 15, 20 and 30wt% cobalt oxide (Co3O4). The addition of Co2+/3+ ions to the BaM increased the permeability and magnetic loss tangent to a value of 3.5 at 5% and reduced to I at 30% doping. With increased Co doping, M-s was reduced from 8758 emu/g, M-r increased from I I to 40 emu/g with 3-5 wt% Co and 9 emu/g for 30% doping. H, sharply increased from 540 to 2200 Oe with a reduction to 280 Oe at 10 K with increasing temperature to 300 K. T-c increased from 740 to 750 K for 30% Co doping. DTA-TGA studies of green body showed decarboxilation to occur at around 825 degrees C and the transformation of residual Co3O4 to Co2O3 at around 577 degrees C. The XRD data confirmed the Co ions substituting into Fe sites until a 10-15% doping level where the structure altered to W-type hexaferrite. The densities of the compounds varied with doping to a maximum of 4.45 g/cm(3) (c) 2006 Elsevier B.V. All rights reserved.