Structural, microstructural and electromagnetic properties of NiO doped manganese-zinc (MnZn) ferrites prepared by a solid-state reaction method have been investigated. The continuous doping of NiO resulted in an increase in lattice constant (a), porosity (P) and electrical resistivity (rho), a decrease in average grain size (D) and bulk density (d(b)), while no obvious change in X-ray density (d(x)). Hysteresis loss (P-h) increased monotonically with increasing NiO, and its variation could be explained by the relation: P-h infinity 1/mu(3)(i), where mu(i) was initial permeability. Eddy current loss (P-e) first reduced before achieving its minimum, and then rose gradually. The variation of P-e was inconsistent with the classical expression, P-e,P-cl infinity D-2/rho, where D and rho were average grain size and resistivity, respectively, and the excess loss (P-e,P-exc) could contribute to the P-e. Residual loss (P-r) reduced slowly due to D's decrease and f(r)'s increase which were caused by the increase in NiO, where f(r) was resonance frequency. Finally, the sample doped with 0.10 wt% NiO showed the lowest losses of 207 kW m(-3) at 100 degrees C, 1 MHz and 30 mT. (C) 2008 Elsevier B.V. All rights reserved.