An investigation was made of combustion synthesis to uniformly incorporate small amounts of samarium additive into nanocrystalline Ni0.5Zn0.5Fe2-xSmxO4 (0.0 less than or equal to x less than or equal to 0.1) nanopowders (approximate to26-20 nm particle size). The effect of the addition of the rare-earth ion samarium on the microstructure, relative density and magnetic properties of the Ni-Zn ferrite obtained by combustion reaction was studied. The samples were uniaxially compacted by dry pressing, sintered at 1200degreesC/2 h, and characterized by bulk and apparent density, XRD, SEM and magnetic properties. The nanopowder samples without additive displayed an average grain size of 2.87 mum, while the addition of 0.05; 0.075 and 0.1 wt% Sm was found to inhibit grain growth, decreasing the average grain size to 0.77; 0.68 and 0.62 mum, respectively. The relative density was found to increase with the addition of samarium (>98.00% of the theoretical density). The samples without additive showed higher hysteresis parameter values. (C) 2004 Kluwer Academic Publishers.