NixCo1-xFe2O4 (0 <= x <= 1) nanoparticles (sizes: 8-52 nm) were synthesized by chemical coprecipitation route. Single domain limit (d(sdl)) for the nanoparticles, determined from the coercivity (H-C) versus particle-size curves, was explored as a function of nickel concentration (x). The coercivity of the particles attains a peak value at d(sdl), and it was found that coercivity decreases linearly with increasing nickel concentration in the samples. The saturation magnetization (M-S) and blocking temperature (T-b) of the system show increasing trends with increasing cobalt concentration in the nanoparticles. (C) 2012 Elsevier B.V. All rights reserved.