NixCo1-xFe2O4 (x approximate to 0.3) nanoparticles were synthesized via chemical co-precipitation method. The particle size (d) was found to increase linearly with increasing annealing temperature. Magnetic studies revealed that the coercivity (H-C) attains a value of 965 Oe at similar to 18 nm and then decreases for larger particles. The behavior of H-C with d was attributed to the superparamagnetic nature of smaller nanoparticles, whereas for larger particles it was attributed to domain transformation mechanism. Both saturation magnetization (M-S) and superparamagnetic blocking temperature (T-b) were found to increase linearly with increasing size of the nanoparticles. (c) 2012 Elsevier B.V. All rights reserved.