epsilon-Fe2.4Co0.6N nanoparticles are prepared by nitridation of ultrafine iron-cobalt oxide in NH3 gas atmosphere at 873 K for 3 h. Their magnetic properties are studied using ac and dc magnetization techniques. Two peaks at 69 and 151 K are found it) ac Susceptibility study. The former peak is assigned to superparamagnetic blocking temperature (T-B similar to 69 K), whereas the origin of the latter one could not be ascertained since it is observed only from imaginary part of ac susceptibility study. The positions of peaks are found to be dependent on the applied fields and frequencies. Also, dc magnetization studies corroborate T-B. Anisotropy constant (K-a) is found to be 2.3 x 10(5) erg/cm(3). The peaks disappear at high-applied field due to the overcome of applied filed over anisotropic energy. Ferromagnetic domains rotate incoherently. At low temperature, Bloch law cannot be applied, but Curie law can be applied above T-B. Interestingly, spin relaxation is faster at 10 K as compared to that at 70 K indicating the presence of frustration of spins at lower temperature. (C) 2009 Elsevier Ltd. All rights reserved.