Material synthesis is a scientific art, considering all parameters and conditions in practical view and applying them in an experiment. Properties of synthesized material can be tuned further by technically playing with concerned parameters individually. In this work, a one to one relation between the formation of cubic spinel ferrite structure with stable phase and morphology has been established, and corresponding changes in the magnetic properties are investigated with temperature. Heat treatment method is adopted for the preparation of nanoparticles of cobalt doped nickel ferrite Ni1-xCoxFe2O4 (x=0.5) and characterized by XRD, FESEM, TGA and VSM. Initially, as synthesized powder sample shows amorphous and weak ferromagnetic nature (Ms = 0.04 emu/g, Hc = 92.42 Oe), but heat treatment at 400ºC exhibits signature of phase formation with irregular particle shape and ferromagnetic (Ms = 6.62emu/g, Hc = 460 Oe) behaviour. On further heating upto 600ºC, the nanoparticles of Ni1-xCoxFe2O4 (x=0.5) is formed with stable cubic spinel crystal structure (lattice constant a=8.35452Å) and enhanced magnetic properties (Ms=8.24emu/g, Hc=1955Oe). Due to higher coercivity obtained of Ni1-xCoxFe2O4 (x=0.5) nanoparticles, it may be useful material in the production of data storage devices, permanent magnet, parts of electronic circuits and also in stealth technology.