Using a model Hamiltonian and the Green's function technique for the Zn doped Mn-ferrite, Mn1-xZnxO center dot Fe2O3, and the Gd doped Zn-ferrite, ZnGdxFe(2-x)O4, nanoparticles of different compositions x were studied. The phase transition temperature, T-C, and the coercive field, H-c, for different samples dependent upon composition, particle size, and shape were investigated. An attempt was made to enhance or to lower the T-C of the nanoparticles to the optimum temperature required in magnetic hyperthermia (42-43 degrees C). (C) 2011 American Institute of Physics. [doi:10.1063/1.3580476]