In the present work, Cu 2+ substituted cobalt ferrite (Co 1-x Cu x Fe 2 O 4 , x = 0, 0.3, 0.5, 0.7

Manganese substituted cobalt ferrite (Co 1−x Mn x Fe 2 O 4 with x = 0, 0.3, 0.5, 0.7 and 1) nanopowders were synthesized by chemical coprecipitation method. The synthesized magnetic nanoparticles were investigated by various characterization techniques, such as X-ray diffraction (XRD), vibrating sample magnetometry (VSM), scanning electron microscopy (SEM) and thermogravimetric and differential thermal analysis (TG/DTA). The XRD results confirmed the presence of cubic spinel structure of the prepared powders and the average crystallite size of magnetic particles ranging from 23 to 45 nm. The VSM results showed that the magnetic properties varied with an increase in substituted manganese while SEM analysis showed the change in the morphology of obtained magnetic nanoparticles. The TG/DTA analysis indicated the formation of crystalline structure of the synthesized samples. The heat transfer rate was measured in specially prepared magnetic nanofluids (nanoparticles dispersed in carrier fluid transformer oil) as a function of time and temperature in presence of external magnetic fields. The experimental analysis indicated enhanced heat transfer rate of the magnetic nanofluids which depended upon the strength of external magnetic field and chemical composition.