Calorimetric method for the determination of Curie temperatures of magnetic nanoparticles in dispersion

MnxZn(1-x)Fe(2)O(4)-based magnetic fluids with x = 0.1-0.9 are synthesized by coprecipitation. The samples are heated in a radio frequency (rf) magnetic field using an rf generator at different powers, and the temperature is measured as function of time using an optical thermometer. The heating effect of the dispersed magnetic nanoparticles is proportional to the imaginary part of the dynamic magnetic susceptibility of the ferrofluid, a quantity that depends on the temperature through the magnetization of the ferrite nanoparticles and the Neel or Brownian relaxation times, respectively. We propose an extrapolation method to actuate the Curie temperatures of the dispersed magnetic nanoparticles. By means of appropriate fitting functions for (dT/dt) versus T for both the heating and the cooling process, we deduce the Curie temperature of the samples under investigation. For MnxZn1-xFe2O4-based magnetic nanoparticles the Curie temperatures decrease with increasing Zn content. They turn out to be lower than the literature values for bulk MnxZn1-xFe2O4, a phenomenon which is generally observed for phase transitions of nanocrystalline materials.