A study of the magnetic properties of mixed ferrite nanoparticles in ferrofluids

Magnetic properties of ZnxCo1−xFe2O4 (x = 0.3, 0.5 and 0.7) nanoparticles in ferrofluids have been investigated by means of Mössbauer spectroscopy in the temperature range 4.2-300 K. The mean sizes of the particles were 10.2, 7.4 and 5.2 nm, respectively. The transition temperatures to the superparamagnetic state and values of crystal magnetic anisotropy have been evaluated. The transition to the superparamagnetic state was observed at temperatures below the freezing point of the carrier liquid for the 5.2 nm and 7.4 nm particles and above its freezing point for the 10.2 nm particles. A multi-level model is used in the analysis of the Mössbauer spectra and in the calculation of the hyperfine magnetic field.