Influence of interactions to the properties of ultrasmall CoFe2O4 nanoparticles estimated by Mössbauer study

• Ultrasmall CoFe2O4 nanoparticles were studied by Mössbauer spectroscopy.
• CoFe2O4 nanoparticles before and after gold plating were compared.
• Influence of interactions to the properties was determined.

Superparamagnetic properties of ultrasmall CoFe2O4 (1–3 nm) nanoparticles before and after gold plating were studied by Mössbauer spectrometry. The nanoparticles in the suspension in water were compared with same dried nanoparticles. Blocking temperature increases by 10–20 K after drying of the suspension. Strong dipolar and indirect exchange interactions between nanoparticles in the dried state were used to explain observed differences. The strength of interactions was estimated by the application of the multilevel model for the description of Mössbauer spectra. The strength of interactions between nanoparticles in the dried state, as it was observed, decreased with temperature. Moreover, the multilevel model allowed us to determine the superparamagnetic relaxation time and the barrier height. The results indicate that magnetic anisotropy of CoFe2O4 nanoparticles should be similar to the anisotropy of bulk CoFe2O4.