CoF₂/C/CF/PTFE nanocomposite synthesized in plasma, nanodispersed Co₃O₄ obtained from it and their magnetic properties

The CoF₂/C/CF/PTFE nanocomposite was synthesized in the plasma of a pulsed high-voltage discharge. CoF₂ nanoparticles are distributed in two size ranges ~20–100 nm and ~3–10 nm. After calcining the composite, the nanodispersed powder was obtained consisting of Co₃O₄ nanoparticles with the same particle size distribution. SQUID magnetometry was used to study the temperature and field dependences of the magnetization of the samples in the range of 300–2 K. On the graphs of temperature dependences, for both types of samples, at two different temperatures, peak-like changes occur, attributed to the transitions of CoF₂ or Co₃O₄ nanoparticles, with decreasing temperature, in antiferromagnetic state. The transition temperature T_N was estimated from the peak maxima. The existence of two transition temperatures is explained by size effects, taking into account the distribution of particle sizes in two different ranges. At temperatures below T_N, a shift of the magnetic hysteresis loops to the region of negative fields was found, which is due to the AFM/FM interaction between the core and shell of nanoparticles. For both types of samples, there is a hysteresis in the temperature dependences of FC and ZFC. The manifestation of hysteresis can be explained in the general case by the presence of the metastable state formed in an external magnetic field during the interaction of the magnetic moments of nanoparticles, in this case, arising due to the uncompensated spins on their surface. It is concluded that the effect of particle size on the shift of the transition temperature to the AFM state is more significant in CoF₂ nanoparticles than in Co₃O₄ nanoparticles.