Abstract. Cobalt-ferrite (CoFe 2 O 4 ) nanoparticles (CFNPs) are obtained using direct plasmachemical synthesis in the plasma of a low-pressure arc discharge. The formation of the CFNPs with an average size of 9 nm and a narrow granulometric composition is established employing the methods of X-ray structure analysis and transmission microscopy. The CFNP behavior upon high-temperature annealing is analyzed. The CFNP functional groups are determined using the infrared Fourier spectrum. The results of the X-ray energy dispersion confirm the correspondence of the ratio of the number of atoms of each material to the nominal stoichiometry. The basic magnetic properties of the obtained and annealed samples are investigated at room temperature using the vibrating spectrum magnetometry (VSM). IntroductionIn recent years, the obtaining and application of ferrite nanoparticles (FNP) has aroused great interest owing to their unique magnetic and electric properties. Ferrite nanoparticles are used for preparing magnetic fluid seals in the chemical industry, as magnetic lubricants for enriching of nonmagnetic materials, for increasing recording density in magnetic devices, and so on. One of the important applications of FNPs is their use in biology and medicine. Thus, FNPs can serve as "couriers" ensuring the delivery of a medicine or a diagnostic agent [1] to the specified area. The method of incorporating magnetic fluids into tumor tissues, where the subsequent action by a high-frequency magnetic field causes a local heating and destruction of cancer cells (so-called hyperthermia), has been successfully tested [2,3]. FNPs are widely employed as catalysts for CO 2 decomposition [4].Magnetic properties of FNPs are mainly determined by superparamagnetic relaxation, which manifests itself in nanostructured systems with nanoparticles smaller than 10 nm in size. At the same time, some distinctive features of FNPs are connected with the presence of a system of aggregates of a certain type, which appear, as a rule, due to magnetodipole interaction between single-domain nanoparticles and the action of magnetic fields. Among spinel ferrites, CoFe 2 O 4 has the cubic structure. The CFNP physical and chemical properties strongly depend on the method of synthesis. The chemical method of the CFNP synthesis is the most widely employed due to high productivity and simplicity. Two different methods of chemical synthesis of CoFe 2 O 4 nanocrystals are known today. The first method implies the use of an oxidizing agent such as H 2 O 2 [5] and KNO 3 [6]. The method without an oxidizing agent is more elaborate [7]. Chemical methods are quite complicated and inefficient, and the obtained nanoparticles are contaminated by the products of chemical reactions. The plasmachemical methods of producing magnetic nanoparticles are the most advanced methods [8][9][10][11][12]. The presence of supersaturated vapor of the initial compounds in the plasma and high rates of their annealing lead to the formation of nanoparticles under certain conditions. Variat...

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