The magnetic properties of monodisperse magnetite in a docosane matrix are investigated. A clus ter-organized nanostructure based on nanomagnetite loses its magnetization by means of the first-order phase transition (jumplike) upon increasing the temperature to over 16 K. The thermodynamic model of such a transition is studied by taking into account magnetostriction and the compressibility of nanoclusters in the nanostructure. A value of 10~(–2) T for the critical field strength of the transition from a paramagnetic to a mag netically ordered (supermaramagnetic) state of the magnetite cluster matching the experimental value is obtained when the thermodynamic model of the magnetic phase transition of a nanocluster in an external magnetic field is analyzed. A model of the oxidized magnetite which differs from γ-Fe_2O_3 is examined.
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