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 2 O 3 is examined.

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