  The experiments on the impact of the size of magnetite clusters on various
magnetic properties (magnetic moment, Curie temperature, blocking temperature
etc.) have been carried out. The methods of magnetic separation, centrifuging
of water suspensions of biocompatible iron oxide nanoparticles (NPs) allow
producing fractions with diameter of nanoparticles in the range of 4{\div}22
nm. The size of NPs are controlled by the methods of dynamic light scattering
(DLS), transmission electron microscopy (TEM) and atomic force microscopy
(AFM). For the first time the DLS method is applied in real time to control the
size during the process of the separation of the NPs in aqueous suspensions.
The changes of the size of NPs cause a shift in the Curie temperature and in
the changes in the specific magnetic properties of the iron NPs. The
experimental data is interpreted on the basis of Monte Carlo simulations for
the classical Heisenberg model with different bulk and surface magnetic
moments. It is demonstrated experimentally and by theoretical modeling that
magnetic properties of magnetite NPs are determined not only by their sizes,
but also by the their surface spin states, while both growing and falling
dependences of the magnetic moment (per Fe3O4 formula unit) being possible,
depending on the number of magnetic atoms in the nanoparticle. Both NPs clean
and covered with a bioresorbable layer clusters have been investigated.
