Abstract
The monodisperse magnetite nanoclusters 13–15 nm in size were synthesized and studied. The nanoclusters were stabilized in colloid solution by means of surface-active-substance (SAS)—oleic acid in the presence, paraffin—docosane and separated from each other by paraffin layers at the distance approximately ∼3 nm. The first order magnetic phase transition was observed in magnetite, when magnetization of a cluster disappears by jump at some critical temperature similar to that of the Curie or Neel temperatures typical of the bulk material. The observed effect is explained by the influence of surface-induced defects in the nanomagnetite. The developed surface of a cluster generates defects and causes surface tension. The model is proposed, and transition from a paramagnetic to magnetic ordered (superparamagnetic) state of a cluster is found experimentally by applying an external magnetic field which is more than some critical one. Nanomagnetite itself exists in a special “oxidized condition”, which elementary cell is the same as in normal magnetite, but it contains only trivalent cations in a B-sublattice.
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