Abstract
The experimental and theoretical concept concerning the magnetic properties of 2-dimensional arrangements of small particles with different strengths of interaction is discussed. Taking into account recent results on isolated small single-domain particles, a representation for a whole scale of possible structures from these to closely packed ferromagnetic films is given, using a molecular field formalism. The interaction considered may be much weaker than the full ferromagnetic exchange coupling. The resultant collective magnetization M of such aggregates of small particles may be considerably lower than their spontaneous magnetization M s . A transition temperature T A towards occurrence of superparamag-netism may appear that is well below the Curie temperature Θ of the individual particles. These effects seem to be consistent with experimental data obtained by a number of authors using various methods of measurement. Observations of time effects at magnetization reversals in very thin films also support the aforementioned model.
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