On magnetic properties of superparamagnets in the Curie point region

The features of the paraprocess in superparamagnetic particles in the Curie point region are discussed. To describe the dependencies of the magnetic p roperties of superparamagnetic particles on temperature and field, the mole cular field theory supplemented with the Langevin model is applied. It is s hown that, as in the case of ordinary ferromagnetic particles, the paraproc ess in superparamagnetic particles in the region of small fields is determi ned by the dominant influence of the exchange field inside the particle. Th e role of the external field is, mainly, to change (or even to give rise to ) the exchange field. However, the quantitative characteristics describing the paraprocess in a superparamagnet are completely different. The magnetic susceptibility of the paraprocess chi(M) (M being the specific magnetizati on of the particle corresponding to its 'relaxation' magnetic moment sigma in the held H) at the Curie temperature is independent of the held strength . Under the same conditions, the 'experimentally measured' magnetization si gma is changed with the field in accordance with the H-3 law. (C) 2000 Publ ished by Elsevier Science B.V. All rights reserved.