Curie temperature of multiphase nanostructures

The Curie temperature and the local spontaneous magnetization of ferromagne tic nanocomposites are investigated. The macroscopic character of the criti cal fluctuations responsible for the onset of ferromagnetic order means tha t there is only one Curie temperature, independent of the number of magneti c phases present. The Curie temperature increases with the grain size and i s, in general, larger than predicted from the volume averages of the exchan ge constants. However, the Curie-temperature enhancement is accompanied by a relative reduction of the spontaneous magnetization. Due to the quadratic dependence of the permanent-magnet energy product on the spontaneous magne tization, this amounts to a deterioration of the magnets performance. The l ength scale on which an effective intergranular exchange coupling is realiz ed (coupling length) depends on the Curie-temperature difference between th e phases and on the spacial distribution of the local interatomic exchange. As a rule, it is of the order of a few interatomic distances; for much big ger grain sizes the structures mimic an interaction-free ensemble of differ ent ferromagnetic materials. This must be compared to the magnetic-anisotro py coupling length, which is of the order of 10 nm. The difference is expla ined by the nonrelativistic character of the Curie-temperature problem. (C) 2000 American Institute of Physics. [S0021-8979(00)63608-3].