MAGNETOSTATIC AND EXCHANGE INTERACTIONS BETWEEN PARTICLES IN PERMANENT MAGNET MATERIALS

The magnetostatic interactions in assemblies of single-domain particles are discussed and it is concluded that large-scale interaction domain structures occur as a result of transient lateral interaction fields during particle switching. Domain structures in a number of Alnico-type alloys are reported and the effects of a.c. demagnetization and rotating field treatments examined. Remanence curves, anhysteretic remanence curves, and the angular variation of coercive force and switching field were measured for Alnico alloys and elongated single-domain materials. The main conclusions are: (i) no direct correlation can be made between domain structure and coercivity; (ii) the structures are formed primarily as a result of magnetostatic interaction; (iii) both magnetostatic and exchange interactions are important in certain alloys; (iv) a uniformly demagnetized structure of alternate oppositely magnetized particles is formed in certain alloys, and in elongated single-domain materials, after a.c. demagnetization; (v) a fanning or buckling mode of reversal occurs in both alloys and elongated single-domain materials; (vi) the form of the anhysteretic curves are governed by negative interactions within the assembly which are stronger in the high-coercivity alloys and elongated single-domain materials than in the low-coercivity alloys.