On the origin of stable remanence in pseudo-single domain grains.

Abstract

The existence of pseudo-single domain (PSD) grains was postulated by F.D. Stacey in 1962 but a viable quantitative model for the magnetic moment of the canonical case of a PSD grain (a two domain grain with a single 180° wall) was explicitly derived only in 1974 by F.D. Stacey and S.K. Banerjee. According to this model, the net magnetic moment of a PSD grain arises from the wall moment alone and the high stability to alternating field demagnetization is mainly due to pinning of the wall by large surface anisotropy in such fine grains. Recent theoretical work in micromagnetics shows not only that the canonical case described above is a crude approximation but that the complexities of spin orientations in such grains are far too large so as to allow a theoretical prediction of the expected moments. Much more hope, however, can be gained from recent experimental studies on rare earth-cobalt alloys and yttrium iron garnet crystals. These studies support the basic contention that the role of surface anistropy is indeed the predominant one in explaining the high coercivity of PSD grains. These studies also provide useful experimental models for future research in this area in rock magnetism.