Nucleation of Domains of Reverse Magnetization and Switching Characteristics of Magnetic Materials
Goodenough, John B.; Menyuk, Norman
The critical requirements for a ferromagnetic memory core are reviewed. Domains of reverse magnetization must form and grow within a core if its induction is to be reversed. The nature of the nucleation centers for these reverse domains will affect the shape of the hysteresis loop and the switching time. Inclusions, grain boundaries, and crystalline surfaces are analyzed as lattice imperfections which could act as nucleating centers. It is concluded that the grain boundaries are the most probable nucleation centers in most polycrystalline materials. It is shown that the criterion for a square hysteresis loop is L(cosϴ[subscript 1] - cosϴ[subscript 2])[superscript 2] < Const. ϴ[subscript w]/I[subscript B][superscript 2] where L is the average grain dianeter, ϴ[subscript 1] and ϴ[subscript 2] are the respective angles made by the magnetization vector of two neighboring grains with the normal to their common surface, ϴ[subscript W] is the surface domain wall energy density, and I[subscript S] is the saturation magnetization of the sample. This explains why loops can be squared by the alignment of a direction of easy magnetization from grain to grain. It also reveals that materials which are not so aligned may have square loops if I[subscript S] is sufficiently small. The switching time T for cores which are driven at fields roughly twice the coercive force (optimum operating conditions for a memory core) is related to the coercivity through the relation H[subscript C] T = S[subscript W] where the switching coefficient S[subscript W] is a constant of the material. Experimental agreement with this model is found.