“Interaction Anisotropy” Model of the Structure of Alnico Magnet Alloys

Abstract

The permanent magnet properties of Alnico alloys have been attributed to the shape anisotropy of an elongated single-domain magnetic precipitate oriented by cooling in a field. This paper reports measurements of the angular variation of the magnetic properties of highly directional crystal-oriented Alnico 5 DG, Columax, and Ticonal XX which are inconsistent with the simple shape anisotropy model. A domain “interaction anisotropy” model is proposed which treats the structure as a complex interconnected single-domain network. The interaction between single-domain elements within this structure minimizes magnetostatic and exchange energy by introducing local variations in particle domain configuration; these nucleate buckling magnetization reversal at particle irregularities and cross-links. In its simplest form this structure can be visualized as a series of H-shaped single-domain size units. The rather complex behavior of such a structure has been analyzed by measuring the magnetic properties of analogous physical models consisting of arrays of pivoted magnets. This analysis suggests that the progressive increase in maximum energy product from 6 million gauss-oersteds for Alnico 5 DG and 8 million gauss-oersteds for Columax to twelve million gauss-oersteds for Ticonal XX is associated with the suppression of buckling through the reduction of particle irregularities and cross-links. Electron micrographs of the structures of these materials and their observed angular variation of magnetic properties are consistent with this new model.