Directional ordering as a source of coercive force and asymmetry of the loop

Abstract

The effect of directional order, formed by annealing in the demagnetized state, on the coercive force of ferromagnetic materials susceptible to magnetic anneal, is examined quantitatively. It is shown that in many magnetic materials, especially Fe-Ni alloys, this is the major source of coercivity. Experimental results are reported on the following alloys: Fe-6.4 percent Si with zero magnetostriction λ and large magnetocrystalline anisotropy constant K ; 60 percent Ni-40 percent Fe with fairly large λ. and low K ; Mumetal (77.5 percent Ni-13.5 percent Fe-5 percent Cu-4 percent Mo) with nearly zero values of both λ and K . The coercive force increase resulting from the formation of a local induced anisotropy K u , with the easy axis along the magnetization direction within each domain and domain wall, is found to vary linearly with the induced anisotropy field K_{u}/J_{s} , through a proportionality factor of the order of 0.1. This figure is shown to be in good agreement with the value predicted by the known theories which relate the coercive force to the fluctuation of the Bloch wall energy, when the contribution of an induced uniaxial term due to the formation of a random directional order is taken into account. Furthermore, in agreement with the fact that the viscosity field is also proportional to the anisotropy energy associated with directional orders a close proportionality is experimentally found to exist between the frozen-in viscosity field measured on a constricted loop and the increase of coercive force which is observed when the wasp-waisted shape of the loop is destroyed by demagnetizing the specimen. It is finally shown that materials characterized by directional anisotropy formation can show asymmetrical hysteresis loops, when properly annealed in a magnetic state corresponding to the vertex of a nonsaturated loop.