Tailoring the magnetization linearity of Finemet type nanocrystalline cores by stress induced anisotropies

Abstract

In this paper a theoretical and experimental investigation will be presented on the linearity of the flat hysteresis loop obtained by continuous stress annealing of Finemet type nanocrystalline ribbon. The stability of the effective permeability versus the magnetizing field depend on the residual random distribution of local magnetization and can be characterized by i) the coefficient a in the expression of magnetization approaching the saturation: M/Ms = 1 − a/H, valid in a restricted region, for Hlin < H < Hsat, where Hlin is the limit of linearity and by ii) the distribution of the anisotropy field ΔHK. The theoretical upper limit of linearity is given by the anisotropy field HK = Bs/µo.µeff. The linearity limit, Hlin, can be expressed as a difference between the value HK and the half width of the anisotropy field distribution, ΔHK. The linearity will be measured by the ratio R = Hlin/HK. The parameter R is almost constant (around 0.72) for large applied stresses and carefully selected annealing parameters (furnace geometry, temperature distribution along the furnace and pulling velocity). For small applied stresses (i.e. for effective permeability’s above 4000) however this linearity parameter is reduced to zero and a potbellied loop appears. A possible explanation will be given for the stress dependence of linearity parameter based on the back stress model of stress annealing.