The fluctuation field of ferromagnetic materials

Starting from the basic constitutive equation that describes the magnetic viscosity of a ferromagnetic material, under the single assumption of a constant external field H, a connection is shown between the different expressions used to determine experimentally the fluctuation field H-f. The simplest method uses the relation H-f = -partial derivative H/partial derivative lnt\(Mirr). If H-f is invariant during the viscous decay of the magnetization, the relation H-f = -partial derivative H/partial derivative ln(M(irr))\(Mirr) may also be employed. The relaxation curves obtained at different fields, in this case, superimpose onto a single curve M(irr)(t) = F[t/t(1/2)] on renormalizing the time. An alternative treatment that considers explicitly the demagnetizing field is also presented. The theory is then applied to magneto-optic thin films, where two activation mechanisms are involved, assuming the absence of dispersion in the energy barriers, and also to the common case of relaxation by a single activation mechanism in the presence of a dispersion of the energy barriers. In both situations, it is shown that the fluctuation field may vary in strength during magnetic reversal. A method of classification of the hard ferromagnetic materials, through experimental means, is suggested.