Starting from the basic constitutive equation that describes the magne
tic viscosity of a ferromagnetic material, under the single assumption
of a constant external field H, a connection is shown between the dif
ferent expressions used to determine experimentally the fluctuation fi
eld H-f. The simplest method uses the relation H-f = -partial derivati
ve H/partial derivative lnt\(Mirr). If H-f is invariant during the vis
cous decay of the magnetization, the relation H-f = -partial derivativ
e H/partial derivative ln(M(irr))\(Mirr) may also be employed. The rel
axation 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 revers
al. A method of classification of the hard ferromagnetic materials, th
rough experimental means, is suggested.