The phenomenological model of magnetization kinetics developed by Estr
in et al. [J. Phys. 1, 4845 (1989)] yields a macroscopic magnetic equa
tion of state or magnetic constitutive equation of functional form mu0
H = f(J(irr), J(irr)), where, mu0H is the internal magnetic field, J(i
rr) is the irreversible magnetic polarization and J(irr) its time deri
vative. In differential form the relation takes the form mu0dH = (1/ch
i(irr))dJ(irr) + LAMBDAd(ln J(irr)). The quantity LAMBDA is related to
the activation volume involved in the process of irreversible magneti
zation. To obtain LAMBDA, three different experimental techniques are
used. One involves the traversal of hysteresis loops at constant J(irr
), from which both chi(irr) and LAMBDA can be obtained most directly,
using a modified version of a technique previously described2. The sec
ond method involves traversing the hysteresis loop at constant mu0H, a
nd subsequently separating the reversible and irreversible components
of magnetization. In the third method, the parameter LAMBDA is obtaine
d from magnetic viscosity measurements, which is the technique most co
mmonly used. Results are presented for all three experimental techniqu
es for measurements performed on Alnico-type permanent magnets. The li
mitations of each method will be reviewed in relation to materials pro
perties.