The Measurement of Irreversible Magnetization and Activation Volumes

The phenomenological model of magnetization kinetics developed by Estrin et al. [J. Phys. 1, 4845 (1989)] yields a macroscopic magnetic equation of state or magnetic constitutive equation of functional form mu0H = f(J(irr), J(irr)), where, mu0H is the internal magnetic field, J(irr) is the irreversible magnetic polarization and J(irr) its time derivative. In differential form the relation takes the form mu0dH = (1/chi(irr))dJ(irr) + LAMBDAd(ln J(irr)). The quantity LAMBDA is related to the activation volume involved in the process of irreversible magnetization. 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 second method involves traversing the hysteresis loop at constant mu0H, and subsequently separating the reversible and irreversible components of magnetization. In the third method, the parameter LAMBDA is obtained from magnetic viscosity measurements, which is the technique most commonly used. Results are presented for all three experimental techniques for measurements performed on Alnico-type permanent magnets. The limitations of each method will be reviewed in relation to materials properties.