Magnetic Viscosity

(1) In ferromagnetism (also called the magnetic aftereffect), the time lag in the change of the magnetic characteristics of ferromagnets (such as magnetization and permeability) relative to changes in the intensity of an external magnetic field. As a result of magnetic viscosity the magnetization of a sample is established in a period ranging from 10^-9 sec to tens of minutes or even hours after the change in field intensity. Upon magnetization of ferromagnets in a variable field, losses to magnetic viscosity, which reach a significant level in high-frequency fields, occur in addition to losses of electromagnetic energy to eddy currents and hysteresis. In conductors, magnetic viscosity is often concealed by the action of eddy currents, which “squeeze” the magnetic flux from ferromagnets. To reduce the influence of eddy currents in experimental studies of magnetic viscosity, specimens of materials are taken in the form of thin wires (see Figure 1).

Figure 1. (a) Experimental curve of the drop in magnetization (in arbitrary units) of an Fe-Ni alloy wire 0.5 mm in diameter, (b) calculated curve of the drop in magnetization of the same specimen in the presence of eddy currents alone. The difference between curves (a) and (b) is due to magnetic viscosity.

Magnetic viscosity may be caused by various factors, depending on the structure of the ferromagnet, the conditions of magnetization, and the temperature. In the case of an aperiodic change in field intensity in a range of values close to the coercive force, where a change in magnetization is usually accomplished by an irreversible displacement of the domain boundaries, the viscous effect in conductors results mainly from eddy microcurrents (type 1 magnetic viscosity). The currents arise during changes in the field that are associated with magnetization reversal of domains. In this case the time required to establish a magnetic state is proportional to the differential magnetic susceptibility, and for pure ferromagnetic metals (iron, cobalt, and nickel) is inversely proportional to the absolute temperature.

Another type of magnetic viscosity results from impurities that reduce the free energy of the domain boundaries. The domain boundaries, which are shifted as a result of the change in the field, are contained at places where impurity atoms are concentrated, and the magnetization process is stopped. With time, after diffusion of the impurity atoms to other places, the boundaries are able to move farther, and magnetization continues (type 2 magnetic viscosity).

Superviscosity, for which the magnetic relaxation time is several minutes or more (type 3 magnetic viscosity), is observed in high-coercivity alloys and some other ferromagnets. This type of magnetic viscosity is associated with fluctuations in energy, mainly thermal fluctuations. The fluctuations cause magnetization reversal of the domains, which upon the change in the field received insufficient energy for immediate magnetization reversal. Diffusion and fluctuation processes depend significantly on temperature; therefore, magnetic viscosity of types 2 and 3 is characterized by a strong temperature dependence: the magnetic viscosity increases with a drop in temperature.

Type 4 magnetic viscosity, which is characteristic primarily of ferrites, is due to the diffusion of electrons between the ions of divalent and trivalent iron. This process is equivalent to the diffusion of the ions themselves but is accomplished much more easily, and therefore the magnetic viscosity of ferrites is usually low. In strong magnetic fields the action of magnetic viscosity is insignificant. Several types of magnetic viscosity often are manifested simultaneously in ferromagnets, and this complicates analysis of the phenomenon. The Soviet physicists V. K. Arkad’ev and B. A. Vvedenskii and the foreign scientists L. Neel and the Dutch physicist J. Snoek have made important contributions to the investigation of magnetic viscosity.