Demagnetization

The reduction or elimination of the magnetic moment in an object; that is, the reverse of magnetization. It is commonly encountered as a procedure for eliminating the inadvertent magnetization of iron (or other ferromagnetic) parts of a sensitive mechanical device that would otherwise result in a malfunction. A suitably intense magnetic field applied in a direction opposite to that of the existing magnetization will serve to reduce or destroy that magnetization. (Alternatively, the material could, if practical, be heated to a temperature above its Curie point, then returned to room temperature, in the absence of any external magnetic field.) The adiabatic (isentropic) demagnetization of paramagnetic materials is a technique used to produce temperatures very near absolute zero. It has been used to cool and study a magnetic substance itself or, through thermal contact, a secondary substance (refrigeration). See Adiabatic demagnetization, Ferromagnetism, Magnetization

Demagnetization

the reduction of the residual, or remanent, magnetization of a ferromagnetic body, such as a specimen of some material or a component of a device, after the external magnetizing field is removed.

Magnetized components made of ferromagnetic materials are usually demagnetized for various reasons prior to the assembly of instruments and devices. It may be desired, for example, to eliminate the effect of residual magnetic fields on measuring devices or the sticking of ferromagnetic particles to components. Specimens used to determine the magnetic properties of materials are also demagnetized, since these properties are dependent on the previous magnetic history of the specimens—a phenomenon called hysteresis. Components or specimens are considered to be demagnetized if the directions of magnetization of the domains, or regions of spontaneous magnetization, are arrayed randomly and if the average magnetization (magnetic induction) in any cross section is equal to zero or is less than a value determined by specifications or established standards.

Demagnetization may be carried out in several ways. Heating the specimen or component above the Curie point causes the substance to lose completely its ferromagnetic properties. Such heating followed by cooling in the absence of any external field results in the most complete demagnetization. In most cases, however, this method of demagnetization cannot be used, since the heating may alter mechanical and other properties of the material.

Figure 1. Curve of the demagnetization of a specimen with residual magnetization I_r, by an alternating field H that decreases to zero

Another widely used technique for demagnetization, called AC demagnetization, consists in the alternating magnetization of the component or specimen by an alternating magnetic field whose amplitude decreases smoothly to zero (Figure 1). Here, the maximum value of the amplitude of the alternating demagnetizing field must generally be no less than that of the magnetizing field. The effectiveness of the demagnetization depends on the frequency of the demagnetizing field, the rate of decrease of the field, the thickness of the component, and the depth of penetration of the field. The thicker the specimen, the lower the frequency of the demagnetizing field. The higher the permeability of the material, the lower the rate of demagnetization—that is, the greater the number of demagnetization cycles. Standard specifications require that a specimen consisting of plates of electrical sheet steel 0.35-0.5 mm thick be demagnetized for 1 min by smoothly decreasing a 50-hertz magnetic field from maximum field strength of 2,000-2,500 amperes/m to zero. From 30 to 60 alternating magnetization cycles are generally sufficient for demagnetization.

REFERENCES

Kifer, I. I. lspytaniia ferromagnitnykh materialov, 3rd ed. Moscow, 1969.
Bozorth, R. Ferromagnetizm. Moscow, 1956. (Translated from English.)

I. I. KIFER