Permanent Magnet

permanent magnet [′pər·mə·nənt ′mag·nət]

(electromagnetism)

A piece of hardened steel or other magnetic material that has been strongly magnetized and retains its magnetism indefinitely. Abbreviated PM.

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Permanent Magnet 

an article of a specific shape (such as a horseshoe or bar) made of previously magnetized ferromagnets or ferrimagnets that are capable of retaining high magnetic induction after removal of the magnetizing field (called hard-magnetic materials). Permanent magnets are widely used as independent sources of a constant magnetic field in electrical engineering, electronic technology, and automation.

The principal physical characteristics of a permanent magnet depend on the nature of the demagnetizing branch in the magnetic hysteresis loop of the material from which the magnet is made. The higher the coercive force H_c and the residual induction B_r of the material (see Figure 1)—that is, the greater the magnetic hardness of the material—the more suitable it is for a permanent magnet. The induction of a permanent magnet can be equal to the maximum residual induction B_r only when the magnet is a closed magnetic circuit.

Figure 1. Curves of demagnetization (a) and magnetic energy (b) for a ferromagnet: (Br) residual magnetic induction, (H_c) coercive force, (H_d) demagnetizing field, (B_a) induction in the field H_d

A permanent magnet is usually used to produce a magnetic flux in an air gap—for example, between the poles of a horseshoe magnet. The air gap reduces the induction (and the magnetization) of the permanent magnet in the manner of an external demagnetizing field H_d. The magnitude of the field H_d that reduces the residual induction B_r to a value B_d depends on the configuration of the permanent magnet. Thus, the magnetic fields produced by permanent magnets may have an induction B < B_r. The greatest effectiveness is achieved if the state of the magnet corresponds to the point on the demagnetization curve where the product of B and H is at a maximum, (BH)max—that is, where the magnetic energy per unit volume of material is at a maximum. Among the materials from which permanent magnets are made are alloys based on iron, cobalt, nickel, aluminum and hexagonal ferrites. The newest and most effective materials include ferrimagnetic intermetallic compounds of the rare earths samarium and neodymium with cobalt (such as SmCo₅). Such compounds have extremely high values of (BH) max (see Table 1).

Table 1. Main characteristics of materials for permanent magnets (data are averaged)
	Hc(oersteds)	Br(gauss)	(BH)max × 106 (gauss·oersted)	Year of first use
Carbon steel ................	50	10,000	0.26	1880
Cobalt steel ................	240	9,200	0.9	1917
Fe-Ni-Al alloy ................	480	6,100	1.05	1933
Barium hexagonal ferrite ................	1,800	2,000	0.9	1952
Pt-Co alloy ................	4,300	6,500	9.5	1958
SmCo5................	9,500	9,000	20.0	1968

An essential condition for achieving the highest magnetic characteristics in a permanent magnet is its preliminary magnetization up to a state of magnetic saturation. Another important requirement is the stability of the magnetic characteristics with time (that is, the absence of magnetic aging). Permanent magnets made from materials subject to magnetic aging undergo special treatments (thermal treatment and treatment in a variable magnetic field) to stabilize their condition.

REFERENCES

Zaimovskii, A. S., and L. A. Chudnovskaia. Magnitnye materialy [3rd ed.]. Moscow-Leningrad, 1957.
Bozorth, R. Ferromagnetizm. Moscow, 1956. (Translated from English.)
Smit, J., and H. P. J. Wijn. Ferrity. Moscow, 1962. (Translated from English.)
Postoiannye magnity: Spravochnik. Moscow-Leningrad, 1963. (Translated from English.)
Rabkin, L. I., S. A. Soskin, and B. Sh. Epshtein. Ferrity. Leningrad, 1968.
Belov, K. P. “Redkozemel’nye magnitnye materialy.” Uspekhi fizicheskikh nauk, 1972, vol. 106, fasc. 2.

K. P. BELOV