Soft-Magnetic Materials

The following article is from The Great Soviet Encyclopedia (1979). It might be outdated or ideologically biased.

Soft-Magnetic Materials

 

magnetic materials that magnetize to saturation and experience a reversal in polarity in relatively weak magnetic fields, with an intensity H ~ 8-800 amperes per m (A/m), or 0.1-10.0 oersteds. At temperatures below the Curie point (for Armco iron, up to 768°C), soft-magnetic materials are magnetized spontaneously but do not manifest magnetic properties externally, since they consist of randomly oriented regions (domains) that are magnetized to saturation. Soft-magnetic materials are characterized by high values of magnetic permeability (initial permeability μa ~ 102 - 105; maximum permeability μmax ~ 103-106). The coercive force Hc of soft-magnetic materials ranges from 0.8 to 8 A/m (0.01-0.1 oersted), and the magnetic hysteresis losses per remagnetization cycle are very small (~ 1-103 joules per cu m, or 10-104 ergs per cu cm). The ability of soft-magnetic materials to magnetize in weak magnetic fields results from the low magnitude of the energy of magnetic crystalline anisotropy, and in some such substances (such as Fe-Ni-based soft-magnetic materials and some ferrites), also from a low magnitude of magnetostriction. This is because magnetization takes place as a result of the displacement of the boundaries between domains and rotation of the magnetization vector of the domains. Mobility of the boundaries, which facilitates magnetization, is reduced in the presence of various discontinuities and stresses that alter the energy of the boundaries upon displacement. Therefore, magnetic materials that have significant energy of magnetic crystalline anisotropy but in which there are no deleterious interstitial impurities such as carbon, nitrogen, or oxygen (or, more accurately, small quantities of them), dislocations and other defects that distort the crystal lattice, and inclusions in the form of other phases or cavities significantly larger than the parameters of the lattice also have the properties of soft-magnetic materials. However, the process of rotation of the magnetization vector in such materials requires application of stronger fields. The production of such nearly flawless materials entails great manufacturing difficulties. A number of alloys (such as Perminvars) and some ferrites with low energy of magnetic crystalline anisotropy but with a well-defined uniaxial anisotropy that is formed during the annealing of the material in a magnetic field are also among the soft-magnetic materials. Some soft-magnetic materials (such as Permendur) are weakly anisotropic but highly magnetostrictive.

Soft-magnetic materials are subdivided into two groups according to their function: materials for weak-current equipment and for electrical steel. Binary and other Fe-Ni alloys (Permalloys), which have low Hc (≈ 0.01 oersted) and very high μa (up to 105) and μmax (up to 106), are the most important representatives of the soft-magnetic materials used in weak-current technology. Alloys based on Fe and Co (such as Permendur), which have the highest Curie point among soft-magnetic materials (950°-980°C) and a saturation value of magnetic induction Bs of up to 2.4 × 104 gauss (2.4 teslas), as well as Fe-Al and Fe-Si-Al alloys, also belong to this group. Alloys with an Fe-CoNi base, and having constant magnetic permeability achieved by heat treatment in a transverse magnetic field that forms induced uniaxial anisotropy, are used for operation at frequencies up to 105 hertz the crystalline magnetic anisotropy must be as small as possible. Constancy of the magnetic permeability (within 15 percent) is maintained for inductions up to 8,000 gauss and takes place because the rotational process is dominant in the magnetization of such soft-magnetic materials. Magnetodielectrics, which are fine powders of carbonyl iron, Permalloy, or Alsifer mixed with a dielectric bonding material, are used in the frequency range 104-108 Hz.

Mixed ferrites (for example, a compound consisting of zinc and nickel ferrites) and ferrite-garnets, whose crystal structure is identical with natural garnets, are used extensively in weak-current technology. They are characterized by exceptionally high electrical resistance and the virtual absence of the skin effect. Ferrite-garnets are used at very high frequencies (if the dielectric losses are small).

Soft-magnetic alloys are smelted in metallurgical furnaces, and the ingots are forged or rolled to impart the required shape to them. Ferrites are produced by sintering metal oxides at high temperatures, and the products are pressed from powder (produced by grinding the ferrite) and baked. Transformer cores (such as microphone, output, intermediary, and peak transformers), magnetic screens, computer memory components, and the cores of magnetic recording heads are made of soft-magnetic alloys. Magnetic antennas, wave guides, and other products are also made of ferrites.

Among the electrical steels are iron-based alloys with silicon (0.3-6.0 percent by weight). The alloys also contain 0.1-0.3 percent manganese. The steels may be hot-rolled (isotropic) or cold-rolled (grain-oriented). Energy losses during a magnetization cycle are lower, and the magnetic induction higher, for grain-oriented steel than for hot-rolled steel. Electrical steels are used in the production of generators, transformers, and electric motors.

All cold-rolled soft-magnetic alloys and steels are heat-treated (at 1100°-1200°C) in a vacuum or in a hydrogen atmosphere to improve their magnetic properties. The Fe-Co, Fe-Ni, and Fe-Al alloys tend to order their structure at temperatures of 400°-700°C; therefore, in this temperature range each alloy must have its own cooling rate, for which the required structure of the solid solution is created.

Thermomagnetic alloys, which are used to compensate for temperature changes of magnetic fluxes in the magnetic systems of instruments, and magnetostrictive materials, by means of which electromagnetic energy is converted to mechanical energy, are among the special-purpose soft-magnetic materials.

The characteristics of the most widely used soft-magnetic materials are presented in Table 1.

I. M. PUZEI

The Great Soviet Encyclopedia, 3rd Edition (1970-1979). © 2010 The Gale Group, Inc. All rights reserved.