Soft magnetic material

The basic magnetic properties expected in soft magnets are high initial and maximum permeabilities, p-, and r,., and low coercivity, (lower than 10 A/m). In many applications, soft magnets are subjected to ac fields their frequency response and, particularly, their ac losses are therefore important. 

A number of the original references are old and sometimes not readily accessible instead, some excellent reviews on soft magnetic materials are given (Chen, 1977 Chin Wernick, 1980 Cullity, 1972). When expressing alloy compositions, an atomic per cent basis is used unless otherwise specified. 

An Iron-Silicon Alloy Used in Transformer Cores 

the grains in polycrystalline materials are oriented randomly, with the result that their properties are isotropic (Section 3.15). However, one way of developing anisotropy in polycrystalline metals is 

The magnetic characteristics of this alloy may be further improved through a series of deformation and heat-treating procedures that produce a (100)[001j texture. 

Material Composition H t%) Initial Relative Permeability pi Saturation Flux Density [tesla (gauss)] Hysteresis Loss/Cycle [J/ui (eig/cm )] Resistivity p (D m) 

Source Adapted from Metals Handbook Properties and Selection Stairdess Steels, Tool Materials and Special-Purpose Metals, 

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The permeabiHty = B jH is important information for soft magnetic materials. The most often quoted values ate the initial permeabiHty and the maximum permeabiHty ]1. These correspond to the initial and maximum slopes of the virgin magnetization curve, respectively (Fig. 1). Because the value of at S/H = 0 needs to be extrapolated from measurements at finite H, the value often is quoted ia commercial catalogues at specific B or H The quoted values usually are relative to the free-space value p.Q. 

Soft magnetic materials are characterized by high permeabiUty and low coercivity. There are sis principal groups of commercially important soft magnetic materials iron and low carbon steels, iron—siUcon alloys, iron—aluminum and iron—aluminum—silicon alloys, nickel—iron alloys, iron-cobalt alloys, and ferrites. In addition, iron-boron-based amorphous soft magnetic alloys are commercially available. Some have properties similar to the best grades of the permalloys whereas others exhibit core losses substantially below those of the oriented siUcon steels. Table 1 summarizes the properties of some of these materials. 

Fig. 2. Magnetization curves of commercial soft magnetic materials.

B. Thomas, Proceedings Conference Soft Magnetic Materials, Cardiff, Wales, 1975, p. 109. 

R. Ball, Soft Magnetic Materials, Heyden and Sun Ltd., London, 1979, handbook of soft magnetic materials. 

C. W. Chen, Magnetism and Metallurgy of Soft Magnetic Materials, North-HoUand, New York, 1977. 

Energy losses in soft magnetic materials arise due to both hysteresis and eddy currents, as described in the previous section. Eddy current losses can be reduced by increasing the electrical resistivity of the magnetic material. This is one reason why solid-solution iron-silicon alloys ( 4% Si) are used at power frequencies of around 60 Hz and why iron-nickel alloys are used at audio frequencies. Some magnetically soft ferrites (see Section 6.2.2.1) are very nearly electrical insulators and are thus immune to eddy current losses. Some common soft magnetic materials and their properties are listed in Table 6.19. Soft magnetic alloys are described further in Section 6.2.1.6. 

Table 6.19 Selected Properties of Some Soft Magnetic Materials...

Zuberek, R., K. Fronc, E. Mosiniewicz-Szablanska and H. Szymczak, 1997b, J. Phys., IV Proc. Soft Magnetic Materials vol. 13, Grenoble, 24-26 Sep. 1997. 

There are two principal uses for glassy metals. Because metal glasses have no barriers for domain wall movement they are excellent soft magnetic materials. Thin ribbons have been used for transformer cores since the 1960s. Metallic glasses have very good corrosion resistance and very low damping. 

A hard magnetic material has a much greater hysteresis than a soft magnetic material. The differences are much greater than shown in this figure. 

A metallic glass containing 80% Fe and 20% B is an excellent soft magnetic material because there are no grain boundaries to obstruct domain wall motion. 

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