Soft Magnetic Materials: Types and Applications | Electrical Engineering

In this article we will discuss about the types of soft magnetic materials and its applications in electrical engineering.

Types of Soft Magnetic Materials:

1. Soft Iron-Silicon Alloys:

The most widely used type of soft magnetic material is the iron-silicon alloy. Before 1900, ordinary low-carbon steel was used for low frequency power applications, in transformers, generators, and motors. Today iron-silicon alloys, which cut power losses by a factor of three, are used instead.

The addition of silicon to iron increases the electrical resistivity, thus reducing eddy current losses and hysteresis. It also increases the magnetic permeability. The presence of silicon in iron makes rolling into sheet (most transformer cores are made of laminated sheet) more difficult.

If the rolling and annealing of silicon-iron sheet is carefully controlled, preferred crystal orientation can be induced. The directions of easy magnetization then lie in the rolling direction as illustrated in Fig. 4.12. It is easier to magnetize the textured sheets shown in Fig. 4.12 (b) and 4.12 (c) in the rolling direction than randomly textured sheet (Fig. 4.12 (a)), for the unfavourably oriented grains of Fig. 4.12 (a) require higher magnetizing fields. Fig. 4.13 illustrates the great advantage of random-textured silicon-iron over plain cast iron (or plain carbon steel), and the even greater advantage of textured silicon-iron over the random form, in permeability. Lower hysteresis also accompanies easier saturation.

2. Soft Iron-Nickel Alloys:

The initial magnetization behaviour of the iron and iron-silicon alloys is magnified in Fig. 4.13. It shows that the permeability of these materials in weak fields is relatively low. Low initial permeability is no real problem in power equipment where core materials are operated at high magnetizations. For high sensitivity and fidelity in communications equipment, iron-silicon alloys are, however, not suitable. Iron-nickel alloys are usually used in such equipment.

Table 4.5 compares the important properties of iron and iron-silicon with three commercial iron-nickel alloys. The Permalloys and Mumetals have higher initial permeability, lower hysteresis and eddy current losses. These advantages are gained, as Table. 4.6 indicate, at the expense of saturation magnetization, but are, nonetheless, useful for operation at audio and low radius frequencies.

The permeabilities of the iron-nickel alloys are sensitive to heat and mechanical treatment, especially for compositions between 50 and 80 per cent nickel. If such an alloy is slowly cooled from above 600°C to below 400°C, its permeability is about one half that of the same alloys rapidly cooled through this temperature range. This behaviour arises from the order-disorder transformation in the nickel-iron system.

Below 500°C, the equilibrium structure of such FCC alloys contains Ni atoms at face centres and Fe atoms at face corner. The perfectly ordered structure has a relatively low permeability; slow cooling past 500°C favours ordering. Quenching on the other hand suppresses the order transformation, and leads to higher permeability.

Higher permeabilities can be attained by magnetic annealing; this requires cooling from 600°C in a magnetic field. The alloy develops a strong magnetic anisotropy with the direction of easy magnetization. Magnetic annealing also affects the hysteresis curve. The square hysteresis loop that results is desired in computer, magnetic amplifier, and pulse transformer applications.

Extreme care is necessary in handling the iron-nickel alloys since plastic strain drastically lowers the permeability. Permalloy sheet or tape is therefore laminated or wound into the desired form and subsequently annealed. Such processing can raise the relative permeability of supermalloy to over one million.

3. Soft Ferrites and Garnets:

The hysteresis curves, domain structure, and domain motion of ferrites is similar to those of ferromagnetic metals. Their large magnetic permeability is due to an antiferromagnetic interaction which rigidly aligns neighbouring magnetic moments in opposite directions.

Since one set of moments is larger than the other, a net moment results. In general, the ferrites are represented by X Fe, O₄, in which X represent divalent metallic element (Fe²⁺, Mn²⁺, Co²⁺, Cu²⁺, etc.). The prototype ferrite is Fe₃O₄, the mineral magnetite, sometimes called lodestone. Most ferrites have the inverse spinel structure.

The oxygen atoms form an FCC lattice; the iron atoms are equally divided between octahedral sites, and one set of tetrahedral sites. The divalent atoms fill the remaining set of either tetrahedral sites or octahedral sites. In the ferrimagnetic ferrites the moments of all tetrahedrally coordinated ions are opposed to those of the octahedrally coordinated ions by antiferromagnetic interaction.

Thus the set of iron moments in the octahedral sites cancel those in the tetrahedral sites. The residual moment is that of the divalent ions in the remaining tetrahedral sites. It is possible to predict the saturation magnetization if the moments of the divalent ions are known.

There are hard and soft ferrites, just as in the case of ferromagnetic materials. Soft ferrites have lower saturation induction than soft ferromagnetic, yet higher resistivities. Consequently, eddy current losses are much lower; they are usually less than one millionth of those in typical silicon-iron. At frequencies above 10⁶ Hz ferrites cores are mandatory.

The net magnetic moment of the ferrite unit cell, is equal to the magnetic moment of the divalent metal ions. The saturation magnetization and, usually the permeability can be increased by divalent ions of larger magnetic moment. The rare earth ions with large moments are mainly trivalent and too large in volume to fit the tetrahedral interstices of the ferrite structure.

The ferrimagnetic oxides garnets can, however, incorporate the rare-earth ions in their structure. The formula for ferrimagnetic garnets is X₃Fe₅O₁₂, where X is any rare-earth element (Sm, Eu, Gd etc.) The garnet, Y₃Fe₅O₁₂, called yttrium iron garnet, or simply YIG, has a high resistivity and very low hysteresis loss at microwave frequencies.

The general electric and magnetic characteristics of ferrites are:

1. A very high resistivity (more than 10³ Ωm),

2. A microwave dielectric constant of the order of 10^-12,

3. Extremely low dielectric loss,

4. A permeability of several tens,

5. A saturation magnetization which is appreciable but noticeably smaller than that of ferromagnetic material and low coercive force,

6. A curie temperature which varies from few °K to several hundred °K (see Table 4.9)

7. Mechanically hard, brittle and difficult to machine.

Ferrites are carefully made by mixing powdered oxides, compacting, and sintering at elevated temperatures. The high frequency transformer in television and fm receivers is almost always made with ferrite cores. Many ferrites, such as (50% MgO, 50% MnO). Fe₂O₃, have square hysteresis loops and are, therefore, useful in computers.

The nickel-zinc ferrites, such as the Ferrox-cube listed in Table 4.8, can be magnetically annealed to improve the squareness of the hysteresis loop. Ferrites with large magnetostrictive effects are sometimes used in electromechanical transducers. In high frequency applications, magnetostriction in ferrites can lead to undesirable noise and even failure.

Applications of Soft Magnetic Materials:

The engineering problems which must be solved are those of obtaining high permeability and low energy loss materials. Cost is, of course, a real production problem as well.

The development of soft magnetic materials has been accomplished mainly with three types of materials:

(1) Fe-Si alloys for low frequency high-power applications;

(2) Fe-Ni alloys for high-quality low-power uses (e.g. for audio transformers); and

(3) Ferrites for high frequency (mega-cycle) uses.

Characteristics typical of several common soft magnetic materials are listed in Table 4.5. Pure iron, which is listed for comparison purpose is seen to have rather ordinary properties compared with those of the alloys listed. Silicon additions to pure iron improve its magnetic quality in two ways—both by increasing the resistivity so that eddy current losses are lower and by decreasing the magnetic hysteresis loss.

Two grades of Si-Fe alloys are listed-one of the common Fe—2 percent Si used for ordinary-quality induction motors, and the other of higher quality, the grain-oriented Fe—4 percent Si alloy. The larger addition of Si also makes the eddy current losses low. Both these effects could be accentuated by using even more Si, but unfortunately the alloy is so hard above 6 percent Si that it becomes almost unworkable.

The permalloy which has very high permeability is more expensive and is used in special applications like recording head and magnetic shields. Sendust (5% Al, 10% Si, and 85% Fe) on account of its high wear resistance and anti-corrosion properties is widely used as recording head material. The magnetic mild steel is used for relays and poles pieces for electromagnets.

A typical ferrite, the Mn-Zn ferrite, is listed mainly for one reason. This ferrite, like all the rest of the ferrites, is a good insulator so that p is relatively high. Thus the eddy-current loss is vanishingly small, even in un-laminated section. The ferrite is extremely useful at ultrahigh frequencies, where metallic solids become virtually useless.

Thus ferrite cores can be used in high Q antennas (the ferri-loops-stick), in pulse transformers operating in the megacycle range, in magnetic deflection yokes for cathode-ray tubes, and choke coils, recording heads, memory switching cores and magnetic amplifiers.

Thermal sensing ferrite material is used as switches in refrigerator, air-conditioners, electronic ovens and fire detection systems. These are Ni-Zn ferrites in which the initial permeability drops sharply to zero near the Curie temperature. Copper-nickel- cobalt ferrite is used in magnetostrictive oscillators which are used for medical as well as industrial purposes. Substituted lithium ferrites and ferrite powder such as -Fe₂O₃ are used for computer memory elements and tapes respectively.