Ferromagnetic materials
Now we would like to talk about the various kinds of magnetic materials that there are in the technical world and to consider some of the problems involved in designing magnetic materials for different purposes. First, the term “the magnetic properties of iron,” which one often hears, is a misnomer—there is no such thing. “Iron” is not a well-defined material—the properties of iron depend critically on the amount of impurities and also on how the iron is formed. You can appreciate that the magnetic properties will depend on how easily the domain walls move and that this is a gross property, not a property of the individual atoms. So practical ferromagnetism is not really a property of an iron atom—it is a property of solid iron in a certain form. For example, iron can take on two different crystalline forms. The common form has a body-centered cubic lattice, but it can also have a face-centered cubic lattice, which is, however, stable only at temperatures above $900^\circ$C. Of course, at that temperature the body-centered cubic structure is already past the Curie point. However, by alloying chromium and nickel with the iron (one possible mixture is $18$ percent chromium and $8$ percent nickel) we can get what is called stainless steel, which, although it is mainly iron, retains the face-centered lattice even at low temperatures. Because its crystal structure is different, it has completely different magnetic properties. Most kinds of stainless steel are not magnetic to any appreciable degree, although there are some kinds which are somewhat magnetic—it depends on the composition of the alloy. Even when such an alloy is magnetic, it is not ferromagnetic like ordinary iron—even though it is mostly just iron. We would like now to describe a few of the special materials which have been developed for their particular magnetic properties. First, if we want to make a permanent magnet, we would like material with an enormously wide hysteresis loop so that, when we turn the current off and come down to zero magnetizing field, the magnetization will remain large. For such materials the domain boundaries should be “frozen” in place as much as possible. One such material is the remarkable alloy “Alnico V” ($51\%$ Fe, $8\%$ Al, $14\%$ Ni, $24\%$ Co, $3\%$ Cu). (The rather complex composition of this alloy is indicative of the kind of detailed effort that has gone into making good magnets. What patience it takes to mix five things together and test them until you find the most ideal substance!) When Alnico solidifies, there is a “second phase” which precipitates out, making many tiny grains and very high internal strains. In this material, the domain boundaries have a hard time moving at all. In addition to having a precise composition, Alnico is mechanically “worked” in a way that makes the crystals appear in the form of long grains along the direction in which the magnetization is going to be. Then the magnetization will have a natural tendency to be lined up in these directions and will be held there from the anisotropic effects. Furthermore, the material is even cooled in an external magnetic field when it is manufactured, so that the grains will grow with the right crystal orientation. The hysteresis loop of Alnico V is shown in Fig. 37–12. You see that it is about $700$ times wider than the hysteresis curve for soft iron that we showed in the last chapter in Fig. 36–8. Let’s turn now to a different kind of material. For building transformers and motors, we want a material which is magnetically “soft”—one in which the magnetism is easily changed so that an enormous amount of magnetization results from a very small applied field. To arrange this, we need pure, well-annealed material which will have very few dislocations and impurities so that the domain walls can move easily. It would also be nice if we could make the anisotropy small. Then, even if a grain of the material sits at the wrong angle with respect to the field, it will still magnetize easily. Now we have said that iron prefers to magnetize along the $[100]$ direction, whereas nickel prefers the $[111]$ direction; so if we mix iron and nickel in various proportions, we might hope to find that with just the right proportions the alloy wouldn’t prefer any direction—the $[100]$ and $[111]$ directions would be equivalent. It turns out that this happens with a mixture of $70$ percent nickel and $30$ percent iron. In addition—possibly by luck or maybe because of some physical relationship between the anisotropy and the magnetostriction effects—it turns out that the magnetostriction of iron and nickel has the opposite sign. And in an alloy of the two metals, this property goes through zero at about $80$ percent nickel. So somewhere between $70$ and $80$ percent nickel we get very “soft” magnetic materials—alloys that are very easy to magnetize. They are called the permalloys. Permalloys are useful for high-quality transformers (at low signal levels), but they would be no good at all for permanent magnets. Permalloys must be very carefully made and handled. The magnetic properties of a piece of permalloy are drastically changed if it is stressed beyond its elastic limit—it mustn’t be bent. Then, its permeability is reduced because of the dislocations, slip bands, and so on, which are produced by the mechanical deformations. The domain boundaries are no longer easy to move. The high permeability can, however, be restored by annealing at high temperatures.  It is often convenient to have some numbers to characterize the various magnetic materials. Two useful numbers are the intercepts of the hysteresis loop with the $B$- and $H$-axes, as indicated in Fig. 37–12. These intercepts are called the remanent magnetic field $B_r$ and the coercive force $H_c$. In Table 37–1 we list these numbers for a few magnetic materials.