662 MAGNETISM [FERROMAGNETISM in the saturation intensity may amount to as much as 30%. The curve for cast cobalt resembles that for cast iron, while some Heusler alloys have magnetic properties similar to those of nickel. The magnetization curves vary considerably as the temperature is increased. The initial permeability generally increases, while the remanence and coercivity decrease. The significance of the changes in the saturation intensity has already been discussed.
In the following table are given some data as to the magnetic characteristics of a number of ferromagnetic materials, selected on account of their technological importance or their purely scientific interest. (Instead of the maximum induction B..., the value of 47r/m.„ is given, as this is characteristic of the ma terial. The maximum induction is obtained by adding to the maximum field, H....) so that the carbon steels are not so satisfactory as the tungsten and chromium steels-tungsten steel being much used. Cobalt steel has the remarkable characteristic of combining a very high coercivity with a high remanence, and, as is evident from the efficiency factor much surpasses other materials in suit ability for powerful permanent magnets. Its costliness has pre vented its extensive use except for small magnets. Silicon steel, as has been mentioned, is most suitable for transformer cores, due to its combining low magnetic hysteresis loss with high specific resistance, so that the eddy current loss is also small. Pure iron silicon alloys have been made, with very small hysteresis losses and high permeability. In general compounds of iron with non ferromagnetic elements are non-ferromagnetic, while mixed crys tals are ferromagnetic when the iron forms the solvent.
Magnetic Constants for Some Ferromagnetic Materials The detailed consideration of the magnetic properties is out side the range of this article, but a few points in connection with some of the materials in the above table may be mentioned. It has been stated already that iron has a number of transition tem peratures, changing from the a to the (3 state at 769° C, and undergoing a further transformation at about 90o° to the 7 state. The transformation temperatures are affected by the amount of carbon, or other materials, present in the iron. With manganese present, for example, the non-ferromagnetic 7 state may persist to ordinary temperatures, so that a "non-magnetic" manganese steel may be produced. At high temperatures carbon dissolves in iron, and the form in which the carbon remains in the iron at low temperatures depends on the amount present and the rate of cooling. With about 1% carbon, slow cooling results in the forma tion of "pearlite" (consisting of a mixture of layers of iron carbide, and iron) ; on rapid cooling, however, the carbon remains in solid solution in the iron. The general character of the corresponding differences in the magnetic properties appears from the figures in the table.
In spite of its low hysteresis loss electrolytic iron would be unsuitable for transformer cores, for owing to its low specific resistance, the eddy current loss would be large. The purest iron obtainable is produced by electrolysis followed by melting in vacuo, and the investigation of its properties is of great importance. The small coercivity and high permeability of dynamo "steel" are connected with the fact that the carbon content is very low (about
.04%), which is due to a secondary action (during the annealing process) of the manganese present (about .4%). Cast iron may have a high coercivity, but it is of little use for permanent magnets owing to its low remanence. Carbon steels, hardened by quenching, were formerly used for magnets (the product H,X.B, can be made fairly large, by suitable carbon content and appropriate heat treat ment), but the remanence falls off as the coercivity is increased, Nickel and cobalt have great interest as forming, with iron, the triad of elements having definite ferromagnetic properties. (Ordi nary manganese is paramagnetic, but after special heat treatment, it shows slight ferromagnetic properties.) Technically, nickel and cobalt are of importance only in alloys. The ferronickels form two series, the "irreversible" alloys with less than 34.4% Ni (cor responding to showing a strong Curie temperature hyster esis. The reversible alloys are characterized by the high value of the initial permeability (as discovered by H. D. Arnold and G. W. Elmen in 1923), which rises to a sharp maximum for the so called "permalloy," containing 78.5% Ni. The initial permea bility, with appropriate heat treatment, may reach 12,00o. The maximum intensity of magnetization is of the value to be antici pated from the composition, being about half that of iron. This saturation value, however, is practically attained in fields of a fraction of gauss (of the order of magnitude of the earth's mag netic field). Permalloy has been applied to the "loading" of cables (a high self induction is required), while other technical applications may be anticipated. Ferrocobalt with 34% Co) is remarkable in that the saturation intensity of magnetization is greater than that of either of its constituents (this being due probably to the way in which the electrons are distributed between the atoms of the complex group), so that it is suitable for the pole pieces of powerful electromagnets.
The fact that a strongly ferromagnetic alloy can be formed from non-ferromagnetic constituents was discovered by Heusler in 1898, and since then many investigations have been made as to the magnetic properties of "Heusler alloys" of different compositions, and on the manner in which these are affected by various heat treatments. The alloys contain three metals, manganese (some of whose binary alloys are ferromagnetic) being an essential con stituent. In the first alloys manganese, tin and copper were used ; but it has been found that tin may be replaced by a number of other metals, aluminium giving the most magnetizable alloys. The saturation intensity rises to a maximum for about 13% Al, and Heusler has suggested that the "carrier" of the ferromagnetism is a group of atoms, Al, containing one atom of alumin ium to three atoms of topper and manganese together.