a physical quantity that characterizes the relation between the magnetic moment (magnetization) of a substance and the magnetic field in the substance.

Volume magnetic susceptibility is equal to the ratio of the magnetization per unit volume of the substance J to the intensity H of the magnetizing field: K = J/H. Magnetic susceptibility is a dimensionless quantity and is measured in dimensionless units. The magnetic susceptibility per kilogram or gram of a substance is called the specific susceptibility (K_SP = k/ρ, where ρ is the density of the substance), and the magnetic susceptibility of 1

mole is the molar susceptibility (X = K_sp-M, where M is the molecular weight of the substance).

Magnetic susceptibility may be positive or negative. Diamag-nets, which are magnetized against rather than with the field, have negative magnetic susceptibility. In paramagnets and ferromagnets the magnetic susceptibility is positive (they are magnetized with the field). The magnetic susceptibility of diamagnets and paramagnets is low (of the order of 10 ^-4-10 ^-6) and depends very slightly on H, and then only in a region of very strong fields (and low temperatures). The values for magnetic susceptibility of some substances are given in Table 1.

Magnetic susceptibility attains particularly high values in ferromagnets (from several tens to many thousands of units) and is very strongly and intricately dependent on H. Therefore, the differential magnetic susceptibility K_d = dJ/dH is introduced for ferromagnets. For H = 0 (see Figure 1), the magnetic susceptibility of ferromagnets is not equal to zero but rather has the value K_a, which is called the initial magnetic susceptibility. As H increases, the magnetic susceptibility increases, reaches a maximum (K_max), and then declines. In the region of very high values of H and at temperatures that are not very close to the Curie point, the magnetic susceptibility of ferromagnets becomes just as insignificant as that of ordinary paramagnets (the region of the paraprocess). The shape of the K(H) curve (the Stoletov curve) results from the complex mechanism of magnetization of ferromagnets. Typical values of K_a and K_max are as follows: for Fe, ~ 1,100 and ~ 22,000; for Ni, ~12 and ~80; and for Permalloy, ~800 and ~8,000 (under normal conditions).

Figure 1. Curve showing dependence of the differential magnetic susceptibility K_d of ferromagnets on the intensity H of the magnetizing field

Magnetic susceptibility usually depends on temperature (most diamagnets and some paramagnets—the alkali metals and, in part, the alkaline earths—are exceptions). The magnetic susceptibility of paramagnets decreases with temperature, conforming to Curie’s law or the Curie-Weiss law. In ferromagnetic solids magnetic susceptibility increases with temperature, reaching a sharp maximum near the Curie point θ. The magnetic susceptibility of antiferromagnets increases with temperature up to the Neel temperature and then decreases according to the Curie-Weiss law.