# Magnetic Susceptibility

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## Magnetic susceptibility

The magnetization of a material per unit applied field. It describes the magnetic response of a substance to an applied magnetic field. See Magnetism, Magnetization

All ferromagnetic materials exhibit paramagnetic behavior above their ferromagnetic Curie points. The general behavior of the susceptibility of ferromagnetic materials at temperatures well above the ferromagnetic Curie temperature follows the Curie-Weiss law. The paramagnetic Curie temperature is usually slightly greater than the temperature of transition. See Curie temperature, Curie-Weiss law, Ferromagnetism

Most paramagnetic substances at room temperature have a static susceptibility which follows a Langevin-Debye law. Saturation of the paramagnetic susceptibility occurs when a further increase of the applied magnetic field fails to increase the magnetization, because practically all the magnetic dipoles are already oriented parallel to the field. See Paramagnetism

The susceptibility of diamagnetic materials is negative, since a diamagnetic substance is magnetized in a direction opposite to that of the applied magnetic field. The diamagnetic susceptibility is independent of temperature. Diamagnetic susceptibility depends upon the distribution of electronic charge in an atom and upon the energy levels. See Diamagnetism

The susceptibility of antiferromagnetic materials above the Néel point, which marks the transition from antiferromagnetic to paramagnetic behavior, follows a Curie-Weiss law with a negative paramagnetic Curie temperature.

## Magnetic Susceptibility

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 (KSP = k/ρ, where ρ is the density of the substance), and the magnetic susceptibility of 1

Diamagnetsx ×106Paramagnetsx ×106
1 Data are in the cgs system of units
ElementsElements
Helium (He) .......-2.02Lithium (Li) .......24. 6
Neon (Ne) .......-6.96Sodium (Na) ........ 16. 1
Argon (Ar) .......-19.23Potassium (K) .......21.35
Copper (Cu) .......-5.41Rubidium (Rb) .......18.2
Silver (Ag) .......-21.5Cesium (Cs) .......29.9
Gold (Au) .......-29.59Magnesium (Mg) .......13.25
Zinc (Zn) .......-11.40Calcium (Ca) .......44.0
Beryllium (Be) .......-9.02Strontium (Sr) .......91.2
Bismuth (Bi) .......-284.0Barium (Ba) .......20.4
Inorganic compounds ....... Titanium (Ti) .......161.0
AgCI .......-49.0Tungsten (W) .......55
BiCI3 .......-100.0Platinum (Pt) .......189.0
C02(gas) .......-21.0Uranium (U) .......414. 0
H20 (liquid) .......-13.0 (0°C)Plutonium (Pu) .......627. 0
Organic compounds ....... Inorganic compounds .......
Aniline, C6H7N .......-62.95CoCI2 .......1 21,660
Benzene, C6H6 .......-54.85EuCI2 .......26,500
Diphenylamine, C12H11N .......-107.1MnCI2 .......14,350
Methane, CH4 (gas) .......-16.0FeS .......1 ,074
Octane.C8H18 .......-96.63UF6 .......43
Naphthalene, C10H8 .......-91.8

mole is the molar susceptibility (X = Ksp-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 Kd = 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 Ka, which is called the initial magnetic susceptibility. As H increases, the magnetic susceptibility increases, reaches a maximum (Kmax), 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 Ka and Kmax 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 Kd 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.

### REFERENCES

Vonsovskii, S. V. Magnetizm. Moscow, 1971.
Bozorth, R. Ferromagnetizm. Moscow, 1956. (Translated from English.)
Foëx, G. Tables de constantes et données numériques, 7: Constantes sélectionnées. Diamagnétisme et paramagnétisme. Paris, 1957.

S. V. VONSOVSKII

## magnetic susceptibility

[mag′ned·ik sə‚sep·tə′bil·əd·ē]
(electromagnetism)
The ratio of the magnetization of a material to the magnetic field strength; it is a tensor when these two quantities are not parallel; otherwise it is a simple number. Also known as susceptibility.
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