Paramagnet


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Paramagnet

 

a substance that is magnetizable in an external magnetic field in the direction of the field. When an external magnetic field is not present, a paramagnetic substance is nonmagnetic. The atoms or ions of a paramagnet have an intrinsic magnetic moment but lack the magnetic structure characteristic of ferromagnetic and antiferromagnetic substances. On exposure to an external magnetic field, the magnetic moments of the atoms or ions of a paramagnet (in paramagnetic metals, the spins of some of the conduction electrons) are oriented largely in the direction of the field. As a result, the paramagnet acquires a total magnetic moment J that is proportional to the field strength H and is aligned with the field. The magnetic susceptibility χ = J/H of a paramagnetic substance is always positive. Its absolute value is low (see Table 1). In weak fields the susceptibility is independent of the magnetic field strength but is strongly dependent on temperature; most metals are exceptions. Paramagnetic substances include molecular oxygen O2, the

Table 1. Magnetic susceptibility of some paramagnetic substances (X is the molar susceptibility under normal conditions)1
Substancex-106
1The numerical data are given in the Gaussian system of units
Al ...................................16.7
Li ..................................24.6
Na .................................16.1
K ..................................21.35
Ti ..................................161.0
V ..................................296.0
U ..................................414.0
Pu..................................627.0
O2 ..................................3,396
NO .................................1,461
MnO .................................4,850
CuCl2.................................1,080
FeCI2.................................14,750
NiS04 ................................4,005
Dy2(SO4)3.8H2O..........................92,760
HO2(SO4)3.8H2O..........................91,600

alkali metals, the alkaline-earth metals, some transition metals with incomplete d and f shells, and such compounds as NO and salts of iron, cobalt, nickel, and the rare-earth elements. There also exist paramagnetic substances whose paramagnetism is due to the magnetic moment of the nuclei, for example, He3 at temperatures of T < 0.1°K.