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12.5: Paramagnetism

Diamagnetism makes itself evident in atoms and molecules that have no permanent magnetic moment. Some atoms or molecules, however, do have a permanent magnetic moment, and such materials are paramagnetic. They must still be diamagnetic, but often the paramagnetism will outweigh the diamagnetism. The magnetic moment of an atom of a molecule is typically if order of a Bohr magneton. (See Chapter VII, Sections 21-23, of Stellar Atmospheres for more details about the Bohr magneton and the magnetic moments of atoms. All that we need note here is that a Bohr magneton is about 9.3 % 10-24 N m T-1.) The presence of a permanent magnetic moment is often the result of unpaired electron spins. An example often quoted is the oxygen molecule O2. Liquid oxygen indeed is paramagnetic. When a paramagnetic material is placed in a magnetic field, the magnetic moments experience a torque and they tend to orient themselves in the direction of the magnetic field, thus augmenting, rather than diminishing, B. Unsurprisingly the effect is greatest at low temperatures, where the random motion of atoms and molecules is low. At liquid helium temperatures (of order 1 K), susceptibilities can be of order +10-3 or +10-2, thus greatly exceeding the small negative susceptibility. At room temperature, paramagnetic susceptibilities are much less – typically about +10-5, barely exceeding the diamagnetic susceptibility.