Diamagnetism

Diamagnetism is a form of magnetism that some materials exhibit in the presence of a magnetic field; characteristic of materials that do not possess a magnetic dipole moment of their own. Diamagnetic materials are characterized by the fact that magnetization has the opposite direction with respect to the magnetic field; therefore these materials are weakly “rejected.” This is a very weak quantum effect, which is voided if the material has other magnetic properties such as ferromagnetism or paramagnetism. In the non-scientific field, diamagnetic materials are often simply called non-magnetic.

These materials respond to the presence of an external magnetic field exclusively by electron motion. The motion of electrons can be, in fact, ideally schematized as a rotation around the nucleus. The rotation of the electron generates a dipole moment on which the external field acts. The dipole moments due to the rotation (clockwise or counterclockwise) of electrons respond differently to the external field, here assumed oriented along the same direction. Those parallel are weakened and those antiparallel to the field are strengthened.

Since the number of parallel dipoles is equal to the number of antiparallel dipoles, the external field results in an overall strengthening of the antiparallel dipole moments with respect to the external field. For this reason, a diamagnetic material is weakly repelled by the magnetic field. The electronic structure of a diamagnetic material is such that it does not result in the presence of a magnetic dipole of its own: in other words, the electronic levels are fully occupied.

In common experience the substances that manifest a diamagnetic behavior are water, many gases (but not oxygen and nitrogen), most organic substances (DNA, oils, plastics) and some metals such as mercury, gold, copper, silver and bismuth. In 1778 S. J. Brugmans first observed that bismuth and antimony were repelled by magnetic fields. However the term diamagnetism was coined by Michael Faraday in September 1845, when he discovered that all materials in nature possess a diamagnetic component in response to an external applied magnetic field.

Langevin diamagnetism

The Bohr-van Leeuwen theorem shows that there can be no diamagnetism in a purely classical physical system. However, the classical theory for Langevin diamagnetism, in which the electron is treated as a charge orbiting the nucleus and whose motion constitutes a microscopic current, provides results in agreement with those obtained through the (exact) approach provided by quantum mechanics. Langevin’s model applies to materials containing atoms whose electronic shells are complete, so it does not apply to metals.

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