Magnetism

Magnetism is a class of physical phenomena that are mediated by magnetic fields. In particular, for stationary phenomena (not variable over time) we speak of magnetostatics (which has some formal analogies with electrostatics when the densities of electric current are replaced by the distributions of electric charge). For time-dependent phenomena, however, the electric and magnetic fields influence each other and it is necessary to resort to a unified description of the two fields obtained in 1864 by the British scientist James Clerk Maxwell within the theory of classical electromagnetism or classical electrodynamics.

The Bohr–van Leeuwen theorem, discovered in the 1910s, showed that classical physics theories are unable to account for any form of magnetism. Magnetism is now regarded as a purely quantum mechanical effect. One of the fundamental properties of an electron (besides that it carries charge) is that it has a magnetic dipole moment, i.e., it behaves like a tiny magnet, producing a magnetic field. This dipole moment comes from the more fundamental property of the electron that it has quantum mechanical spin. Due to its quantum nature, the spin of the electron can be in one of only two states; with the magnetic field either pointing “up” or “down” (for any choice of up and down). The spin of the electrons in atoms is the main source of magnetism, although there is also a contribution from the orbital angular momentum of the electron about the nucleus. When these magnetic dipoles in a piece of matter are aligned, (point in the same direction) their individually tiny magnetic fields add together to create a much larger macroscopic field.

However, materials made of atoms with filled electron shells have a total dipole moment of zero: because the electrons all exist in pairs with opposite spin, every electron’s magnetic moment is canceled by the opposite moment of the second electron in the pair.

Types of magnetism

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References

  1. C. D. Stanciu, A. V. Kimel, F. Hansteen, A. Tsukamoto, A. Itoh, A. Kirilyuk, and Th. Rasing, Ultrafast spin dynamics across compensation points in ferrimagnetic GdFeCo: The role of angular momentum compensation, Phys. Rev. B 73, 220402(R) (2006).
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