Dirac particle (spin magnetic moment)

Description

The spin magnetic moment of a charged, spin-1/2 particle that does not possess any internal structure (a Dirac particle).

In physics, mainly quantum mechanics and particle physics, a spin magnetic moment is the magnetic moment induced by the spin of elementary particles. For example, the electron is an elementary spin-1/2 fermion. Quantum electrodynamics gives the most accurate prediction of the anomalous magnetic moment of the electron.

“Spin” is a non–classical property of elementary particles, since classically the “spin angular momentum” of a material object is really just the total orbital angular momenta of the object’s constituents about the rotation axis. Elementary particles are conceived as concepts which have no axis to “spin” around.

In general, a magnetic moment can be defined in terms of an electric current and the area enclosed by the current loop. Since angular momentum corresponds to rotational motion, the magnetic moment can be related to the orbital angular momentum of the charge carriers in the constituting the current. However, in magnetic materials, the atomic and molecular dipoles have magnetic moments not just because of their quantized orbital angular momentum, but the spin of elementary particles constituting them (electrons, and the quarks in the protons and neutrons of the atomic nuclei). A particle may have a spin magnetic moment without having an electric charge; the neutron is electrically neutral but has a non–zero magnetic moment, because of its internal quark structure.