In physics, mainly
quantum mechanics and
particle physics, a spin magnetic moment is the
magnetic moment caused by the
spin
Spin or spinning most often refers to:
* Spinning (textiles), the creation of yarn or thread by twisting fibers together, traditionally by hand spinning
* Spin, the rotation of an object around a central axis
* Spin (propaganda), an intentionally b ...
of
elementary particles. For example, the
electron is an elementary
spin-1/2 fermion
In particle physics, a fermion is a particle that follows Fermi–Dirac statistics. Generally, it has a half-odd-integer spin: spin , spin , etc. In addition, these particles obey the Pauli exclusion principle. Fermions include all quarks an ...
.
Quantum electrodynamics gives the most accurate prediction of the
anomalous magnetic moment of the electron.
In general, a
magnetic moment can be defined in terms of an
electric current
An electric current is a stream of charged particles, such as electrons or ions, moving through an electrical conductor or space. It is measured as the net rate of flow of electric charge through a surface or into a control volume. The moving pa ...
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 carrier
In physics, a charge carrier is a particle or quasiparticle that is free to move, carrying an electric charge, especially the particles that carry electric charges in electrical conductors. Examples are electrons, ions and holes. The term is used ...
s in the constituting current. However, in
magnetic material
A magnet is a material or object that produces a magnetic field. This magnetic field is invisible but is responsible for the most notable property of a magnet: a force that pulls on other ferromagnetic materials, such as iron, steel, nickel ...
s, the atomic and molecular dipoles have magnetic moments not just because of their
quantized orbital angular momentum, but also due to the spin of elementary particles constituting them.
"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 point objects with no axis around which to "spin" (see
wave–particle duality).
History
The idea of a spin angular momentum was first proposed in a 1925 publication by
George Uhlenbeck and
Samuel Goudsmit to explain
hyperfine splitting
In atomic physics, hyperfine structure is defined by small shifts in otherwise degenerate energy levels and the resulting splittings in those energy levels of atoms, molecules, and ions, due to electromagnetic multipole interaction between the nucl ...
in atomic spectra. In 1928,
Paul Dirac provided a rigorous theoretical foundation for the concept in the
Dirac equation for the
wavefunction of the
electron.
Spin in chemistry
Spin magnetic moments create a basis for one of the most important principles in chemistry, the
Pauli exclusion principle. This principle, first suggested by
Wolfgang Pauli, governs most of modern-day chemistry. The theory plays further roles than just the explanations of
doublet
Doublet is a word derived from the Latin ''duplus'', "twofold, twice as much",