page_title: Neutron magnetic moment
section_title: Physical significance
hierarchical_section_title: Neutron magnetic moment / Physical significance
caption_attribution_description: English: Larmor precession for negative charged particle: magnetic field black, spin red, precession green. Created with POVRAY.
context_page_description: The neutron magnetic moment is the intrinsic magnetic dipole moment of the neutron, symbol μₙ. Protons and neutrons, both nucleons, comprise the nucleus of atoms, and both nucleons behave as small magnets whose strengths are measured by their magnetic moments. The neutron interacts with normal matter through either the nuclear force or its magnetic moment. The neutron's magnetic moment is exploited to probe the atomic structure of materials using scattering methods and to manipulate the properties of neutron beams in particle accelerators. The neutron was determined to have a magnetic moment by indirect methods in the mid 1930s. Luis Alvarez and Felix Bloch made the first accurate, direct measurement of the neutron's magnetic moment in 1940. The existence of the neutron's magnetic moment indicates the neutron is not an elementary particle. For an elementary particle to have an intrinsic magnetic moment, it must have both spin and electric charge. The neutron has spin 1/2 ħ, but it has no net charge. The existence of the neutron's magnetic moment was puzzling and defied a correct explanation until the quark model for particles was developed in the 1960s.
context_section_description: When a neutron is put into a magnetic field produced by an external source, it is subject to a torque tending to orient its magnetic moment parallel to the field (hence its spin antiparallel to the field).  Like any magnet, the amount of this torque is proportional both to the magnetic moment and the external magnetic field.  Since the neutron has spin angular momentum, this torque will cause the neutron to precess with a well-defined frequency, called the Larmor frequency. It is this phenomenon that enables the measurement of nuclear properties through nuclear magnetic resonance. The Larmor frequency can be determined by the product of the gyromagnetic ratio with the magnetic field strength. Since the sign of γₙ is negative, the neutron's spin angular momentum precesses counterclockwise about the direction of the external magnetic field.
  The interactions of the neutron's magnetic moment with an external magnetic field were exploited to finally determine the spin of the neutron.  In 1949, Hughes and Burgy measured neutrons reflected from a ferromagnetic mirror and found that the angular distribution of the reflections was consistent with spin 1/2.  In 1954, Sherwood, Stephenson, and Bernstein employed neutrons in a Stern-Gerlach experiment that used a magnetic field to separate the neutron spin states.  They recorded the two such spin states, consistent with a spin 1/2 particle.  Until these measurements, the possibility that the neutron was a spin 3/2 particle could not have been ruled out.
  Since neutrons are neutral particles, they do not have to overcome Coulomb repulsion as they approach charged targets, as experienced by protons or alpha particles. Neutrons can deeply penetrate matter.  The magnetic moment of the neutron has therefore been exploited to probe the properties of matter using scattering or diffraction techniques. These methods provide information that is complementary to X-ray spectroscopy. In particular, the magnetic moment of the neutron is used to determine magnetic properties of materials at length scales of 1–100 Å using cold or thermal neutrons.  Bertram Brockhouse and Clifford Shull won the Nobel Prize in physics in 1994 for developing these scattering techniques.
  Without an electric charge, neutron beams cannot be controlled by the conventional electromagnetic methods employed for particle accelerators. The magnetic moment of the neutron allows some control of neutrons using magnetic fields, however, including the formation of polarized neutron beams.  One technique employs the fact that cold neutrons will reflect from some magnetic materials at great efficiency when scattered at small grazing angles.  The reflection preferentially selects particular spin states, thus polarizing the neutrons.  Neutron magnetic mirrors and guides use this total internal reflection phenomena to control beams of slow neutrons.
  Since an atomic nucleus consists of a bound state of protons and neutrons, the magnetic moments of the nucleons contribute to the nuclear magnetic moment, or the magnetic moment for the nucleus as a whole.  The nuclear magnetic moment also includes contributions from the orbital motion of the nucleons.  The deuteron has the simplest example of a nuclear magnetic moment, with measured value 0.857 µN.  This value is within 3% of the sum of the moments of the proton and neutron, which gives 0.879 µN. In this calculation, the spins of the nucleons are aligned, but their magnetic moments offset because of the neutron's negative magnetic moment.
caption: Direction of Larmor precession for a neutron.  The central arrow denotes the magnetic field, the small red arrow the spin of the neutron.
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