Nuclear magnetic moment illustration

Nuclear magnetic moment vectors aligned with or against an external magnetic field.

An electron in an atomic or molecular orbital can produce magnetic fields both by their orbit around the nucleus and by their spin (the electron revolving about its own axis). In most materials, the magnetic field produced in the first way is not significant, cancelled out by an oppositely circulating electrons. Magnetism due to spin is significant in atoms with odd numbers of electrons, where there is at least one unpaired electron, so there is a corresponding spin magnetic moment.

A paramagnetic material has the capacity for weak interactions with an external magnetic field due to the net magnetic moment of unpaired electron spin. In addition, a nucleus can have a net magnetic moment due to unpaired proton spin, a fact underpinning nuclear magnetic resonance spectroscopy. Nuclear magnetic resonance makes it possible to observe transitions between two spin states, which cause a net absorption of energy at a characteristic frequency of oscillating field that depends on the interaction of nucleus and its chemical environment.
Precessional frequency of nuclear magnetic moment

Absorption of radio waves in NMR occurs at the precessional frequency of a nuclear magnetic moment which can lead to spin-flip.

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