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How do spinning charged particles behave in an external magnetic field?

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magnetic resonance Figure 1: Precession of a magnetic dipole moment μ in the presence of a constant field H and a rotating field H′ (see text)

When a spinning charged particle is placed in an external magnetic field, it behaves similarly to a gyroscope due to its intrinsic spin and associated magnetic moment.12

The external magnetic field exerts a torque on the particle’s magnetic moment, attempting to align it with the field's direction. However, because the particle also possesses angular momentum, this interaction causes the spin axis to undergo Larmor precession, where the axis describes a cone around the direction of the magnetic field.1

According to quantum mechanics, the particle cannot assume an arbitrary orientation; instead, it is restricted to a limited set of discrete magnetic energy states. In magnetic resonance, a weak oscillating field can be applied to induce transitions between these energy states. When the frequency of this oscillating field matches the Larmor frequency, the particle's orientation changes, resulting in the absorption or emission of energy that can be measured.1