gyromagnetic ratio (redirected from Magnetogyric ratio)

gyromagnetic ratio [¦jī·rō·mag′ned·ik ′rā·shō]

(physics)

The ratio of the magnetic dipole moment to the angular momentum for a classical, atomic, or nuclear system.

Occasionally, the reciprocal of the quantity in the first definition.

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Gyromagnetic ratio

The ratio of angular momentum to magnetic moment for atomic systems. This ratio is usually expressed in terms of the magnetomechanical factor g^′, as in Eq. (1).

(1) 

The ratio is written here in electromagnetic units; thus, e/c and m are the charge and mass of the electron. The factor g^′ is sometimes loosely called the gyromagnetic ratio.

The magnetomechanical ratio is the inverse of the gyromagnetic ratio. It is usually denoted by γ and is equal to g^′e/2mc. The magnetomechanical ratio of a substance identifies the origin of the magnetic moment. For example, for electron spin the angular momentum is ½ℏ, where ℏ is Planck's constant divided by 2π. The magnetic moment is the Bohr magneton eℏ/2mc. Thus, the magnetomechanical ratio

(2) 

is given by Eq. (2). Since γ = g^′e/2mc, for electron spin g^′ = 2. For orbital angular momentum, γ = e/mc and g^′ = 1. The experimental values of g^′ for most ferromagnetic materials are in the neighborhood of 2, showing that the major contribution to the magnetization comes from the electron spin. In superconductors, on the other hand, the fact that g^′ = 1 shows that the diamagnetic currents which cause the Meissner effect are caused by electrons. See Meissner effect, Superconductivity