Gyromagnetic ratio

The gyromagnetic ratio (sometimes magnetogyric ratio), $\gamma$, is the constant of proportionality between the magnetic moment (μ) and the angular momentum(J) of an object:

${\boldsymbol {\mu }}=\gamma \mathbf {J} \ .$

Its SI units are radian per second per tesla (s⁻¹·T ^-1) or, equivalently, coulomb per kilogram (C·kg⁻¹). When the object is placed in a magnetic flux density B, because of its magnetic moment it experiences a torque and precesses about the field.

Examples

The electron gyromagnetic ratio is:

$\gamma _{\rm {e}}=2|\mu _{\rm {e}}|/\hbar =\mathrm {1.760\ 859\ 770\ \times \ 10^{11}\ s^{-1}T^{-1}} \ ,$

where μ_e is the magnetic moment of the electron (-928.476 377 x 10^-26 J T^-1), and ℏ is Planck's constant divided by 2π and ℏ/2 is the spin angular momentum.

Similarly, the proton gyromagnetic ratio is:

$\gamma _{\rm {p}}=2\mu _{\rm {p}}/\hbar =\mathrm {2.675\ 222\ 099\ \times \ 10^{11}\ s^{-1}T^{-1}} \ ,$

where μ_p is the magnetic moment of the proton (1.410 606 662 x 10^-26 J T^-1). Other ratios can be found on the NIST web site.

Theory and experiment

The relativistic quantum mechanical theory provided by the Dirac equation predicted the electron to have a gyromagnetic ratio of exactly two Bohr mangetons, where the Bohr magneton is:

$m_{B}={\frac {e\hbar }{2m_{e}c}}\ ,$

with c the speed of light in ideal vacuum, and e the elementary charge.

Subsequently (in 1947) experiments on the Zeeman splitting of the gallium atom in magnetic field showed that was not exactly the case, and subsequently this departure was calculated using quantum electrodynamics.