The Transient Field is a strong magnetic field (several 1000 Tesla) used in nuclear spin precession measurements of short lived states.
Its presence and effect was observed 1967 in magnetic moment measurements of ions when implanted in ferromagnetic solids for their large internal fields. A precession was observed while the ions were moving, before being stopped, therefore the name "transient" was coined.
The transient magnetic field (TF) is the strongest field available for magnetic moment measurements in nuclei. The TF is not a macroscopic field, one cannot put a Hall probe in and get a number. It is a hyperfine field acting at the nucleus inside an atom originating mainly from the spins of the electrons in the atomic shell. By far the largest contribution comes from single s electrons. For instance, one single 1s electron in oxygen produces a field of 8.5 kT. Fully occupied orbits do not contribute!
The field has a direction. By definition it is opposite to the spin direction of the electron. When all spin directions are equally present, then there is no net field direction.
So, to have a field at the nucleus of an ion, the electron shell of that ion should be highly in disarray, and hopefully, have some s shell vacancies. And since a net field direction is desirable, an excess of electrons with a given spin direction is required.
Such conditions occur when ions move fast within a polarized ferromagnetic solid.
The TF field strength can then be thought of as
|n||are the major quantum numbers|
|qn(Z,v)||the charge state distributions|
|pn(Z,v,host)||their degree of polarization|
|Bns||the Hyperfine fields ~(Z/n)3, assuming only s electron contributions.|
Although the exact charge state distributions, their state of excitation or degree of polarization and how the polarization transfer from the medium to the moving ions is facilitated (electron capture and loss, spin-spin scattering) are still unknown, from calibration measurements the Z (charge) and v (velocity) dependence of the field strength is measured and parameterized. A quantitative or microscopic description of the observed field strength is still outstanding.
A more detailed account on the subject can be found in a recent review article in J.Phys.G.Nucl.Part.Phys. 34 R321-R358 (2007).
Ref. R.R.Borchers etal., Phys.Rev.Lett.20,424-427(1968)