Velocity dependence of the dynamic magnetic field acting on swift O and Sm ions

The hyperfine dynamic field acting on fast ions traversing polarized iron foils has been measured as a function of the ion velocity in the region $0.03<\mathrm{}\frac{v}{Z{v}_0}<0.06\mathrm{}$ for Sm ions. The dynamic field increases with velocity but not linearly. The same conclusions were obtained from measurements on oxygen ions traversing thin iron foils or stopping in thick iron foils. All available data on dynamic-field measurements at ions with $8<Z<\mathrm{}62\mathrm{}$ traversing thin foils were examined and fitted to a single general form for the dynamic field. The region of validity of this parametrization of the dynamic field, as well as possible explanations of its origin, are discussed.