It is well known that the magnetic properties of rare earth impurities in crystals are due to the localized character of the 4
f
electrons. The degeneracy of the |
L
,
S
> ionic levels may be lifted by the exchange interaction between the rare earth and the host or by crystal field effects, and the level sequence depends on which of these mechanisms dominates (see, for example, Campbell, this volume, p. 131). Experiments on various rare earth ions implanted into Fe, Co and Ni have shown that the exchange mechanism is important in ferromagnetic transition metal hosts (Grodzins, Borchers & Hagemann 1966; Boehm, Hagemann & Winther 1966) where the main contribution to the hyperfine field is due to the orbital momentum
L
of the 4
f
shell, but there is no reason to expect that crystal field effects are negligible in all cases. In fact, at very low temperatures, the hyperfine field, which is proportional to the average <
J
Z
> of the ground state, may be rather sensitive to the relative strength of the two mechanisms. This is shown by the measurement, reported here, of the hyperfine field at Pr in Fe, using nuclear orientation.
Nuclear orientation of 206Bi in nickel
