NNR IN RAPIDLY ROTATED METALS By - Nottingham eTheses ...

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and Slichter(104) reported that at higher temperatures their measure-
ments of the Hahn echo T2 were consistent with a nuclear quadrupole
coupling to long-range, slowly varying lattice strains of some sort.
Similar results obtained by Fradin and Rowland (112) have supported
this contention. Anderson and Redfield
(18)
considered that their
anomalously high measured value for the ratio of the spin-lattice
relaxation time in high and low external magnetic fields (S) arose
from a quadrupolar coupling, possibly even due to the presence of
impurities. More recently Tunstall and Brown (72)
interpreted a
measured variation of S against temperature in terms of a quadrupolar
contribution to the local internal magnetic fields and showed how
agreement between theory and experiment could then be improved.
However in each of these cases the precise nature of the lattice
defects producing the quadrupole coupling has still to be isolated
satisfactorily.
The results reported here imply that lattice strains are intro-
duced by the cold working of the metal to produce foil or fine part-
icles. There are two ways in which such strains might arise:
either through the introduction of impurities or by a distortion
of the crystal lattice. Before annealing the second moments of
the sprayed and filed powders were about 50% higher than the values
stated in Table 7.1. (The same reduction caused by the annealing
process has been reported by other workers in filed powders (110)
and cold rolled foils(105). ) Therefore it would appear that anneal-
ing successfully removes'some strain mechanisms but leaves others.
The presence of impurities introduced by the cold working process
cannot be ruled out. Such impurities
might exist separately as
macroscopic particles or diffuse into the grains of aluminium them-
selves. Certainly our use of a tungsten carbide coated file did