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

- 25 -
(2.25), can then be assumed to apply directly.
If.
S
is regarded as the sum of the local dipolar interact-
ion energies between all combinations of pairs of nuclei, this
energy will change if either of the spins in a single pair is
relaxed. By definition the relaxation of single spins proceeds
at a rate 1/Tlz, so from this simple model it follows that
Tis = Tlz/2. The ratio of the spin-lattice relaxation times
S Tlz/Tis is therefore equal to 2.
In practice the situation is more complicated because Tls
depends both upon the terms making up. °S and the degree of correlat-
ion between the fluctuating fields at the nuclear spin sites.. If
we consider. aP°S
as the sum of a dipolar interaction and a scalar
exchange interaction thus
ALS - X- + A.. I . I1
then if the second term dominates and the conduction electrons have
long wavelengths, the local fields experienced by near-neighbours
are identical. Consequently the product 11.13 remains constant
and the relaxation time Tis will be very long. Generally, however,
(and particularly in light metals) the first term of equation (2.26)
dominates. 6 is then a measure of the correlation in the conduct-
ion electron spin system. It is given by
where
8e2+E
E Klj(6)/1 (6)
ij rij ij rij
(2.26)