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(3.55) |
In a simple metal we finally obtain the Korringa law (see [#!patrik!#]):
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(3.56) |
In the case where there are magnetic moments in the sample the situation becomes much more complicated. A detailed theoretical analysis for a dilute magnetic alloy is given in [#!giovannini!#]. This model applies in the case where the coupling between the momenta of the impurities is small compared with the coupling to the external field. It has first been testet experimentally by McHenry et al. in La1-xGdxAl2 (see [#!mchenry!#]). After the relaxation due to the paramagnetic impurities has been separated from the Korringa relaxation, they found very good coincidence of the experimental curves with the theoretical prediction.
There are four different mechanisms through which the spin of the
impurities (S) can couple with the spin of the nucleus under investigation
(I). There are the Benoit, de Gennes and Silhouette mechanism and the
Giovannini and Heeger mechanism which both couple via the conduction electrons
and rely on the RKKY model. In our case one would expect these mechanisms
not be be dominant. On the other hand there is the direct dipolar
coupling, wich is called either longitudinalor transversal.
At high temperatures the longitudinalcoupling is expected to be
dominant (see [#!abragam!#], chapter IX). We only want to quote the
general result from [#!abragam!#] here:
):
