Principles of nuclear magnetic ordering

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Abstract
We describe in this article the principles of production and observation of magnetic ordering in systems of nuclear spins subjected to dipole-dipole interactions. The cooling necessary for producing ordering, which concerns only the nuclei, is obtained by a two-step process: dynamic polarization in a high field followed by adiabatic nuclear demagnetization. The study is mostly limited to adiabatic demagnetization in the rotating frame, for which the effective nuclear spin-spin interactions are truncated dipolar interactions. We list briefly a number of measurements that can be made, mostly relevant to magnetic resonance techniques, together with the information they yield on the ordering. The prediction of the nature of ordered structures, both at positive and negative temperatures, is made through the use of the local Weiss-field approximation. In the case of simple cubic systems of spins 1/2, one predicts the occurrence of three different antiferromagnetic structures. The Weiss-field approximation, occasionally supplemented with high-temperature approximation to spin temperature theory, is finally used for predicting as a function of entropy various properties of the antiferromagnetic states: sublattice magnetizations, transition entropy, transverse and longitudinal susceptibilities, transition field, and shape of the fast-passage dispersion signal.

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