Nuclear spins in metals provide excellent models to investigate magnetism. The nuclei are well localized, their spins are isolated from the electronic and lattice degrees of freedom at low temperatures, and the interactions between nuclear spins can be calculated from first principles. Therefore, nuclear magnets are particularly suitable for testing theory against experiments. Because the nuclear magneton is small, the critical temperatures for spontaneous magnetic ordering are in the submicrokelvin range; this makes the experimental studies very demanding.
In nuclear spin systems it is possible to achieve negative temperatures. At T < 0, the spins tend to the highest energy states and the energy of the spin system is maximized in this case. The tendency to maximize energy, instead of minimizing it, is the basic difference between negative and positive temperatures. This difference will influence the collective magnetic behavior of the spins and lead to different magnetically ordered configurations.