Negative absolute temperatures: "Hot" spins in spontaneous magnetic order

Depending on the sign of the spin temperature in the picokelvin range, antiterromagnetic nuclear and ferromagnetic nuclear orders in silver are caused by the same interactions. In rhodium, the antiferromagnetic state is preterred both at temperatures greater than and temperatures less than zero. The lowest and 'highest' temperatures ever produced on this scale and measured, 280 and -750 picokelvin, respectively, have been reached in the course of these experiments. The results on silver, in particular, show that negative temperatures are real, not fictitious, quantities.