NEGATIVE NANOKELVIN TEMPERATURES IN THE NUCLEAR-SPIN SYSTEMS OF SILVER AND RHODIUM METALS

Using a dilution refrigerator and two nuclear cooling stages in series , we have reached the nuclear spin temperatures T = 500 pK and T = -1. 6 nK in silver and T = 280 pK and T = -750 pK in rhodium; the latter p air of values are the lowest and 'highest' temperatures ever produced. Spontaneous antiferromagnetic spin ordering has been observed in silv er at T > 0 and ferromagnetic ordering at T < 0, with the transitions occurring at T(N) = 560 pK and T(c) = -1.9 nK. In rhodium, the transit ion points have not been reached but the metal tends towards antiferro magnetism both at T > 0 and at T < 0. The experimental procedure emplo yed for reaching the negative spin temperatures will be illustrated. T his new domain offers interesting possibilities for studies of magneti sm.