Nuclear ferromagnetic ordering in Ag at negative temperatures

In silver, the nearest-neighbor antiferromagnetic force favors antiparallel alignment of the nuclear magnetic
moments and leads to antiferromagnetism when T > 0. At T < 0, since the energy now is maximized, parallel
orientation is favored and the very same interactions produce ferromagnetic nuclear order.
 
Silver is the only metallic nuclear magnet where spontaneous spin ordering at T < 0 has been reached [PRL68, 365 (1992)]. The ordered state can be explained as a ferromagnetic domain configura- tion, caused by the long-range character of the dipolar forces. Since the energy of the spins tends to maximum at negative tempera-tures, the domain configurations are fundamentally different from those at T > 0.

FIGURE Phase diagram in the field (B) vs. reduced entropy (S/Rln2) plane for silver nuclear spins at negative (·) and positive (·) absolute temperatures.

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