Ferromagnetism of ${}_{}{}^{19}{}_{}{}^{}\mathrm{F}$ nuclear spins in a monocrystalline ${\mathrm{KMgF}}_3$ sample has been observed by NMR in the presence of a high magnetic field and in the microkelvin temperature range. The three ${}_{}{}^{19}{}_{}{}^{}\mathrm{F}$ spins of one crystalline cell are equivalent when the field is along the [111] axis of the crystal and inequivalent for the field along the [100] axis. At negative temperature, the structures are ferromagnetic with domains for both orientations. In the [100] orientation, it is a two-sublattice ferromagnetism: Spins belonging to different sublattices of ${}_{}{}^{19}{}_{}{}^{}\mathrm{F}$ have different polarizations within the same domain.

• Received 18 December 1990

DOI:https://doi.org/10.1103/PhysRevLett.66.2810