Ferrimagnetism in CsMn1−xNixF3 and CsMn1−xCoxF3

CsMn_1?xNi_xF₃ with 0.3≦x≦0.5 and CsMn_1?xCo_xF₃ with 0.4≦x≦0.65 have been found to be ferrimag with a Curie temperature 50±1K. The magnetic moment at 4.2 K is 1.42μ_B per formula-unit for CsMn_0.65Ni_0.35F₃, and 1.77μ_B for CsMn_0.6Co_0.4F₃. The crystal structure has been determined to be rhombohedral; it probably has a stacking of twelve CsF₃-layers in the unit cell in hexagonal representation. Magnetic properties have been explained on the assumption that a Ni(Co) ion has a strong preference for occupying one of three inequivalent sites in the structure. It has been suggested that the magnetic moments of one-quarter of the magnetic ions couple antiparallel to those of the other three-quarters so that ferrimagnetism appears. The Curie temperature has been discussed in the molecular field approximation.