Magnetic properties of the dipole-coupled singlet-singlet system HoF3

Experimental investigations of HoF3 have revealed that it is a unique magnetic system. The electronic properties of the Ho-ions are dominated by two singlet states well separated from the remaining 4f-levels. The classical dipole coupling is an order of magnitude larger than any other two-ion interaction between the Ho-moments. The hyperfine-coupled electronic and nuclear moments on the Ho-ions order co-operatively in a ferrimagnetic structure at TC=0.53 K. A theory has been developed for describing this system, and it is found that the leading-order corrections to the mean-field behaviour, obtained from the high-density 1/z-expansion, are significant. The magnetic heat capacity between 90 mK and 7-8 K is accounted for accurately by the theory, as is the paramagnetic excitation spectrum. The only discrepancy of importance is that the calculated moment in the zero-temperature limit is about 9% smaller than the neutron diffraction result of 5.7μB. ((orig.))