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 dominate d by two singlet states well separated from the remaining 4f-levels. T he classical dipole coupling is an order of magnitude larger than any other two-ion interaction between the Ho-moments. The hyperfine-couple d electronic and nuclear moments on the Ho-ions order co-operatively i n a ferrimagnetic structure at T-c = 0.53 K. A theory has been develop ed for describing this system, and it is found that the leading-order corrections to the mean-field behaviour, obtained from the high-densit y 1/z-expansion, are significant. The magnetic heat capacity between 9 0 mK and 7-8 K is accounted for accurately by the theory, as is the pa ramagnetic excitation spectrum. The only discrepancy of importance is that the calculated moment in the zero-temperature limit is about 9% s maller than the neutron diffraction result of 5.7 mu(B).