The hyperfine splitting of the ground state and first excited state of holmium in yttrium hydroxide

We have studied the field dependence of the hyperfine splitting of ¹⁶⁵Ho in yttrium hydroxide by spin echo NMR at liquid helium temperatures and in fields up to 8 T. Hyperfine spectra have been obtained from the electronic ground state and from the first excited state. The measured hyperfine splittings are compared with theoretical results obtained by diagonalizing the complete electronic-nuclear Hamiltonian for the nominally ⁵I₈ ground multiplet of Ho³⁺, using crystal field parameters derived from optical spectroscopy. There is close agreement between the measured and calculated quadrupolar and octupolar splittings, but we find a significant difference between the measured and calculated dipolar splittings, especially when allowance is made for J mixing. Agreement is restored, to within the uncertainties, if the currently accepted value of the dipolar hyperfine coupling coefficient for the free Ho³⁺ ion is reduced by 1.3%. Independent evidence for a reduction of that order will be presented in a later publication. The ratio of nuclear to electronic antishielding factors for Ho³⁺ is found to be gamma _N/ gamma _E=148+or-10, in close agreement with the value obtained in pure holmium hydroxide.