Experimental and theoretical studies of NMR in PrFe4P12. Suggestion of antiferro-monopole type ordering

Experiments on the NMR signal splitting of 31P are reported for the low temperature phase of PrFe4P12, whose nature has been controversial. The observed NMR signal splitting is consistent with the field-indicated antiferromagnetic moment observed by the neutron diffraction. For a theoretical analysis an invariant coupling form is first derived for the hyperfine interaction between multipolar moments and the nuclear spin in the filled skutterudite structure, and then the experimental results are analyzed. Two scenarios for the order parameter are examined: One is a Γ3-type antiferro-quadrupolar (AFQ) ordering. Another is an antiferro-monopole (AF-monopole) type ordering. Experimental facts, especially the field-direction dependence in the (11-bar0) plane, are most naturally explained by the latter model, suggesting the AF-monopole ordering. Both the field-direction dependence and the field-strength dependence of the NMR splitting can be consistently accounted for it low order terms in the magnetic field are taken into account. This supports applicability of expansion regarding the magnetic field based on the AF-monopole model. Experiments can be also qualitatively reproduced by the AFQ model when the field is not near the [111] direction. It is very likely that the order parameter in PrFe4P12 is the AF-monopole, although we cannot completely rule out a possibility of the Γ3-type AFQ order at present. (author)