Ferromagnetic quantum critical point in heavy-fermion iron oxypnictide Ce(Ru_1-xFe_x)PO

We have performed P31-NMR measurements on Ce(Ru_1-xFe _x)PO in order to investigate ferromagnetic (FM) quantum criticality, since a heavy-fermion (HF) ferromagnet CeRuPO with a two-dimensional structure turns into a HF paramagnet by an isovalent Fe substitution for Ru. We found that Ce(Ru_0.15Fe_0.85)PO shows critical fluctuations down to ∼0.3K, as well as the continuous suppression of Curie temperature and the ordered moments by the Fe substitution. These experimental results suggest the presence of a FM quantum critical point (QCP) at x∼0.86, which is a rare example among itinerant ferromagnets. In addition, we point out that the critical behaviors in Ce(Ru_0.15Fe_0.85)PO share a similarity with those in YbRh₂Si₂, where the local criticality of f electrons has been discussed. We reveal that Ce(Ru _1-xFe_x)PO is a new system to study FM quantum criticality in HF compounds.