Ferromagnetic Quantum Critical Point in Heavy-Fermion Iron Oxypnictide Ce(Ru$_1-x$Fe$_x$)PO

We have performed ^31P-NMR measurements on Ce(Ru_1-xFe_x)PO in order to investigate ferromagnetic (FM) quantum criticality, since heavy-fermion (HF) ferromagnet CeRuPO with a two-dimensional structure turns to 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 ∼ 0.3 K, 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 the similarity with those in YbRh_2Si_2, 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.