Ferromagnetic Quantum Critical Point in Heavy-Fermion Iron Oxypnictide Ce(Ru1-xFex)PO

We have performed P-31-NMR measurements on Ce(Ru1-xFex)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(Ru0.15Fe0.85)PO shows critical fluctuations down to similar 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 similar to 0.86, which is a rare example among itinerant ferromagnets. In addition, we point out that the critical behaviors in Ce(Ru0.15Fe0.85)PO share a similarity with those in YbRh2Si2, where the local criticality of f electrons has been discussed. We reveal that Ce(Ru1-xFex)PO is a new system to study FM quantum criticality in HF compounds.