Metamagnetic Behavior and Kondo Breakdown in Heavy-Fermion CeFePO

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We report that nonmagnetic heavy-fermion (HF) iron oxypnictide CeFePO with two-dimensional XY-type anisotropy shows a metamagnetic behavior at the metamagnetic field HM≃4 T perpendicular to the c axis and that a critical behavior is observed around HM. Although the magnetic character is entirely different from that in other Ce-based HF metamagnets, HM in these metamagnets is linearly proportional to the inverse of the effective mass, or to the temperature where the susceptibility shows a peak. This finding suggests that HM is a magnetic field breaking the local Kondo singlet, and the critical behavior around HM is driven by the Kondo breakdown accompanied by the Fermi-surface instability.