Metamagnetic behavior and Kondo breakdown in heavy-fermion CeFePO

We report that nonmagnetic heavy-fermion (HF) iron oxypnictide CeFePO with two-dimensional XY-type anisotropy shows a metamagnetic behavior at the metamagnetic field H _M4T perpendicular to the c axis and that a critical behavior is observed around H _M. Although the magnetic character is entirely different from that in other Ce-based HF metamagnets, H _M in these metamagnets is linearly proportional to the inverse of the effective mass, or to the temperature where the susceptib+ility shows a peak. This finding suggests that H _M is a magnetic field breaking the local Kondo singlet, and the critical behavior around H _M is driven by the Kondo breakdown accompanied by the Fermi-surface instability.