  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_M \simeq 4 T 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 susceptibility 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.
