We report the temperature-pressure-magnetic field phase diagram made from electrical resistivity measurements for the ferromagnetic (FM) Kondo lattice CeRuPO. The ground state at zero field changes from the FM state to another state, which is suggested to be an antiferromagnetic (AFM) state, above ∼ 0.7 GPa, and the magnetically ordered state is completely suppressed at ∼ 2.8 GPa. In addition to the collapse of the AFM state under pressure and a magnetic field, a metamagnetic (MM) transition from a paramagnetic state to a polarized paramagnetic state appears. CeRuPO will give us a rich playground for understanding the mechanism of the MM transition under comparable FM and AFM correlations in the Kondo lattice.KEYWORDS: CeRuPO, ferromagnetic Kondo lattice, metamagnetism, pressureThe metamagnetic (MM) transition in correlated electron systems has been an interesting subject related to magnetic instability and Fermi surface instability. An excellent example is the heavy-fermion system CeRu 2 Si 2 and doped systems.1-11 In the antiferromagnetic (AFM) system with Rh, La, and Ge doping, a magnetic field suppresses the ordered phase in the critical field, inducing the MM transition.2-7 Even in the absence of the AFM phase, if the system is located close to the AFM instability, the proximity of the AFM critical field yields the MM transition. In addition to the field evolution of the AFM correlation, the presence of the ferromagnetic (FM) correlation also plays a vital role in the MM transition in CeRu 2 Si 2 , 8 and the FM state is realized on the Ge-rich side in CeRu 2 (Si,Ge) 2 .6, 7 Another key factor for the MM transition is a change in the Fermi surface related to the breakdown of the Kondo effect. If the Kondo temperature T K is low, a magnetic field corresponding to T K can quench the Kondo effect, inducing the MM transition. The interpretation of the change in the Fermi surface is still a subject of debate.9, 10 In Rh-doped CeRu 2 Si 2 , a clear separation of two MM transitions demonstrates the presence of two mechanisms for the MM transition.11 On the other hand, in the system with the FM instability, the MM transition from the paramagnetic (PM) state to the FM state is realized because of strong FM correlations. If the system has a tricritical point (TCP) where the second-order phase transition at Curie temperature T C changes into the first-order phase transition, a wing structure of the first-order MM transition appears in the pressure(P )-temperature(T )-magnetic field(H) phase diagram.12, 13 Such a phase diagram can be explained in the framework of itinerant FM systems and has been confirmed in 5f systems such as UGe 2 14, 15 and UCoAl
16and 3d systems such as ZrZn 2 .
17CeRuPO is a rare example of the FM Kondo lattice among 4f electron systems.18, 19 Its FM ordered moments are aligned along the c-axis in the tetragonal structure below T C = 14.5 K, although a larger magnetization is induced along the ab plane in high-temperature or high-field regions owing to the effect of a crystal electric field (CEF)....
