Magnetic phase diagram and fermi surface properties of CeRu ₂(Si_1-xGex)₂

We report the magnetic, transport and de Haas-van Alphen (dHvA) effect measurements to investigate the evolutions of metamagnetic behavior and electronic structure with Ge concentration x in CeRu₂(Si _1-xGex)₂. With decreasing x, the ground state changes from ferromagnetic state to antiferromagnetic state at xa = 0.58, then to paramagnetic state at xc = 0.065. When a magnetic field is applied parallel to the [001] direction, first order metamagnetic transition takes place in the anitferromagnetic state and it changes to metamagnetic crossover in the paramagnetic state at xc. It is also found that the metamagnetic behavior as well as the magnetic phase diagram qualitatively change at xb = 0.29. The transport and dHvA effect measurements have been performed with fields applied in the (001) plane for which no metamagnetic transition takes place. With decreasing x, the magnitudes of the residual resistivity at 0.5 K, the coefficient A of T2 term in the temperature dependence of resistivity, the magnetoresistivity and the Hall resistivity increase discontinuously across xa. In the antiferromagnetic states they do not change much between xa and xb and then increases rapidly with decreasing x down to xc. Then, in the paramagnetic state they decrease with decreasing x, indicating that they are enhanced around xc in the antiferromagnetic and paramagnetic states. However, they seem to be maximum between x = 0.2 and xc, but not at xc. There may be also discontinuous changes in the transport properties across xb and xc. In the ferromagnetic state, we can observe the dHvA oscillations which are attributed to the electron surfaces as well as from the ellipsoidal hole surfaces in CeRu₂Ge2 where the f electron is localized. In the antiferromagnetic state for x < xa we can not observe the dHvA oscillations from the electron surface, while we can observe those from the ellipsoidal hole surfaces which are also present in CeRu₂Si₂ where the f electron is itinerant. The effective mass is found to be enhanced around xc. We discuss the characteristic behavior of the metamagnetic transition in this system compared with other system. From the discontinuous changes at xa found in the transport and dHvA effect measurements, we argue that the f electron is likely to change its nature form localized to itinerant at xa.