* Corresponding author. E-mail address: mucio@if.u!.br (M.A. Continentino). Journal of Magnetism and Magnetic Materials 226}230 (2001) 152}154 Phase diagram of Ce(Co  Fe  )Ge  : from complex magnetic ordering to a non-magnetic Fermi liquid S.N. de Medeiros, S.L. Bud'ko, M.B. Fontes, M.A. Continentino*, E.M. Baggio-Saitovitch Instituto de Fn & sica, Universidade Federal Fluminense, Av. Litora L nea s/n } Boa Viagem, Nitero & i, RJ 24210-340, Brazil Centro Brasileiro de Pesquisas Fn & sicas, rua Dr. Xavier Sigaud 150, Urca, Rio de Janeiro, RJ 22290-180, Brazil Ames Laboratory and Department of Physics and Astronomy, Iowa State University, Ames, IA 50011, USA Abstract The evolution from an antiferromagnetic Kondo lattice, with two magnetic ordering transitions (CeCoGe  ), to a non-magnetic, moderate heavy fermion compound (CeFeGe  ) in the Ce(Co  Fe  )Ge  series was studied by magnetic and transport measurements. A tentative magnetic x}T phase diagram for the series was constructed. The experimental results suggest the existence of a quantum critical point close to x  "0.6.  2001 Elsevier Science B.V. All rights reserved. Keywords: Quantum critical point; Kondo lattice; Magnetic order The unusual properties of strongly correlated systems in the vicinity of a quantum critical point (QCP), includ- ing non-Fermi-liquid (NFL) behavior in transport and thermodynamic properties, have attracted a lot of atten- tion of theorists and experimentalists [1,2]. Very few stoichiometric compounds are close to a QCP at ambient pressure. The common approach to bring some Ce, Yb, and U containing materials to the QCP is to modify their properties by applying pressure or by isoelectronic sub- stitution, which often acts as a `chemical pressurea [3]. In this brief communication, we will describe the evolu- tion of the physical properties of CeCoGe  , a Kondo lattice with a complex magnetic order [4,5], towards a non-magnetic heavy fermion CeFeGe  [6,7], caused by non-isoelectronic FePCo substitution. In this case, since there are no changes in the lattice parameters, it is the variation of the band "lling and possibly slight modi"ca- tion of the band structure which are governing the vari- ation of the physical properties. Samples of the Ce(Co  Fe  )Ge  series were prepared by conventional arc-melting of the stoichiometric amounts of the elements in a partial argon atmosphere and subsequent annealing at &7503C for "ve days. Room temperature powder X-ray di!raction con"rmed the single-phase BaNiSn  -type structure of the samples. Resistivity (T, H) of the samples were measured on rec- tangular slabs by conventional four-probe technique us- ing LR-700 AC resistance bridge. AC susceptibility, (T), was measured by homemade susceptometer with modu- lation "eld h  "1G and frequency range 10}10 Hz. Both types of measurements were performed in the 1.7K(T(300 K temperature range. AC susceptibility results for several Ce(Co  Fe  )Ge  samples are shown in Fig. 1. Double transitions could be resolved by careful examination of the susceptibility curves for x)0.2. Temperature-dependent resistivity for representative concentrations are shown in Fig. 2. For pure CeCoGe  there is a clear decrease of resistivity due to the loss of spin disorder scattering at the magnetic ordering transition. Analysis of the derivatives d/dT reveals the presence of both transitions [5]. Similar ap- proach shows the magnetic transitions for Fe-doped samples. From transport and magnetic measurements a tentative magnetic x}T phase diagram was constructed 0304-8853/01/$-see front matter  2001 Elsevier Science B.V. All rights reserved. PII:S0304-8853(00)00653-3