Phase diagram of the CeNi 1 Àx Cu x Kondo system with spin-glass-like behavior favored by hybridization J. Garcı ´ a Soldevilla and J. C. Go ´ mez Sal CITIMAC, Facultad de Ciencias, Universidad de Cantabria, Avenida de los Castros, 39005 Santander, Spain J. A. Blanco Departamento de Fı ´sica, Facultad de Ciencias, Universidad de Oviedo, Calvo Sotelo, 33007 Oviedo, Spain J. I. Espeso and J. Rodrı ´ guez Ferna ´ ndez CITIMAC, Facultad de Ciencias, Universidad de Cantabria, Avenida de los Castros, 39005 Santander, Spain Received 21 June 1999 We present the magnetic phase diagram of the Kondo ferromagnetic and antiferromagnetic CeNi 1-x Cu x series revealing the existence of a ‘‘spin-glass-like’’ state above the Curie temperature T C . The stability temperature range of this magnetically disordered phase increases when approaching the magnetic localized- delocalized crossover point ( x 0.1). This phenomenology is first discussed considering a classical model of spin-glass phase diagram including the effect of Kondo interactions. The similarities to the scenario described by recent theoretical analysis of strongly correlated electron systems, considering disorder and competing Ruderman-Kittel-Kasuya-Yosida and Kondo interactions, are also pointed out. I. INTRODUCTION The competition of interactions in strongly correlated f-electron systems is attracting a great deal of interest be- cause of the physics involved: Kondo lattice, heavy fermi- ons, and recently non-Fermi liquids NFL’s or quantum phase transitions at zero temperature. 1,2 An adequate way to approach the problem is to study a system where the compe- tition between the different interactions, needed to induce a priori those behaviors, could be modified by a single param- eter: this is the case of CeNi 1 -x Pt x Ref. 3 and CeRu 2 Ge 2 Ref. 4 both evolving from ferromagnetism to Fermi-liquid behavior by changing composition or pressure, respectively, or the well-known Y 1 -x U x Pd 3 Ref. 5 and CeCu 5.9 Au 0.1 Ref. 6 systems which are some of the paradigmatic NFL’s. Additionally, an increasing number of authors are referring to the relevance of disorder effects 7,8 in the low-temperature thermodynamic and magnetic properties of these substitu- tional strongly correlated systems and they find ground states such as spin-glass ones induced by structural disorder or ran- dom competing interactions. In such a way, recent experi- mental research on the ground state of the CeNi 0.4 Cu 0.6 compound 9 shows that this Kondo lattice compound, with low local symmetry, presents a simple collinear ferromag- netic long-range order below T C =1.1 K with the magnetic moments lying in the b direction. In addition, from all the measured bulk properties a spin-glass SG state below T f =2 K is found, while at higher temperatures the compound becomes paramagnetic. In this context the general CeNi 1 -x Cu x series presents many noticeable features, pro- viding a singular and attractive system to investigate the competition of the different involved interactions, i.e., indi- rect Ruderman-Kittel-Kasuya-Yosida RKKY exchange interactions, large 4 f -conduction-band hybridization Kondo and strong crystalline electric-field CEF local an- isotropy combined with the presence of disorder effects. The previously obtained results for CeNi 1 -x Cu x compounds 10,11 are summarized in Fig. 1. The stability do- main of the orthorhombic FeB-type structure Pnma extends for 1 x 0.2. CeNi is CrB type Cmcm. Both structures differ only in the relative disposition of trigonal prisms. A change from antiferromagnetism AFM for CeCu and CeNi 0.1 Cu 0.9 to ferromagnetism FM for x 0.8 is observed, as in other R Ni 1 -x Cu x series. 12 Simultaneously, the 4 f -conduction-band hybridization increases with the Ni con- tent, evolving in a similar way to that in the much studied CeNi 1 -x Pt x series. 3 The stronger hybridization effects appear on the Ni-rich side, with CeNi being an intermediate valence compound. Consequently, these CeNi 1 -x Cu x compounds are FIG. 1. Concentration dependence of the cell volume full circles and the Kondo temperature estimated from different tech- niques: magnetic susceptibility ( |  p | /10, full squares, quasielastic neutron scattering QENS, open squares for the CeNi 1-x Cu x se- ries. The broken lines separate the FeB-CrB crystallographic struc- tures and AFM-FM magnetic states. Full lines are guides for the eyes. PHYSICAL REVIEW B 1 MARCH 2000-II VOLUME 61, NUMBER 10 PRB 61 0163-1829/2000/6110/68215/$15.00 6821 ©2000 The American Physical Society