~) Solid State Communications, Vol. 77, No. 5, pp. 317-321, 1991. Printed in Great Britain. 0038-1098/91 $3.00+.00 Pergamon Press plc MAGNETIC PHASE DIAGRAM OF Ge-SUBSTITUTED CeRu2Si2 IN AN APPLIED FIELD J.-M. Mignot* and Ph. Boutrouille LLB, CEN Saclay, 91191 Gif sur Yvette, France L.-P. Regnault DRF/SPh-MDN, CEN Grenoble, 85X, 38041 Grenoble C&lex, France and P. Haen and P. Lejay CRTBTt CNRS, 166X, 38042 Grenoble C6dex, France (Received 5 November 1990 by P. Burlet) Long-range magnetic order can be induced in the heavy-fermion compound CeRu2Si2 by slightly expanding its lattice through the substitution of a few percent of germanium for silicon. For a concentration z = 10%, the material orders in the same collinear, incommensurate, modulated structure as that already reported for (Ce,La)Ru2Si2. The (H, T) phase diagram of this system has been determined by elastic neutron scattering. Various commensurate-incommensurate transitions take place as a function of temperature and applied field. The results are discussed in connection with the problem of magnetic correlations in pure CeRu2Si2. I1 est possible d'induire un ordre magn6tique ~ longue port6e dans le compos6 ~tfermions lourds CeRu2Si2 en dilatant 16g~rement son r~seau cristallin par la substitution de quelques pour cent de germanium au silicium. Pour une concentration de z = 10%, le mat6riau s'ordonne dans une structure modul6e colin6aire incommensurable identique ~t celle d6crite ant6rieurement pour (Ce,La)Ru2Si2. Le diagramme de phases (H, T) de ce syst~me a 6t6 d6termin6 par diffusion 61astique de neutrons. Diff6rentes transitions incommensurable-commensurable se produisent en fonction de la temp6rature et du champ appliqu6. Les r6sultats sont discut6s en relation avec le probl~me des corr61ations magn6tiques/t courte port6e dans le CeRu2Si2 pur. 1. INTRODUCTION One of the major questions concerning heavy-fermion (I-IF) systems is the role played by intersite magnetic inter- actions, particularly at low temperatures where a competi- tion takes place between different possible ground states: a magnetically ordered phase, an (unconventional) superconductor, or a Fermi liquid state consisting of quasiparticles with extremely high effective masses. The latter situation appears to prevail in CeRu2Si2, a tetragonal (I4/mmm) compound in which neither superconductivity, nor long-range order was observed down to 20inK 1,2. However, neutron scattering experiments3,4 have shown that short-range magnetic correlations occur below = 60 K. Their in-plane characteristic length, Ca, increases on cooling, first slowly, then more rapidly below 8 K, but finally it saturates to a value of only 10-15 A. The failure of a static long-range structure to develop in this compound was ascribed to the existence of strong, Kondo-type, local fluctuations. Indeed, magnetic ordering can be induced by slightly expanding the unit cell through the substitution of ~8% La on the Ce sublatticeS, 6, thereby decreasing the Kondo temperature TK 7. The resulting structure was * Permanent address: CRTBT, CNRS, Grenoble 1" Laboratoire asoci6 /1 l'Universit6 Joseph Fourier, Grenoble. 317 shown 6 to be collinear (~ II c), and to consist in an incommensurate modulation of the moment amplitude along the a-direction. Quite remarkably, the associated wavevector, ~ = [0.31, 0, 0], corresponds to the position of the correlation peaks observed in pure CeRu2Si28. Furthermore, it does not depend on temperature for a given alloy, and very little on the La concentration over a wide range ofx from 0.08 to 0.809,10. On the other hand, it should be borne in mind that, besides exerting a "chemical pressure" on the system, La substitution has the undesired effect of disturbing the periodic arrangement of the Ce magnetic moments and reducing their concentration. This may lead to difficulties in analysing the results, especially at low temperatures. It thus seemed of interest to investigate another alloy-system, CeRu2(Sit.zGez)2, in which the expansion of the unit cell is achieved while leaving the Ce sublattice unaffected. Previous experiments performed on polycrystals have shown that, for 0.20 ~ z < 0.40, these alloys undergo an antiferromagnetic transition at a critical temperature TN--- 5- 8K 11,]2. In the ordered state, the magnetic isothermsl2, t3 suggest some kind of metamagnetic behavior in an applied field, similar to that already reported for Cel.xLaxRu2Si214. This paper describes elastic neutron scattering experiments performed on a single crystal of CeRu2(Sit.~Gez)2. The magnetic structure at H = 0 and the (H, T) phase diligram were determined. The results are compared to bulk magnetic measurements .on the same material and to the neutron data for (Ce,La)Ru2Si2.