INCOMMENSURABILITIES AND METAMAGNETISM IN THE HEAVY-FERMION ALLOYS (CE0.8LA0.2)RU2SI2 AND CERU2(SI0.9GE0.1)2

Long-range magnetic order can be induced in the heavy-fermion compound CeRu2Si2 by slightly expanding the unit-cell volume through alloying (La substituted for Ce, or Ge for Si). The resulting magnetic state has been investigated in (Ce0.8La0.2)Ru2Si2 and CeRu2(Si0.9Ge0.1)2 single crystals by neutron-scattering experiments, as a function of temperature and magnetic field. Very similar phase diagrams, consisting of various regions with either commensurate or incommensurate modulations of the Ce moments, are obtained for both alloys. The second magnetic transition below T(N) that occurs at H = 0 in (Ce0.8La0.2)Ru2Si2 appears to be closely related to the persistence of short-range magnetic fluctuations in the ordered state of this material. It is suggested that the leading effect may be a locking of the phase of the modulation.