INCOMMENSURATE AND FERRIMAGNETIC PHASES IN U(NI,M)(2)SI-2 WITH MINOR M=CO OR CU

UNi2Si2 orders magnetically at 124+/-1 K in an incommensurate (IC) pha se, undergoes transition at 103+/-1 K to AF-I (+-+-)phase, and then an other transition at 53+/-1 K to a ferrimagnetic (++-) phase. A.c.-susc eptibility and neutron-diffraction studies of polycrystalline U(Ni,M)( 2)Si-2 solid solutions, with minor M=Co and Cu (U(Co1-yNiy)(2)Si-2 wit h y=0.75, 0.90, and 0.95, and U(Ni1-zCuz)(2)Si-2, with z=0.05, 0.10, a nd 0.25) confirm an IC phase for y=0.90 below 119+/-2 K down to 105+/- 2 K and suggest (by a.c.-susceptibility only) IC phases below T-N for compositions between y approximate to 0.85 and z approximate to 0.05. The above ferrimagnetic phase is observed at T less than or equal to 1 2 K also for compositions between y approximate to 0.93 and z approxim ate to 0.03. The magnetic phase diagram in the vicinity of UNi2Si2 is redrawn and discussed. (C) 1998 Elsevier Science S.A.