MAGNETIC-ORDERING IN UCONISI2 AND UCOCUSI2 STUDIED BY AC-SUSCEPTIBILITY AND NEUTRON-DIFFRACTION MEASUREMENTS

Polycrystalline samples of intermediate solid solutions of the UM2Si2 compounds (M=Co,Ni,Cu), namely UCoNiSi2 and UCOCuSi2, Were prepared an d were found to have body-centered tetragonal ThCr2Si2-type crystallog raphic structure. In UCoNiSi2 ac susceptibility indicates a single ant iferromagnetic (AF) transition at T(N) = 115 +/- 5 K, confirmed by neu tron-diffraction observation of the AF-I structure down to 10 K (with uranium moments of 1.6 +/- 0.2mu(B), along the tetragonal c axis). In UCoCuSi2 ac susceptibility indicates ferromagnetic transition at T(C) = 107 +/- 5 K, and implies an AF transition at lower temperature, conf irmed by the AF-I structure, observed in neutron diffraction below T0 = 95 +/- 5 K down to 10 K (with uranium moments of 1.6 +/- 0.1muB, alo ng the c axis). The magnetic properties are discussed in comparison Wi th UM2X2 and U(M,M')2X2 materials (X = Si,Ge).