MAGNETIC-PROPERTIES OF U(NI1-XCUX)2SI2 SOLID-SOLUTIONS IN THE VICINITY OF X=0.50 STUDIED BY NEUTRON-DIFFRACTION AND AC-SUSCEPTIBILITY

The magnetic properties of two U(Ni1-xCux)2Si2 solid solutions in the vicinitY of x = 0.50 (denoted I and II) have been studied by neutron d iffraction and ac-susceptibility. Both materials have ThCr2Si2-type cr ystallographic structure. AC-susceptibility shows antiferromagnetic tr ansitions at T(N) = 150+/-5 K in UNiCuSi2(I) and 155+/-5 K in UNiCuSi2 (II), followed by several transitions with ferro/ferrimagnetic (F) cha racter at lower temperatures. Apart from the transitions to AF-I struc ture (+ - + -) at T(N) (150+/-1 K and 152+/-2 K, respectively), none o f the F transitions is observed by neutron diffraction. Short-range ma gnetic order, involving several consecutive ferromagnetic planes or fe rrimagnetic groups of planes in the AF-I phase,detected by ac-suscepti bility and not by neutron diffraction in both materials and therefore significant to x almost-equal-to 0.50, is proposed to explain the unus ual susceptibility.