3 COMMENSURATE MAGNETIC-STRUCTURES IN THE SOLID-SOLUTION U(NI0.25CU0.75)2SI2, OBSERVED BY NEUTRON-DIFFRACTION

A polycrystalline sample of the solid solution U(Ni0.25Cu0.75)2Si2 Was prepared and found to have the tetragonal ThCr2Si2-type crystallograp hic structure. The observation of a ferrimagnetic transition at 112 +/ - 5 K by ac-susceptibility measurements, is confirmed by neutron diffr action, which also indicated three commensurate magnetic structures (b elow almost-equal-to 110 K). Two structures are antiferromagnetic (AF) , 40% each, and one is ferrimagnetic, 20% in volume. The structures ar e AF-I (+ - + -), ferrimagnetic (+ + -), and AF-IA (+ + - -), with wav e vectors k = (0, 0, 1), (0, 0, 2/3), and (0, 0, 1/2), and T(N) = 120 +/- 3, 115 +/- 4, and 110 +/- 3 K, respectively. The uranium ordered m agnetic moments (2.2 +/- 0.1 mu(B) at 1.5 K) are along the tetragonal axis in all three structures. These phases fit well into the proposed magnetic phase diagram of the U(Ni1-xCux)2Si2 system, and are compared with corresponding solid solutions in the parallel U(Ni1-xCux)2Ge2 sy stem.