Magnetic Properties of U2Co17-xSix Compounds

Introduction Uranium and cobalt do not form a binary compound with the hexagonal Th2Ni17-type crystal structure but this structure can be stabilized by a small substitution of Co by a third element, e.g. Si or Ge [Berlureau et al., 1993; Chevalier et al., 1996; Shiokawa et al., 1994]. The homogeneity range extends from x = 1.3 to 3 in the U2Co17-xGex system [Chevalier et al., 1996]. The related U2Co17-xSix system has not been studied systematically. Only the existence of U2Co15Si2 with the Th2Ni17-type of crystal structure has been reported [Berlureau et al., 1993]. Its magnetic properties have not been studied yet. There is no doubt that the Co atoms carry large magnetic moment in this compound. It is of interest whether U is magnetic or not in U2Co17-xSix because a coexistence of ordered magnetism of 5fand 3dmetals is a rather rare case. It is known that U is non-magnetic and, moreover, reduces considerably the Co magnetic moment in the binary compound UCo5.3 [Deryagin et al., 1976]. On the other hand, there is an indirect evidence (strong magnetic anisotropy and peculiarities of spontaneous magnetostriction) for magnetic state of U in UCo10Si2 [Andreev et al., 1995; 1997]. In the compound, isostructural to U2Co15Si2, U2Co15Ge2 [Shiokawa et al., 1994], an enhanced magnetic anisotropy can also serve as a proof for magnetic state of U, however, no data are available on the Co-sublattice anisotropy that is needed to estimate the U contribution. Later on, a single crystal with a higher Si content (x = 3.4) was grown and its magnetic properties were measured [Andreev et al, 2004]. A nonmagnetic state of U was deduced from the comparison with the isostructural Lu2Co13.6Si3.4 compound with the same Si content. The aim of the present work was to determine the width of the homogeneity range, to grow single crystals of compounds within the homogeneity range, to study their magnetic properties and to estimate the uranium contribution to the magnetism of U2Co17-xSix compounds.