Magnetization study of the UNi1-xCoxGa solid solutions

Results of a crystal-structure and magnetization study of UNi1-xCoxGa solid solutions are presented. All compounds over the entire concentration range have been found to crystallize in the hexagonal ZrNiAl-type structure. The concentration dependences of the lattice parameters a and c are linear on intervals x<0.6 and x>0.7 but they considerably change their slope around x=0.6-0.7, which is probably due to preferential occupation of the two non-equivalent transition-metal sites by the Ni and Co atoms. The lattice volume, however, does not show such an anomaly. Magnetization data are interpreted in terms of a sudden conversion of antiferromagnetism in UNiGa to ferromagnetism induced by substituting 2% of Co atoms for Ni without apparent change of the magnetic moment and magnetic ordering temperature. All compounds with xgreater than or equal to0.02 show clear attributes of ferromagnets. Whereas the magnetic moment more or less monotonously decreases with increasing Co concentration the values of Curie temperature pass through a broad minimum in the intermediate Ni-Co concentration region. Since the high-field differential susceptibility exhibits a pronounced maximum in this concentration region, microscopic magnetic inhomogeneities (freezing of antiferromagnetically coupled regions) due to substitutional disorder in the Ni-Co sublattice may be a possible mechanism responsible for the non-monotonous evolution of T-C values. (C) 2002 Elsevier Science B.V. All rights reserved.