BREAKDOWN OF DE-GENNES SCALING IN HOXLU1-XNI2B2C

The temperature dependence of magnetic ordering and the superconductin g transition have been studied for polycrystalline HoxLu1-xNi2B2C comp ounds by susceptibility and resistivity measurements as well as neutro n diffraction. For Ho concentrations in the range 0 less than or equal to x less than or equal to 0.7, the superconducting transition temper ature, T-c, decreases from 16.5 K at x = 0 linearly with increasing x and, consequently, with increasing effective de Gennes factor. This is in accordance with a generalized Abrikosov-Gor'kov theory. At x = 0.7 , T-c reaches 8.5 K i.e. the value of HoNi2B2C. Pair breaking by the H o magnetic moments is stronger in HoxLu1-xNi2B2C compared to HoxY1-xNi 2B2C. This is attributed to the difference between the lattice constan ts of the Ho-Y and the Ho-Lu systems resulting in different electronic structure parameters. A complete breakdown of the scaling behaviour o f T-c occurs for x > 0.7, where T-c becomes independent of the effecti ve de Gennes factor. In this range of x re-entrant behaviour and the p resence of an incommensurate a-axis, modulated antiferromagnetic struc tures have been observed below a characteristic temperature T-m. Comme nsurate and incommensurate c-axis modulated antiferromagnetic structur es that are observed both in the Ho-Y and the Ho-Lu systems coexist wi th superconductivity. (C) 1998 Elsevier Science B.V. All rights reserv ed.