Suppression of superconductivity by nonmagnetic impurities, structural properties and magnetic ordering in Ho xLa 1− xNi 2B 2C

Abstract

Superconducting and magnetic properties as well as the lattice structure of polycrystalline Ho x La 1− x Ni 2B 2C samples have been studied by susceptibility, X-ray, and neutron diffraction measurements, respectively. A miscibility gap has been found between x=0.4 and x=0.7 if the samples are prepared by standard arc-melting. Samples in this concentration range have been successfully prepared by melt-spinning. All samples exhibit the LuNi 2B 2C-type structure and the lattice parameters depend linearly on the La content. Already a low lanthanum concentration of about 10% leads to a rapid depression of T c from 8.5 K to 2 K. Furthermore this concentration leads to a complete depression of the a-axis modulated incommensurate antiferromagnetic structure. Two other magnetic structures, one with commensurate (0 0 1) and one with incommensurate wave vector (0 0 0.915) were found to be less sensitive to the La concentration. The neutron-diffraction peak intensities connected to these structures decrease with increasing La content and disappear at x=0.75 (commensurate structure) and x=0.55 (incommensurate structure). A comparison with Ho x R 1− x Ni 2B 2C compounds (R=Y, Lu) shows that these two magnetic structures are influenced in different way by the size of the nonmagnetic rare-earth ion R. No size effect was observed for the magnetic ordering temperature of the commensurate antiferromagnetic structure, whereas the incommensurate spiral structure along c-axis was found to be sensitive to the difference in the size of the R and the Ho ions.