Suppression of superconductivity by nonmagnetic impurities, structural properties and magnetic ordering in Ho sub(x)La sub(1-x)Ni sub(2)B sub(2)C

摘要

Superconducting and magnetic properties as well as the lattice structure of polycrystalline Ho sub(x)La sub(1-x)Ni sub(2)B sub(2)C samples have been studied by susceptibility, X-ray, and neutron diffraction measurements, respectively. A miscibility gap has been found between x identical with 0.4 and x identical with 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 sub(2)B sub(2)C-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 sub(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 identical with 0.75 (commensurate structure) and x identical with 0.55 (incommensurate structure). A comparison with Ho sub(x)R sub(1-x)Ni sub(2)B sub(2)C compounds (R identical with 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.