| Investigations of the physical properties of the recently discovered rare-earth nickel borocarbides, RNi{dollar}rmsb2Bsb2C,{dollar} continue to provide further insight into the interplay between superconductivity and magnetism. In several of these materials, long range magnetic order coexists with superconductivity over some temperature range, and the large variety of magnetic structures observed in neutron and x-ray experiments indicate long range order of localized magnetic moments coupled via a RKKY-type interaction. The strong anisotropic behavior present in some members of the family indicates that crystalline electric field energies also play an important role in the formation of the magnetically ordered state.; We performed x-ray resonant exchange (XRES) scattering experiments on {dollar}rm NdNisb2Bsb2C,{dollar} {dollar}rm SmNisb2Bsb2C,{dollar} {dollar}rm GdNisb2Bsb2C, HoNisb2Bsb2C{dollar} and {dollar}rm ErNisb2Bsb2C.{dollar} For temperatures below 5 K, the Ho compound orders in a simple antiferromagnetic state with modulation wave vector q = (0, 0, 1), while between 5 K and 6.1 K an incommensurate antiferromagnetic phase with two modulation wave vectors, q{dollar}sb{lcub}a{rcub}{dollar} = (0.59, 0, 0) and q{dollar}sb{lcub}c{rcub}{dollar} = (0, 0, 0.92) is observed. {dollar}rm GdNisb2Bsb2{dollar}C orders antiferromagnetically below T{dollar}sb{lcub}N{rcub}{dollar} = 19.4 K with q = (0.55, 0, 0). Between 19.4 K and 13.6 K, the magnetic moments are aligned along the b-axis of the crystal, while below 13.6 K and additional component along the c-axis develops. In {dollar}rm ErNisb2Bsb2C,{dollar} the paramagnetic-to-antiferromagnetic phase transition at T{dollar}sb{lcub}N{rcub}approx{dollar} 6 K is accompanied by a lowering of the lattice symmetry from tetragonal to orthorhombic. The antiferromagnetic structures of the Nd and Sm compounds are characterized by a modulation wave vector q = ({dollar}{lcub}1over2{rcub},{dollar} 0, {dollar}{lcub}1over2{rcub}{dollar}), while the moment directions were determined as (1, 0, 0) and (0, 0, 1), respectively.; Our measurements of these materials have not only demonstrated that high resolution synchrotron x-rays measurements can confirm and refine magnetic structure determinations originally carried out by neutron diffraction, but also that XRES is a tool capable of ab initio structure determinations. In particular, we have shown that by measuring the integrated intensity of selected magnetic Bragg reflections and comparing them with model calculations of their Q-dependence, it is possible to determine the orientation of moments in antiferromagnetically ordered phases. |
