NATURE OF THE MAGNETIC ORDER IN SUPERCONDUCTING AND NONSUPERCONDUCTING HONI2-XCOXB2C

Neutron-diffraction measurements have been carried out to investigate the magnetic properties of superconducting (T-c similar to 8 K) HoNi2B 2C and nonsuperconducting (T-c<3 K) HoNi1.98Co0.015B2C. Both systems b ecome magnetically long-range ordered below similar to 8 K, with three types of magnetic order being present. The low-temperature structure is a commensurate antiferromagnetic state that consists of Ho3+ moment s aligned ferromagnetically in the a-b plane, with the sheets coupled antiferromagnetically along the c axis. The magnetic state that initia lly forms on cooling, however, is dominated by an incommensurate spira l antiferromagnetic state along the c axis, with wave vector q(c) appr oximate to-0.054 Angstrom(-1), in which the relative alignment of each ferromagnetic sheet is rotated in the a-b plane by similar to 17 degr ees from the low-temperature antiparallel configuration. The intensity for this spiral state reaches a maximum near similar to 5 K; the spir al state then collapses at lower temperature in favor of the commensur ate antiferromagnetic state. A smaller amplitude a-axis modulation, wi th q(a) approximate to 0.73 Angstrom(-1), is also observed above the s piral antiferromagnetic transition, but over a narrower temperature ra nge than the spiral state. The identical sequence of phase transitions is observed for both the superconducting and nonsuperconducting sampl es, demonstrating that the reentrant superconductivity and the coexist ence of long-range antiferromagnetic order and superconductivity at lo w temperatures are both controlled by the nature of the magnetic struc tures present. In the temperature regime where the three magnetic stru ctures are observed simultaneously, the sample dependence of the inten sities strongly suggests that they occur in spatially separate regions .