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) HoNi2B2C and nonsuperconducting (T-c<3 K) HoNi1.98Co0.015B2C. Both systems become 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+ moments aligned ferromagnetically in the a-b plane, with the sheets coupled antiferromagnetically along the c axis. The magnetic state that initially forms on cooling, however, is dominated by an incommensurate spiral antiferromagnetic state along the c axis, with wave vector q(c) approximate 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 degrees from the low-temperature antiparallel configuration. The intensity for this spiral state reaches a maximum near similar to 5 K; the spiral state then collapses at lower temperature in favor of the commensurate antiferromagnetic state. A smaller amplitude a-axis modulation, with q(a) approximate to 0.73 Angstrom(-1), is also observed above the spiral antiferromagnetic transition, but over a narrower temperature range than the spiral state. The identical sequence of phase transitions is observed for both the superconducting and nonsuperconducting samples, demonstrating that the reentrant superconductivity and the coexistence of long-range antiferromagnetic order and superconductivity at low temperatures are both controlled by the nature of the magnetic structures present. In the temperature regime where the three magnetic structures are observed simultaneously, the sample dependence of the intensities strongly suggests that they occur in spatially separate regions.