Neutron diffraction from HoNi₂b₂C

RENi₂B₂C (RE = rare-earth) are quaternary compounds which exhibit a considerable degree of interaction between their superconducting and magnetic properties. The Ho variant is found to become superconducting at T_c = 9 K, but anomalous behaviour is observed in the low-field magnetic properties at T < 7 K (increasing susceptibility with decreasing temperature); at temperatures less than 5.6 K more conventional behaviour is regained. Neutron diffraction measurements demonstrate that at low temperatures (T < 5 K) the magnetic order consists of ferromagnetic planes antiferromagnetically coupled up the c-axis with a Ho moment of 8.9 ± 0.2 μ_B canted at 15 ± 5° from the a-b plane. At high temperatures (5 K < T < 8.5 K) satellites, together with the principal Bragg reflections, are observed which correspond to a modulation of the magnetic order with a wavelength of ∼ 136 Å up the c-axis. In the intermediate temperature (5 K < T < 7 K) range further Bragg reflections become evident. An additional modulation along the a-axis becomes important in this temperature range.