Neutron scattering study of crystal field energy levels and field dependence of the magnetic order in superconducting HoNi₂B₂C

Elastic and inelastic neutron scattering measurements have been carried out to investigate the magnetic properties of superconducting (T_c ∼ 8 K) HoNi₂B₂C. The inelastic measurements reveal that the lowest two crystal field transitions out of the ground state occur at 11.28(3) and 16.00(2) meV, while the transition of 4.70(9) meV between these two levels is observed at elevated temperatures. The temperature dependence of the intensities of these transitions is consistent with both the ground state and these higher levels being magnetic doublets. The system becomes magnetically long range ordered below 8 K, and since this ordering energy kT_N ≈ 0.69 meV ≪ 11.28 meV the magnetic properties in the ordered phase are dominated by the ground-state spin dynamics only. The low temperature structure, which coexists with superconductivity, consists of ferromagnetic sheets of Ho³⁺ moments in the a-b plane, with the sheets coupled antiferromagnetically along the c-axis. The magnetic state that the initially forms on cooling, however, is dominated by an incommensurate spiral antiferromagnetic state along the c-axis, with wave vector q_c ∼ 0.054 A ̊^-1, in which these ferromagnetic sheets are canted from their low temperature antiparallel configuration by ∼ 17°. The intensity for this spiral state reaches a maximum near the reentrant superconducting transition at ∼ 5 K; the spiral state then collapses at lower temperature in favor of the commensurate antiferromagnetic state. We have investigated the field dependence of the magnetic order at and above this reentrant superconducting transition. Initially the field rotates the powder particles to align the a-b plane along the field direction, demonstrating that the moments strongly prefer to lie within this plane due to the crystal field anisotropy. Upon subsequently increasing the field at constant T the antiferromagnetic and spiral states are both observed to decrease in intensity, but at modest fields the spiral state decreases much less rapidly. Approaching the superconducting phase boundary from high fields, we find that the spiral state is strongly preferred, in diference to the superconductivity, again demonstrating a direct coupling between these two cooperative phenomena. The magnitude of the spiral wave vector q_c, on the other hand, shows very little field dependence. A magnetic moment of 8.2±0.2 μ_B for the Ho³⁺ is obtained from the observed field dependence of the induced moment at high fields (7T).