Antiferromagnetic ordering and crystal-field splittings of the Ho3+ions in HoBa2Cu4O8

Abstract
The antiferromagnetic structure of the high-temperature superconductor HoBa2Cu4O8 with Tc=77 K was determined by means of neutron diffraction at T=9 mK. Imperfect three-dimensional antiferromagnetic ordering of the Ho3+ ions corresponding to propagation vector K=(0, 1/2 , 0) is found below TN=100 mK with magnetic moments of magnitude 1.5 mu B at saturation and oriented parallel to the a-axis. The antiferromagnetic sublattice is characterized by stacking faults corresponding to a finite correlation length xi c=18 AA along the c-axis. The majority of the 17 crystal-field level within the ground-state J-multiplet of the Ho3+ ions is established by inelastic neutron scattering, which allowed an unambiguous determination of the nine assignable crystal-field parameters. The observed temperature dependence of the order parameter is found to be in excellent agreement with mean-field calculations including crystal-field, exchange and hyperfine interactions.