ANTIFERROMAGNETIC ORDERING AND CRYSTAL-FIELD SPLITTINGS OF THE HO(3+) IONS IN HOBA2CU4O8

The antiferromagnetic structure of the high-temperature superconductor HoBa2Cu4O8 with T(c) = 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 T(N) = 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 angstrom along the c-axis. The majority of the 17 crystal-field levels 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.