Combined Electronic-Nuclear Magnetic Ordering of the Ho 3+ Ions and Magnetic Stacking Faults in the High- T c Superconductor HoBa 2 Cu 3 O 7

Abstract
Highly unusual magnetic ordering stabilised by nuclear 165Ho polarisation is found to exist below TN = 190 mK in HoBa2Cu3O7 by means of low-temperature neutron scattering experiments. The three-dimensional antiferromagnetic ordering corresponding to K = [0,1/2,1/2] is imperfect, due to finite correlation length ξc = 29 Å along the c=axis. The observed temperature dependence of the magnetic order parameter is in excellent agreement with mean-field calculations taking into account crystal field, exchange and hyperfine interactions.