Neutron-Diffraction Evidence for 3D Long-Range Antiferromagnetic Ordering in DyBa₂Cu₃O_6,95 and for Antiferromagnetic Correlations in HoBa₂Cu₃O_6,8

Polycrystalline samples of the high-temperature superconductors RBa2Cu3O7−x (R = Dy, Ho) were investigated by means of neutron diffraction in the temperature range from 80 mK to 295 K. Profile refinements show that these compounds are isostructural with YBa2Cu3O7−x, corresponding to space group Pmmm, and yield the oxygen-defect concentrations x = 0.05(6) andx = 0.22 (6) for the Dy and the HO compound, respectively.

No significant change was detectable in the crystal structure of HoBa2Cu3O6.8 during the phase transition to superconductivity sat Tc = 94(1) K. In agreement with an independent recent neutron-diffraction study DyBa2Cu3O6.95 shows below the Néel point TN = 0.90(3) K three-dimensional (3D) long-range antiferromagnetic order, similar to the previously reported S-state system GdBa2Cu3O7, corresponding to Q = [12,12,12].

The Dy3+ ordered magnetic moments are aligned parallel to the c-axis and saturate to 6.8(1) μB. In HoBa2Cu3O6.8 a broad peak, presumably due to antiferromagnetic correlations between Ho ions, appears below a characteristic temperature of 140(15) mK, in agreement with heat-capacity measurements. The peak resembles the one observed in the 2D antiferromagnet ErBa2Cu3O7 but its shape is considerably less asymmetric.