Neutron-diffraction evidence for 3D long-range antiferromagnetic ordering in DyBa 2Cu 3O 6.95 and for antiferromagnetic correlations in HoBa 2Cu 3O 6.8

Abstract

Polycrystalline samples of the high-temperature superconductors RBa 2Cu 3O 7− 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 YBa 2Cu 3O 7− x , corresponding to space group Pmmm, and yield the oxygen-defect concentrations x = 0.05(6) and x = 0.22 (6) for the Dy and the HO compound, respectively. No significant change was detectable in the crystal structure of HoBa 2Cu 3O 6.8 during the phase transition to superconductivity sat T c = 94(1) K. In agreement with an independent recent neutron-diffraction study DyBa 2Cu 3O 6.95 shows below the Néel point T N = 0.90(3) K three-dimensional (3D) long-range antiferromagnetic order, similar to the previously reported S-state system GdBa 2Cu 3O 7, corresponding to Q = [ 1 2 , 1 2 , 1 2 ] . The Dy 3+ ordered magnetic moments are aligned parallel to the c-axis and saturate to 6.8(1) μ B. In HoBa 2Cu 3O 6.8 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 ErBa 2Cu 3O 7 but its shape is considerably less asymmetric.