Two-dimensional spin fluctuations of Ho3+ in HoBa2Cu3O7.

Abstract

Neutron diffraction and inelastic neutron scattering results on ${\mathrm{HoBa}}_{2}$${\mathrm{Cu}}_{3}$${\mathrm{O}}_{7}$ are presented. In the neutron diffraction experiments two-dimensional (2D) short-range order has been observed in the ${\mathrm{Ho}}^{3+}$ sublattice at 1.6 K, which is at a temperature far above the hyperfine-induced ordering temperature ${\mathit{T}}_{\mathit{N}}$=190 mK. A strong enhancement of the effective susceptibility (including the hyperfine interaction) compared with a Curie-Weiss susceptibility is found above 400 mK. This enhancement explains why short-range ordering of the ${\mathrm{Ho}}^{3+}$ spins with an in-plane correlation length of 13 \AA{} are detectable at these temperatures. In addition, a mean-field shift of the lowest lying crystal-field transition was observed between 9 and 300 mK, which is well described within our formalism. The anisotropic line shape of this transition could be understood by a Gaussian field distribution on the ${\mathrm{Ho}}^{3+}$ site, originated in the dynamic 2D short-range ordering. These results are in good agreement with muon spin rotation results. \textcopyright{} 1996 The American Physical Society.