Crystal Field and Flux Pinning Magnetostriction in REBa 2 Cu 3 O 7 _ x high- T c Superconductors

Abstract

High-field (up to 12 T) magnetostriction measurements have been carried out on single crystals of high-T_c superconductors REBa₂Cu₃O₇__x (RE = Dy, Ho and Er) at the paramagnetic and superconducting phase, between 3.8 and 100 K. The magnetostriction has been measured both parallel and perpendicular to the main crystal symmetry directions (a- and c-axes). We have isolated the basal plane symmetry breaking magnetostrictive mode, εγ, which becomes large (=-300 x 10⁻⁶ at 10 K and 12 T), except for the Er compound, where magnetostriction is weak. Also a linear combination of the exchange and crystal field origin modes distorting the volume and the c/a ratio could be determined, from the A(c, a) strain. A main result is the obtainment of the ratio M^γ/C^γ₀ where M^γ is the single-ion crystal field irreducible magnetoelastic coupling parameter and C^γ₀ the irreducible elastic constant, which amounts 1.3 x 10⁻⁴ (Ho compound) and is of the order of 5 x 10⁻⁴ (Dy compound), as well as the quadrupolar coupling parameter G^γ for the Ho compound, which amounts -35 mK/ion. Low-temperature strong contributions to the magnetostriction, related to the vortex flux pinning by the crystal lattice are clearly evidenced. This flux pinning magnetostriction shows a scaling behavior which has been analyzed in terms of Kramer's flux-lattice shear stress model, and represents a new method to study pinning force mechanisms using magnetostriction measurements.