Elastic properties of polycrystalline (La, Sr)₂CuO₄ and YBa₂Cu₃O_7−δ doped with iron

Abstract

The elastic modulus of polycrystalline (La, Sr)₂CuO₄ and YBa₂Cu₃O_7−δ with copper replaced by iron has been measured in the temperature range from room temperature to 4.2 K using the cube resonance method. For the former system, 0.5% Cu is displaced by iron and the content of strontium is varied. The incorporation of iron gives rise to an apparent coexistence of magnetic order and superconducting order in samples La_2−x Sr _x (Cu, Fe)O₄ withx=0.15 and 0.3, which is reflected in the temperature dependence of Young's modulus. For samples withx=0 and 0.05, an anomalous behaviour of Young's modulus, which is thought to correspond to orthorhombic-monoclinic transition, is observed. For the YBa₂(Cu_1−γFe_γ)₃O_7−δ system, softening and a thermal hysteresis loop are observed in the temperature dependence of shear modulus of the sample withγ=0. The softening also appears in the temperature variation of Young's modulus. The degree of softening becomes smaller as the iron content which displaces copper is increased. These phenomena are discussed on the basis of the microstructure of the samples and the change of crystal structure with the iron content.