Effect of disorder on the passage from bulk superconductivity to spin glass behaviour in RuSr2GdCu2O8

In this paper we analyse the structural and physical properties of rutheno-cuprate samples of the phase Ru–Gd(1212) subjected to different thermal treatments. We have observed the variations induced by these treatments by structural measurements (x-ray powder diffraction and high-resolution transmission electron microscopy) and physical measurements (dc magnetic measurements and magnetoresistive measurements). Such variations involve both the intrinsic properties of the Ru–Gd 1212 phase (for example the temperatures of the magnetic and superconducting ordering), and the extrinsic properties, i.e. those related to the microstructure of the samples, such as the domain dimension, and to their granular nature. In particular, while a prolonged annealing in flowing oxygen induces an extended microstructural homogeneity, annealing in vacuum, besides producing loss of oxygen, leads to the formation of intragranular disordered microdomains and extended planar defects. We verify how the physical properties are dependent on the level of microstructural order that determines the passage from metallic to semiconducting behaviour and, possibly, from bulk to spin glass superconducting behaviour. By saying ‘bulk’ superconducting behaviour we mean the behaviour commonly observed in high-Tc superconductors where, as is well known, granularity is present and intergranular and intragranular properties of the sample may be observed: in appropriate experimental conditions it is possible to separate the two contributions to study the intrinsic properties of the material or to observe the character of the Josephson coupling between grains (or clusters). In contrast, the ‘spin glass’ is modelled as superconducting grains weakly linked into closed loops, where different supercurrent carrying states of nearly equal energy may install, producing, among other effects, metastable configurations. We suggest that the microstructure is an important cause of the different behaviour of samples otherwise very similar: strong microstructural disorder decreases the domain size inside the grains until they are comparable with the coherence length ξ (and much smaller than the penetration depth λ). In such conditions the superconducting volume of the sample becomes smaller and smaller; granularity or weak-link behaviour becomes ubiquitously present and practically determines the physical properties in the whole phase diagram.