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

Abstract

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-T c 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.

The ruthenocuprates: natural superconductor–ferromagnet multilayers

“…Regarding the resistively measured transition a characteristic feature of Josephson networks is that, while the transition strongly broadens with increasing magnetic field the onset of the superconducting transition (given by the temperature where the grains become superconducting) shifts only slightly. This effect has been seen in transport experiments both on Ru-1212 and Ru-1222 [79,80,81,82]. A similar broadening occurs also in susceptibility measurements.…”

High magnetic field study of RuSr2GdCu2O8

“…T intra = T S corresponds to the intrinsic critical temperature where the grains become superconducting whereas at T inter superconducting (Josephson) coupling between the grains is established. It should be noted that the magnetic and superconducting properties of Ru1212Gd depend strongly on the preparation conditions and long annealing in oxygen atmosphere is necessary in order to achieve the properties described previously [8]. We conclude that the sample investigated is representative of high quality rutheno-cuprates similar to those investigated previously [1,2].…”

Pairing state in the ruthenocuprate superconductorRuSr2GdCu2O8: A point-contact Andreev reflection sp

“…In the resistivity measurements versus temperature, the onset of the superconducting transition was observed at T on c ≃ 43K with T c (ρ = 0) ≃ 24K and ∆T c = 12K (∆T c is defined as the difference between the temperatures measured at 90% and 10% of the normal state resistance). We notice that a broadening of the superconducting transition is often observed in polycrystalline samples and it is usually related to the formation of intergrain weak Josephson junctions 5,12,13 . We address this point in the next section.…”