Oxidation state and superconducting properties of Y1−xPrxSr2Cu2.85Re0.15O7+δ (original) (raw)
Lecture Notes in Physics, 2002
We discuss the properties of new superconducting compositions of Ru 1-x Sr 2 RECu 2+x O 8-d (RE=Gd, Eu) ruthenocuprates that were synthesized at 600 atm. of oxygen at 1080°C. By changing ratio between the Ru and Cu, the temperature of superconducting transition (T C ) raises up to T C max =72 K for x=0.3, 0.4. The hole doping achieved along the series increases with Cu→Ru substitution. For x≠0, T C can be subsequently tuned between T C max and 0 K by changing oxygen content in the compounds. The magnetic characteristics of the RE=Gd and Eu based compounds are interpreted as indicative of constrained dimensionality of the superconducting phase. Muon spin rotation experiments reveal the presence of the magnetic transitions at low temperatures (T m =14-2 K for x=0.1-0.4) that can originate in the response of Ru/Cu sublattices. RuSr 2 Gd 1-y Ce 1-y Cu 2 O 8 (0≤y≤0.1) compounds show the simultaneous increase of T N and decrease of T C with y. The effect should be explained by the electron doping that occurs with Ce→Gd substitution. Properties of these two series allow us to propose phase diagram for 1212-type ruthenocuprates that links their properties to the hole doping achieved in the systems. Nonsuperconducting single-phase RuSr 2 GdCu 2 O 8 and RuSr 2 EuCu 2 O 8 are reported and discussed in the context of the properties of substituted compounds. Recent reports of the apparent coexistence of superconductivity (SC) and ferromagnetism (FM) in ruthenocuprates [1,2] have triggered intense interest in the properties of these materials. The compounds that exhibit this unusual behavior are RuSr 2 RECu 2 O 8 (Ru-1212) [3] and RuSr 2 (RE 2-x Ce x )Cu 2 O 10-y (Ru-1222) (RE=Gd, Eu) [2] and they belong to the family of high temperature superconductors (HTSC). Structurally similar to the well-known GdBa 2 Cu 3 O 7 (Gd123) superconductor, the ruthenocuprate RuSr 2 GdCu 2 O 8 is a layered perovskite containing both CuO 2 and RuO 2 planes in its crystal structure. The correspondence between the two structures can be described by replacing the so called chain -Cu atoms in the Gd123 by Ru ions coordinated with full octhaedra of oxygens thus forming RuO 2 planes. The positions of Sr atoms in Ru-1212 correspond to Ba positions in Gd123, and the structural block containing CuO 2 double planes remains similar for both compounds. In RuSr 2 GdCu 2 O 8 , the magnetically ordered state manifests itself at temperatures T N =130-136 K, much higher than the superconducting transition reported at 45 K for the highest T C samples. The magnetic order persists in the superconducting state [2]. What makes these compounds unique in the family of HTSC is that the magnetic ordering originates in the sublattice of the d-electron Ru ions. Recent muon spin rotation and magnetization results provided evidence for the coexistence of the magnetic ordering of Ru moments with superconductivity at low temperatures [2]. Although the ferromagnetic ordering was initially proposed for the Ru sublattice below T N [2], recent neutron diffraction experiments show that the dominant magnetic interactions are of the Gtype antiferromagnetic (AFM) structure [4, 5]. Based on these results, the observed ferromagnetism should originate from the canting of the AFM lattice that gives a net moment perpendicular to the c-axis [5]. This scenario resembles the description of the properties of Gd 2 CuO 4 , a non-superconducting weak ferromagnet, where the distortions present in the CuO 2 plane permit the presence of antisymmetric superexchange interactions in the system of Cu magnetic moments [6, 7]. In the ruthenocuprates, however, the weak ferromagnetism originates in the RuO 2 planes and should result in the effective magnetic field being parallel to the CuO 2 planes [2, 5, 8]. 1. Ru 1-x Sr 2 GdCu 2+x O 8-d (0 ≤ ≤ ≤ ≤ x ≤ ≤ ≤ ≤ 0.75) In order to address how the properties of RuSr 2 RECu 2 O 8 can be affected by the dilution of the magnetic sublattice of Ru, we attempted to partially substitute Ru with Cu ions. With this substitution the nominal formula of the resulting compound should change toward the hypothetic GdSr 2 Cu 3 O 7 , a Sr containing analogue of the GdBa 2 Cu 3 O 7 T C ≈92 K superconductor. We have found that for Ru 1-x Sr 2 GdCu 2+x O 8-d series, the layered Ru-1212 type structure becomes stable only during synthesis at high pressure oxygen conditions. Polycrystalline samples of Ru 1-x Sr 2 GdCu 2+x O 8-d (x=0, 0.1, 0.2, 0.3, 0.4, 0.75) were prepared by solid-state reaction of stoichiometric RuO 2 , SrCO 3 , Gd 2 O 3 and CuO. After