E. Brecht

Thermal treatment of YBa2Cu3−xAlxO6+δ single crystals in different atmospheres and neutron-diffraction study of excess oxygen pinned by the Al substituents

YBa2Cu3−xAlxO6+δ single crystals with different Al contents (x = 0.06, 0.14 and 0.19) were heat treated in different atmospheres. The oxidized as-grown crystals show a macroscopically tetragonal tweed-structure. The oxygen content δ in the Cu(1) layer of these oxidized samples is significantly lower than 1, which results from the Al coordination disturbing the ordering in the chain-oxygen sublattice. Neutron-diffraction data show that upon reduction at 1044 K the oxygen content δ within the Cu(1) layer remains at 0.25 due to pinning of excess oxygen by the aluminum at x = 0.19. Crystals annealed in reducing atmospheres at temperatures above 973 K develop orthorhombic long-range order after reoxidation at 673 K, probably due to short-range clustering of the AlOn defects. The obtained orthorhombicity of the reoxidized material decreases with decreasing temperature of the reduction treatment. No change in the structural state is obtained when the reducing treatment is performed below 950K; the reoxidized samples remain macroscopically tetragonal. Annealing the samples at 1073 K in oxygen restores the tweed structure. An evolution of the tweed structure with annealing time towards orthorhombic long-range order was not observed.

Intergrain flux-pinning in relation to structural phase transformation and tweed formation in YBa2(Cu1-xFex)3O7-y and NdBa2(Cu1-xFex)3O7-y

Title full: Intergrain flux-pinning in relation to structural phase transformation and tweed formation in YBa2(Cu1-xFex)3O7- y and NdBa2(Cu1-xFex)3O7-y. Fe-doped superconductors REBa2(Cu1-xFex)3O7-y (RE=Y, Nd; 0≤x≤0.1) were investigated by conventional and synchrotron X-ray diffraction, thermogravimetric analysis and AC susceptibility measurements. The transition from orthorhombic to macroscopically tetragonal symmetry is analogous for both systems and occurs at xc≈0.030 for YBa2(Cu1-xFex) 3O7-y and at xc≈0.021 for NdBa2(Cu1-xFex)3O7-y. The spontaneous strain elunate=2(b-a)/(a+b) shows tricritical-like scaling as elunate∼|x-xc|1 4, and tweed formation is indicated by diffraction-line-broadening effects. For YBa2(Cu1-xFex)3O7-y the onset temperature Tc of superconductivity is almost constant below x=0.03 (i.e., in the orthorhombic regime) and falls linearly for x > 0.03 in the tweed regime, extrapolating to zero near x=0.2. Samples which were treated twice in flowing O2 at 400°C for 10 h show a significantly steeper linear decrease of Tc with x in the tweed regime. Contrary to the Y-compound, NdBa2(Cu1-x Fex)3O7-y shows no plateau of Tc in the orthorhombic regime, and Tc drops rapidly with dopant concentration x. The low-field AC susceptibility of each sintered sample is strongly dependent upon applied AC RMS and superimposed DC bias fields, owing to the weak-link network of the superconducting grains. Critical transport current densities, J0, and pinning force densities αJ for the intergranular vortices were estimated from AC susceptibility measurements. Both quantities are strongly suppressed by Fe-doping and the substitution of Nd ions for Y ions. A distinct change of the slope dαJ/dx at the critical composition xc for both the YBCO and the NdBCO series indicates a softening of the free energy gradient between superconducting grains and non-superconducting intergrain material due to the formation of twin-domain boundaries in the grains.

Orientational Changes in the ab plane of YBA2Cu3O7−δ Films on Different Substrates

YBa2Cu3O7 (YBCO) thin films with c-axis orientation on (001) SrTiO3, (001) MgO and (110) NdGaO3 substrates were examined by four-circle diffractometry with Co Kα and synchrotron radiation (λ = 0.1540 nm) using the q-scan technique. Small local film rotations about [001] have been introduced by two different mechanisms: (1) the orthorhombic/tetragonal phase transition driven by iron substitution or changes of the oxygen content; (2) heavy-ion irradiation (1.4 GeV 238U UNILAC, GSI Darmstadt). In the original orthorhombic state, the (110) axes of film and substrate are parallel so that the (100)film axes form an angle of 0.45° to those of the substrate. With decreasing orthorhombicity, this angle decreases to zero for tetragonal symmetry. Irradiation of the orthorhombic film by heavy ions produces detwinning and leads to a macroscopic preferential parallel alignment of the 〈100〉 axes of film and substrate. This reorientation is completed for a dose between 1011 and 1012 particles cm−2. High-resolution transmission electron microscopy investigations reveal a rotation of crystallites in the ab plane of irradiated samples of up to 3°. For irradiation with 200 (456) MeV 40Ca, no detwinning is observed.

No antiferromagnetic reordering at low temperature in pure YBa2Cu3O6+x

Abstract Magnetic ordering has been investigated by neutron scattering on an YBa 2 Cu 3 O 6+ x single crystal with x =0.1 and x =0.18, and an Al doped YBa 2 (Cu 2.86 Al 0.14 )O 6.25 crystal. For the undoped crystal an antiferromagnetic ordering transition (AFI) was observed at T N =410 K and 368 K (respectively for x=0.1 and x=0.18), but no second antiferromagnetic transition was found down to 2 K. In contrast, an antiferromagnetic reordering (AFII) was observed for the aluminum doped sample below T 2 =8 K. This shows the high sensitivity of this second phase to impurities and indicates that this reordering is not necessarily an intrinsic property of YBa 2 Cu 3 O 6+ x .

Reduction and reoxidation experiments on YBa2Cu3-xAlXO6+δ single crystals

We performed thermal treatments in different atmospheres on tetragonal YBa2Cu3-xAlxO6+δ single crystals. The transformation from tetragonal tweed to orthorhombic twin structure of YBa2Cu3-xAlxO6+δ single crystals by reduction and subsequent reoxidation is reversible. The orthorhombicity of reoxidized crystals depends on the temperature during reduction.

Antiferromagnetic ordering in reduced Al doped YBa2Cu3−xAlxO6+δ single crystals

Abstract In YBa 2 Cu 3−x Al x O 6+δ single crystals antiferromagnetic AFII ordering has been observed below 18 K by neutron diffraction. The transition temperature T 2 to the AFI ordering increases with x and δ.