V.P.S. Awana

Oxygen non-stoichiometry in Ru-1212 and Ru-1222 magnetosuperconductors

Abstract We report here the results of thermogravimetric (TG) analysis on the oxygen non-stoichiometry of RuSr 2 GdCu 2 O 8− δ (Ru-1212) and RuSr 2 (Gd 0.75 Ce 0.25 ) 2 Cu 2 O 10− δ (Ru-1222) samples. With TG measurements carried out in O 2 and Ar atmospheres it is found that the oxygen content in Ru-1212 remains less affected upon various annealings, while for Ru-1222 the wider-range oxygen-content tuning is possible. When heated in H 2 /Ar atmosphere the both phases release oxygen upon breaking down to mixtures of metals (Ru and Cu) and simple oxides (CeO 2 , Gd 2 O 3 , and SrO) in two distinct steps around 300 and 450 °C. This reductive decomposition reaction carried out in a thermobalance is utilized in precise oxygen content determination for these phases. It is found that the 100-atm O 2 -annealed Ru-1212 sample is nearly stoichiometric, while the similarly treated Ru-1222 sample is clearly oxygen deficient. X-ray absorption near-edge structure (XANES) spectroscopy is applied to estimate the valence of Ru in the samples. In spite of the fact that the Ru-1212 phase was shown to possess less oxygen-deficient RuO 2− δ layer, the valence of Ru as probed with XANES is found to be lower in Ru-1212 than in Ru-1222.

Comparison of magneto-superconductive properties of RuSr2GdCu2O8−δ and RuSr2Gd1.5Ce0.5Cu2O10−δ

Abstract Samples of compositions, RuSr2GdCu2O8−δ (Ru-1212) and RuSr2Gd1.5Ce−0.5Cu2O10−δ (Ru-1222), were synthesised through solid-state reaction routes. The former crystallized in the space group P4/mmm and the latter in I4/mmm. The magnetization vs. temperature curve measured in 5-Oe field showed a clear branching into zero-field-cooled (ZFC) and field-cooled (FC) curves for Ru-1212 and Ru-1222, respectively, around 140 and 100 K with a cusp at 135 and 80 K and a diamagnetic transition around 20 and 30 K only in the ZFC branch. The two down-turn cusps at 135 and 80 K may be attributed to the antiferromagnetic nature of Ru spins. The isothermal magnetization possessed a non-linear contribution due to a ferromagnetic component at low temperatures below 50 K for both samples. The resistance vs. temperature behaviour of the Ru-1212 samples in applied fields of 0, 3 and 7 T confirmed superconductivity, exhibiting a different type of broadening of the superconductivity transition under magnetic fields from that known for conventional high-Tc superconductors. The magnetoresistance (MR) was negative above the Ru magnetic ordering temperature at 135 K. Below the Ru magnetic ordering temperature MR displayed a positive peak at low fields and became negative at higher fields for Ru-1212. For Ru-1222 MR remained negative both above and below the ordering temperature. A maximum of 2% was observed for the negative MR value at the Ru magnetic ordering temperature. An electron diffraction (ED) pattern from the Ru-1212 sample showed two types of superstructure: one has a weak spot at the centre of the a ∗ –b ∗ rectangle, and the other at the center of the b ∗ edge only. The latter indicates that either Ru/Cu or vacancy ordering of 2b periodicity takes place along the b-direction. Interestingly, Ru-1222 showed only ED patterns with clean a ∗ –b ∗ and a ∗ –c ∗ planes without any superspots.

Oxygen Content and Valence of Ru in RuSr2(Gd0.75Ce0.25)2Cu2O10−δ (Ru-1222) Magnetosuperconductor

The valence of Ru was analyzed for two RuSr2(Gd0.75Ce0.25)2Cu2O10−δ samples with different oxygen contents by Ru LIII-edge x-ray absorption near-edge structure (XANES) spectroscopy. For the sample as-synthesized in 1 atm O2 the DC magnetization data measured in an applied field of 5 Oe showed a clear branching of the zero-field-cooled (ZFC) and field-cooled (FC) curves around 140 K, an up-turn for both around 100 K and a cusp at 85 K and a diamagnetic transition around 20 K in the ZFC part. A further confirmation for the superconductivity at 28 K was obtained from a resistance vs. temperature measurement. Annealing the as-synthesized sample in 100-atm O2 atmosphere at 420°C increased the diamagnetic transition temperature from 20 K to 40 K. According to a thermogravimetric analysis, the oxygen content increased accordingly by ca. 0.1 oxygen atoms per formula unit. Quantitative analysis of the XANES spectra using Sr2RuO4 (RuIV) and Sr2GdRuO6 (RuV) as reference materials revealed a valence value of +4.74 and +4.81 for Ru in the as-synthesized and the 100-atm O2-annealed sample, respectively. The obtained result suggests that the valence of Ru in Ru-1222 is affected by the change in oxygen content.

Triple perovskite CoSr2YCu2O7-δ and its peculiar structural features: a neutron diffraction study

Abstract Samples of the composition CoSr 2 YCu 2 O 7− δ (Co-1212) were synthesized in a single-phase form and studied using neutron diffraction (ND), chemical analyses and magnetization measurements. The structure of Co-1212 is best described in the space group Ima2 that yields good fit to the observed ND data. No cross-migration between Cu and Co is seen for Co-1212. The overall oxygen content, as determined from the refined oxygen occupancies, comes out to be around 7.0. This is close to the values determined from wet-chemical and thermogravimetric analyses. Unlike in Cu-1212, the complete reduction of Co-1212 in 5% H 2 /95% Ar atmosphere occurs in a single step, revealing that the amount of reversibly removable oxygen is considerably less than that in Cu-1212. Zero-field-cooled and field-cooled magnetizations, measured in an applied field of 100 Oe, show branching at around 30 K, presumably due to the magnetic ordering of Co moments. The present Co-1212 sample is found not to be superconducting down to 5 K.

Synthesis and magnetism of Pr-based rutheno-cuprate compound RuSr2PrCu2O8−δ

Abstract We have successfully synthesized RuSr 2 PrCu 2 O 8− δ in an essentially single-phase form. Structural analysis from room-temperature powder neutron diffraction pattern shows that this compound crystallizes in tetragonal Ru-1212 structure (space group P 4/ mmm ). Magnetization measurements show clear branching of zero-field-cooled and field-cooled magnetization around 150 K indicating magnetic ordering below this temperature (presumably associated with Ru moments), similar to that reported for RuSr 2 GdCu 2 O 8− δ . Magnetization versus applied field hysteresis loops indicate presence of a small ferromagnetic component at 5 K. Though magnetic behaviour of RuSr 2 PrCu 2 O 8− δ is similar to that of reported magneto-superconductor RuSr 2 GdCu 2 O 8− δ , the Pr-based compound is non-superconductive down to 5 K. Results of presently studied RuSr 2 PrCu 2 O 8− δ compound are compared with reported data on both superconductive and non-superconductive RuSr 2 GdCu 2 O 8− δ samples.

Oxygen stoichiometry in Co-1212, Co-1222 and Co-1232 of the homologous series Co-12s2 of “category-B” layered copper oxides

Abstract The results of a systematic study on oxygen stoichiometry are reported for the first three members of the novel CoSr 2 (Y,Ce) s Cu 2 O 5+2 s ± δ or Co-12 s 2 homologous series, i.e. Co-1212, Co-1222 and Co-1232 phases, with a SrO–CoO 1± δ –SrO–CuO 2 –(Y,Ce)–[O 2 –(Y,Ce)] s −1 –CuO 2 layered structure. The oxygen content was precisely determined by two independent chemical techniques: coulometric Cu + /Cu 2+ titration and iodometric titration. Furthermore, oxygen stability/tunability was investigated by means of oxygenative and reductive annealings carried out in a thermobalance. It was revealed that all the three phases are rather stoichiometric in oxygen content ( δ ≈0) and stable against both oxygenative and reductive annealings. The present results provide us with an indication that the nominally oxygen-stoichiometric B–[O 2 –B] s −1 “fluorite block”, the characteristic structural element for all layered copper oxides of “category-B” [H. Yamauchi, M. Karppinen, Superlatt. Microstructr. 21A (1997) 128], is non-tunable in terms of the oxygen content not only in the Co-12 s 2 ( s >1) phases but also in other category-B phases.

Intermixing of Fe at Cu(1)-chain and Cu(2)-plane sites in FeSr2YCu2O7.30 system : A neutron diffraction and Mössbauer spectroscopic study

Abstract The presently prepared FeSr 2 YCu 2 O z (Fe-1212) sample crystallizes in a tetragonal structure (space group P4/mmm). Rietveld structural refinement of the room temperature neutron diffraction data reveal that nearly half of the Fe remains at the Cu(1)-chain site while the other half goes to the Cu(2)-plane site in the nominal Cu(1)Sr 2 YCu(2) 2 O 7 or Cu-1212 structure. In this compound, Cu is distributed over the chain and plane sites appropriately. Oxygen content of the present Fe-1212 sample, as determined from the oxygen occupancies, is 7.30(2) per formula unit. Existence of Fe at the two different lattice sites is also confirmed from the room temperature 57 Fe Mossbauer spectrum of the compound.

Glass component induced hysteresis/memory effect in magnetoresistance of Pr0.9Sr0.1CoO2.99

Abstract Pr1−xSrxCoO3−δ exhibits magnetoresistivity (MR) of up to 40% at 5xa0K with a strong hysteresis/memory effect. Magnetization measurements on Pr0.9Sr0.1CoO2.99 in an applied field of 100xa0Oe show that, as temperature decreases, the zero field-cooled (ZFC) and field-cooled (FC) magnetization curves branch clearly at 50xa0K, and a cusp appears in the ZFC branch at Tcusp∼20xa0K. Magnetization measurements in various fields between 100 and 10,000xa0Oe show that both the ZFC–FC branching temperature and, Tcusp, decrease with increasing field. The magnetization-field isotherms at 5 and 10xa0K show hysteresis loops typical of ferromagnets. No appreciable MR is seen in this compound at 50xa0K, i.e. at a temperature close to ZFC–FC branching temperature. At 20xa0K, negative MR of above 16% is observed without any hysteresis effect. We believe that the appearance of a ferromagnetic component at 5xa0K and 10xa0K (i.e. at temperatures below Tcusp) within the spin glass state of Co spins is responsible for both large MR and the prominent hysteresis/memory effect in MR.

Magnetoresistance peak in the vicinity of the charge disproportionation/ordering transition in the R1/3Sr2/3FeO3-δ (R = La, Pr) perovskite

Abstract In this letter, we present evidence that charge disproportionation (CD) and/or charge ordering (CO) states have influence upon magnetotransport in both compounds, R 1/3 Sr 2/3 FeO 3− δ (R=La and Pr). Field-dependent resistivity versus temperature measurements reveal the existence of a magnetoresistance (MR) peak in the temperature regime where the CD/CO transition takes place. Further increased MR of up to 6.5% is observed at temperatures as low as 25xa0K. These compounds are basically semiconductors which below the CD/CO temperature follow the variable range hopping process down to around 50xa0K and at even lower temperatures again turn to moderately doped semiconductors. Magnetization measurements show that the CD/CO is accompanied by an antiferromagnetic spin ordering of Fe ions with a further weak ferromagnetic component at low temperatures. While an ordering of Fe(III) and Fe(V) species seems to cause the MR peak near the CD/CO temperature, the appearance of some ferromagnetic component within an antiferrognatically ordered state may be largely responsible for the MR at low temperatures.