Comment on magnetism and superconductivity in rutheno cuprates: RuSr2GdCu2O8 and RuSr2Gd1.5Ce0.5Cu2O10

Both RuSr₂GdCu₂O_8-δ (Ru-1212) and RuSr₂Gd_1.5Ce_0.5Cu₂O_10-δ (Ru-1222) exhibits magnetism and superconductivity, as seen by magnetization vs. temperature behavior measured in 5 Oe field. Zero-field-cooled (ZFC) and field-cooled (FC) magnetization data show branching at around 140 K and 100 K with a cusp at 135 K and 80 K and a diamagnetic transition around 20 K and 30 K in the ZFC part, for Ru-1212 and Ru-1222, respectively. The isothermal magnetization possesses a non-linear contribution due to a ferromagnetic component at low temperatures below 50 K for both samples. The resistance vs. temperature behavior of the samples in applied fields of 0, 3 and 7 T confirmed superconductivity, with a different type of broadening of the superconductivity transition under magnetic fields for Ru-1212 from that known for conventional high-T _c superconductors. The magnetoresistance (MR) is negative above the Ru magnetic ordering temperature at 135 K. Below the Ru magnetic ordering temperature, MR displays a positive peak at low fields and becomes negative at higher fields for Ru-1212. For Ru-1222, MR remains negative both above and below the ordering temperature. A maximum of 2% is observed for the negative MR value at the Ru magnetic ordering temperature. An electron diffraction pattern obtained for the Ru-1212 sample shows two types of superstructure: one has a weak spot at the centre of the a–b rectangle, and the other only along the b direction. Interestingly, Ru-1222 shows only clean a–b and a–c planes, without any superstructures.     

Magnetization scaling in the ruthenate-cuprate RuSr2Eu1.4Ce 0.6Cu2O10-δ (Ru-1222)

The magnetic properties of the superconducting ruthenate-cuprate RuSr₂Eu _1.4Ce_0.6Cu₂O_10-δ (Ru-1222) have been studied by a scaling analysis of the dc magnetization and ac susceptibility measurements. Non-linear M(H) curves reveal the presence of nano-size clusters with a net magnetic moment of ∼10² μ_B at 180 K, near the deviation from a Curie-Weiss behavior. On cooling, no scaling was observed down to 90 K, discarding the possibility of magnetic phase separation of collinear ferromagnetic particles. We explain this result in terms of a variable number of contributing particles, with a temperature dependent net magnetic moment. For 70 K ≤T≤ 90 K the scaling plots evidence the emergence of a system of non-interacting particles, which couple on further cooling. The observed cluster-glass features are preserved down to the lowest measured temperature (10 K); no signature of long-range order was detected. The frequency shift of the peak in the real part of the ac susceptibility does not follow the Vogel-Fulcher type dependence, as previously reported. The puzzling temperature dependence of the coercive field, H_C(T), is correlated with the changes in the scaling factors.     

超导铁磁体RuSr2Gd1.4Ce0.6Cu2O10-δ的输运性质和磁共振研究

本文对RuSr2Gd1.4Ce0.6Cu2O10-δ样品的微结构、输运性质及磁共振性质进行了实验研究。研究表明RuSr2Gd1.4Ce0.6Cu2O10-δ样品的晶格参数自280K至30K缓慢下降。长时间流动氧气退火可提高零电阻转变温度（Tc(0)），但不影响超导起始转变温度（Tc(onset)）。正常态电阻在110K和180K附近有一微小波动，可能是由于RuO2面载流子与Ru磁矩之间交换作用引起的自旋散射降低所至。电子顺磁共振测量显示有两个只收峰，其中一个对应Gd离子的电子顺磁共振（EPR），另外一个在铁磁转变温度以下出现，它对应Ru离子的铁磁共振（FMR）。本文对磁共振谱的线宽、强度和g因子随温度的变化进行了探讨。     

Magnetic field dependence of critical current density in YBa2Cu3O7

We have measured the transport critical current densityJ _cof sintered YBa₂Cu₃O₇, in various applied fields up to 185 Oe at 77 K. We find a sharp decay ofJ _cwith magnetic field. We show that this sharp decay is consistent with the low field hysteresis results of Groveret al. We argue that the observed field dependence is not caused by intragranular weak links.     

Magnetism and superconductivity in R1.5Ce0.5MSr2Cu2O10‐δ (M=Ru and Fe) compounds

It is shown here, that the superconducting (SC) R_1.5Ce_0.5RuSr₂Cu₂O_10-δ (RCeRuSCO, R= Eu and Gd) materials (T_c ～ 32 and 42 K) are also antiferromagnetically (AFM) ordered at T _N(Ru) ～ 122 and 180 K, respectively, thus, T_N ? T_c, a trend which is contrary to that obtained in “magnetic‐SC” intermetallic systems. Mössbauer spectroscopy (MS) on 0.5% ⁵⁷Fe doped samples shows that all Fe ions reside in the Ru site which is magnetically ordered, whereas SC is confined to the CuO₂ planes. On the other hand, for Y_1.5Ce_0.5FeSr₂Cu₂O₉ (YCeFSCO) no SC is found and the Cu–O planes order AFM at T _N(Cu)=31 K. MS studies reveal two in equivalent Fe sites, and that Fe resides predominantly (60%) in the Cu(1) site. In both sites, the Fe does not order magnetically, and the low T _N(Cu) obtained is due to frustration of the Cu moments by the presence of Fe. T _N is sensitive to oxygen concentration and shifts toward 260 K when oxygen is depleted.     

Weak link problem and intragrain current density in polycrystalline Bi1Ca1Sr1Cu2Ox and Tl2Ca2Ba2Cu3O10

Polycrystalline sintered specimen were investigated by means of ac susceptibility and inductive critical current measurements. Main objects are whether the new highT _c oxides have a weakly coupled intergranular structure and to get information about the intragrain critical current density. The Tl₂Ca₂Ba₂Cu₃O₁₀ specimen shows a connective nature similar to YBa₂Cu₃O₇ with low intergrain currents (100 A/cm² at 100K,B=0 T) and high intragrain ones (4.5·10⁵ A/cm² at 100K,B=0 T). The investigation of the Bi₁Ca₁Sr₁Cu₂O_x specimen becomes complex due to the presence of two superconducting phases and the low intragrain critical current density of the lowerT _c phase.     

Microscopic model of the coexistence of superconductivity and ferromagnetism in the hybrid ruthenate-cuprate oxide RuSr2GdCu2O8

A microscopic t-J-I model with competing antiferromagnetic (J) and ferromagnetic (I) exchange interactions is proposed for strongly correlated electrons in RuSr₂GdCu₂O₈. It is assumed that for CuO₂ layers and for RuO₂ layers. A superconducting solution of $d_{x^2 - y^2 }$ symmetry was obtained for the CuO₂ layers while competition between ferromagnetism and p-type triplet superconducting pairing is obtained for RuO₂ layers. It is shown that the RuO₂ layers have a lower carrier concentration in the Hubbard subband formed by coupled ((d _xy -p)-π) orbitals compared with a bulk Sr₂RuO₄ crystal, which leads to stabilization of the ferromagnetic state in the RuO₂ layer.     

Lower critical field and superfluid density of highly underdoped YBa2Cu3O6+x single crystals

The lower critical field H(c1) for highly underdoped YBa2Cu3O(6+x) with T(c) between 8.9 and 22 K has been determined by measurements of magnetization M(H) curves with applied field parallel to the c axis. H(c1) is linear in temperatures below about 0.6T(c), and H(c1)(0) is proportional to T(1.64+/-0.04)(c), clearly violating the proportionality between rho(s)(0) and T(c). Moreover, the slope -dH(c1)/dT decreases steeply toward zero as T(c) approaches zero, indicating that the effective charge of the quasiparticles vanishes as the doping is decreased toward the insulating phase.     

X-ray irradiation enhanced critical current density and strong pinning sites created in Gd1Ba2Cu3O7−x thin films

An enhancement of the critical current density (to more than 10⁶ A/cm² below 65 K) of Gd₁Ba₂Cu₃O_7–x superconducting thin films was achieved by X-ray irradiation with oxygen annealing. This process of X-ray irradiation with oxygen annealing introduced stable pinning centers into the crystal and also increased the activation energy from 0.1 eV to 0.25 eV.     

Critical current and upper critical field of sintered and powdered superconducting YBa2Cu3O7

A sintered bulk sample of YBa₂Cu₃O₇ has been investigated resistively and inductively with respect to its resistivity (B, T), upper critical fieldB _c2 (T), and current carrying capabilityj _c (B, T). The inductive measurements were repeated after powdering the sample to a maximum particle size of 60 m. Results show that the intrinsic-intragrain properties as approximately revealed by the powder measurements, are severely masked in bulk measurements by the intergrain weak-link structure. As an example, the current densityj _c (0 T, 4.2 K) in the powdered YBa₂Cu₃O₇ has a value of 4·10⁶ A/cm², which is about three orders of magnitude higher than in the bulk material. The high values of the intragrainj _c are probably due to pinning at twin boundaries. — Besidesj _c the normal state resistivity just aboveT _c , _n , is influenced by the interconnecting structure, both properties being directly related: a smaller _n leads to a higherj _c in bulk samples. — The upper critical field shows a pronounced anisotropyB _c2 |/B _c2 of about 20 to 30, whereB _c2 |=330 T is the extrapolated upper critical field at zero temperature for the field orientation parallel to the (Cu–O)-planes. — For the time being, theB _c2 anisotropy and the weak-link structure of sintered bulk samples are the main obstacles for high field applications.     

An experimental approach to the improvement of critical current density in high-TC YBa2Cu3O x ceramics

A “grain coating” technique was developed to improve the intergranular links in YBa₂Cu₃O _x ceramics. Thin layers of additives such as Ag, SnO₂, Bi₂O₃ were introduced into the grain boundaries by vacuum evaporation. Notable J_c increments due to grain coating were observed. TEM investigation revealed that dislocations accompanied by stacking faults existed in these samples.     

Microstructure and thermal properties of quasi-equal rare earth substitution Y0.5Gd0.5Ba2Cu3O6.94 superconductor

Sintering effects in YBa₂Cu₃O_z samples with quasi-equal rare earth substitution have been investigated. It has been shown that the Y-123 type compound can be formed when gadolinium is partially substituted (in this case 50% atomic substitution) for yttrium. The superconducting compound was obtained by the optimized ceramic method, including solid-state reaction, melting and sintering, controlled by X-ray diffraction. The microstructure was investigated by SEM and energy dispersive X-ray analyses. An AC susceptibility measurement has shown that T _c =93 K. Temperature-dependece of the thermal conductivity (4.5–300 K) of the polycrystalline Y_0.5Gd_0.5Ba₂Cu₃O_6.94 sample was also measured.      

Effect of low-temperature treatment and subsequent high-temperature annealing on the critical current density of YBa2Cu3O y

The field dependences of the critical current density of the HTSC compound YBa₂Cu₃O _y recovered at T = 920?C950°C after the low-temperature treatment have been investigated. At T = 200°C, structural defects are formed in a wet environment, which are capable of initiating pinning of magnetic vortices. A short-term (1?C3 h) recovery annealing performed at T = 930?C950°C leaves in the samples a fairly large amount of structural defects formed during the low-temperature treatment, which results in a substantial increase in the critical current density in magnetic fields of ??2 T as compared to the ceramics not subjected to double annealing. A longer high-temperature treatment removes the structural defects formed and brings the electrophysical properties of YBa₂Cu₃O _y to the level characteristic of the ceramics produced by standard technology.     

The transition width and critical current density measurements on slow-cooled YBa2Cu3O7−x superconductor

It is shown that by adopting a very gradual programmed cooling procedure in oxygen environment one can obtain a sharp transition (ΔT _c⋍1 K) in YBa₂Cu₃O_7−x while retaining the high-T _c value (⋍105 K) of samples prepared by a semi-wet route. This is attributed to a maximum occupancy of oxygen at 0 1/2 0 sites and a near-perfect ordering of vacancies at 1/2 0 0 sites in the orthorhombic unit cell which maximizes the availability of conduction paths in the form of continuous CuO₄ chains. Critical current densities (J _c) of 204 A cm⁻² are obtained for bulk samples at 77 K. It is suggested that the intergrain coupling is weak and thus limits the transportJ _c-values.