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Bulk ceramic samples with chemical composition (Y 1-x Nd x )Ba 2 Cu 3 O 7-δ , where x = 0, 0.1, 0.2, 0.5, and 1 were sintered by a proposed acetate method. The samples were then submitted to structural characterization by X-ray diffraction with crystal structure refining using the Rietveld method. AC electrical resistivity measurements from ambient temperatures down to 4.2 K were performed, as well as magnetic measurements of magnetic moment as function of temperature and magnetic hysteresis loop. Results have shown superconductivity with critical temperatures of about 91 and 60 K for pure YBa 2 Cu 3 O 7-δ (x = 0) and pure NdBa 2 Cu 3 O 7-δ (x = 1) samples, respectively. Scanning electron microscopy images were also analyzed, revealing a regular parallel-piped grain shape. The proposed method showed as an advantage to reduce the grinding time without any considerable degradation on structural or superconducting properties of pure YBa 2 Cu 3 O 7-δ (x = 0) or pure NdBa 2 Cu 3 O 7-δ ( x = 1), as related to samples prepared by a conventional solid state route.

Two superconducting intermetallic compounds in the Ni-Bi system, namely NiBi and NiBi3, can be formed as result of interdiffusion, if Ni/Bi are produced as bilayers at room temperature by pulsed laser deposition (PLD). These two equilibrium phases are responsible for the observed superconductivity in this kind of bilayer material. Due to different behavior in external magnetic fields, there appear two-step superconducting transitions in the electrical transport measurements. In this work we studied the superconducting and magnetic properties of the Ni-Bi bilayers with the formation of the two intermetallic compounds, NiBi and NiBi3, which have similar Tc ~ 4.0 K but very different upper critical magnetic fields at zero temperature [Bc2(0)]. The magnetic hysteresis loops of the pure NiBi3 phase show the behavior of a weak type II superconductor without strong pinning effect. Upon formation of the NiBi phase caused by highly energetic atoms during pulsed laser deposition, strong pinning effects appeared in the hysteresis loops below Tc. The isotherm V(I) curves show that the residual Ni layer does not induce spontaneous vortices in the NiBi or NiBi3 phases.

The effect of Sn-doping on (La 1− x Sr x ) 2 Cu 1− x Sn x O 4 ( x =0.075, 0.090 and 0.110) has been studied. Lattice parameters monotonically decrease with Sn dopant increasing. Sn atoms dominantly occupy Cu sites and form Sn–O clusters as point-like defects. Flux pinning properties of ceramic samples of (La 1− x Sr x ) 2 Cu 1− x Sn x O 4 have been investigated by magnetic measurements from 6 to 30 K up to 5 T. The current carrying scale-length analysis has been determined from the reverse leg of the magnetization hysteresis loops and compared to the actual dimensions of the sample. Magnetization hysteresis loops of the more richly Sn-doped sample exhibit stronger pinning behavior, especially in the higher temperature range. The intra-granular critical current J c in the rich Sn-doping sample has a significant increase. We discuss the experimental results within a pinning model of point-like defects. From our data we infer that the Sn–O cluster near Cu–O layers provides an effective pinning center.

Temperature dependent Mössbauer spectroscopy on 57Fe doped (1 at % of Ni) RNi2B2C and RNiBC provided clear evidence of a pair-breaking field at the Ni site for non-superconducting compounds. This field is not present in the superconducting collinear AF DyNi2B2C, however it appears when this compound is diluted with non-magnetic Lu (Dy1-xLu xNi2B2C). Important local information on the spin structure of the R magnetic moments is obtained for both systems. The local symmetries of RNi2B2C and RNiBC compounds, measured through the deltaE Q, were found to be linearly correlated with the c¢/a ratio (c´ is the distance of the R-C layers between which the Ni2-B2 layers are sandwiched). A scaling of Tc with the local symmetry (c¢/a) was found to be the same for all the RNi2B2C and RNiBC (except for Y) compounds.

In this work, we report on the physical properties of good-quality polycrystalline superconducting samples of La 2-x Sr x Cu 1-y Zn y O 4-δ (y = 0, 0.02) prepared by a wet-chemical method, focusing on the temperature dependence of the critical current. Using the wet-chemical method, we were able to produce samples with improved homogeneity compared to the solid-state method. A complete set of samples with several carrier concentrations, ranging from the underdoped (strontium concentration x≈0.05) to the highly overdoped (x≈0.25) region, were prepared and investigated. The X-ray diffraction analysis, zero-field cooling magnetization and electrical resistivity measurements were reported on earlier. The structural parameters of the prepared samples seem to be slightly modified by the preparation method and their critical temperatures were lower than reported in the literature. The temperature dependence of the critical current was explained by a theoretical model which took the granular structure of the samples into account.

The simultaneous measurements of Doppler-broadened radiation spectra and lifetime spectra have been performed on (Y 1− x Eu x )Ba 2 Cu 3 O 7− y ( x =0, 0.1, 0.2, 0.3, 0.4, 0.5, 0.6, 0.7, 0.8, 0.9, and 1) superconductors. The mean lifetime and S -parameter, respectively show a characteristic significantly symmetric to Eu-doping concentration. Since lifetime and Doppler broadening spectroscopies are rather sensitive to the variation of the lattice distortion and defects induced by the stress-field of lattice mismatch, such symmetric behaviors of the S -parameter and the mean lifetime suggest a striking characteristic of stress in Y 1− x Eu x -123 superconductor. When the doping concentration x reaches 30%, the density of defects corresponding to stress-field appears the maximum, and there is an optimum distribution of the stress-field so as to increase critical current J c in Eu-doped Y-123 superconductors. Our results of the critical current density obtained by means of measurements of susceptibility χ ′( T , H AC ) and χ ″( T , H AC ) also suggest that the enhancement of the critical current density to reach an optimum when Eu doping x is 30%. We also qualitatively discuss experimental results in the framework of the model of stress-field pinning. The model calculating results also show the stress-field induced by lattice mismatch is an effective pinning center, and further conform that the stress pinning mechanism is dominant in RE-doped Y-123 superconductors.

For La 2−2 x Sr 2 x Cu 1− x Sn x O 4 ( x =0.075, 0.090 and 0.110) superconductors, the effects of Sn-doping on local microstructure is studied by means of Mossbauer spectroscopy. The results show that Sn ions in the 4+ valence state dominantly occupy Cu sites rather than La(Sr) site. In the view of the effective oxygen content (or excess oxygen), the dual role of Sn dopant on the carrier concentration is discussed. We attempt to explain the high transition temperature T c in (Sr,Sn)-doped La 2−2 x Sr 2 x Cu 1− x Sn x O 4 samples under the model of amphoteric effect of Sn-doping on the concentration.

We prepared samples with starting composition Ba 8 –Ge 46− x –Co x ( x =0, 4 and 6) by direct melting. These Ge-based compounds were characterized by X-ray diffraction and WDS, and we found two superconducting transitions at T C =10 and 4 K in the Co-free sample. Co-doping results in the suppression of T C to 7 K. The superconducting volume fraction also decreases with increasing Co-doping. For Co-doped samples, there is no 4 K superconducting transition. X-ray refinement shows that the compounds are mixtures of several phases. The dominant phase is diamond Ge, and we found no Ge-clathrate phases. Besides diamond Ge, there are also several weak diffractions from an unknown Ba–Ge phase, and most of them were indexed on the basis of a monoclinic unit cell. Diffraction peaks for Ba 2 Ge, BaGe, BaGe 2 , BaGe 2 O 5 , BaGeO 3 , Ba 3 GeO 5 , α-BaGeO 3 , BaGe 2 O 5 , Ba 2 Ge 5 O 12 , β-BaGeO 3 , Ba 2 GeO 4 and BaGe 4 O 9 were carefully searched for but not seen in the samples. For the Co-doped sample, besides the main diamond Ge phase, there is also a semiconducting phase CoGe 2 . With increasing Co content, the CoGe 2 content increases. The WDS results agree with this result. The main phase composition for the Co-free sample is Ba 0.01 Ge 99.9 . We also discuss the origin of two superconducting transitions in Ge-based compounds.

The magnetic and transport properties of the quaternary intermetallic HoNiBC compound (space group P4/nmm, a = 3.563(1) A , c = 7.546(1) A ) were studied for 1.2 K ⩽ T ⩽ 300 K and H ⩽ 80 kOe. The compound orders antiferromagnetically at 9.8(3) K and, in the ordered state, the Ho moment saturates to 8.5(1) μB. In contrast with the structurally-related reentrant superconductor HoNi2B2C, superconductivity is not found nor are there indications of a helical ground-state in the whole temperature range. These features are attributed, respectively, to the positioning of the Fermi level at a DOS minimum and the presence of the HoC double layers that inhibits the establishment of a helical spin arrangement.

Alloying influences on the superconducting and magnetic properties of the (Gd x Y 1-x Ni 2 B 2 C series are studied. For x 0.25 and the coexistence of superconductivity with long-range antiferromagnetic order for 0.1 < x < 0.25 are discussed.

Magnetic properties of Y 1− x Gd x Co 2 B 2 C series of compounds were studied. Two magnetically ordered phases were observed for 0.7 ≤ x ≤ 1. Change of the magnetic ordering temperatures T 1 and T 2 with the Gd concentration is argued to be related to the dilution of magnetic Gd by nonmagnetic Y and to the reduction of the distance between ( Y 1− x Gd x ) C planes. However, significant contribution to the changes in T 1 between pure GdNi 2 B 2 C and GdCo 2 B 2 C samples is suggested to be due to the different 3 d band filling in these compounds.

The superconducting and magnetic phase diagram (characteristic temperatures versus effective ionic radii) of the RNi 2 B 2 C (R Lu, Tm, Er, Ho, Dy, Tb, Gd) compounds are considered. Although the gradual degradation of superconductivity can be scaled to the de Gennes factor, ( g −1) 2 J ( J + 1), the unique reentrant behavior of the HoNi 2 B 2 C compound and the abrupt quenching of superconductivity for R lighter than Ho are most probably unaccountable within this scheme. Rather, it is argued that their low- T magnetic and transport properties as well as the main features of the interplay between magnetism and superconductivity can be accounted for if the low- T magnetism of HoNi 2 B 2 C, as reported by Grigereit et al., is generalized to the other isomorphous R members. Thus the onset of the 4f moments antiferromagnetic state at T 1 is accompanied by an oscillatory component, which transforms to a commensurate antiferromagnetic state at T 2 . For HoNi 2 B 2 C, the pressure and magnetic-field influence on T c , T 1 and T 2 will be discussed.

Structural and superconducting properties of Y(Ni1−xMx)2B2C compounds with MCo, Fe and Ru have been studied. A fast decrease of Tc with the concentration of the dopants was found. This effect can be attributed to the shift of the Fermi level induced by the dopant concentration with the additional contributions from the chemical-pressure effects and from the decrease of the electronic mean free path.

Static magnetic order of quasi-two-dimensional FeAs compounds Sr${}_{4}$$A$${}_{2}$O${}_{6\ensuremath{-}x}$Fe${}_{2}$As${}_{2}$, with $A=$ Sc and V, has been detected by $^{57}\mathrm{Fe}$ M\"ossbauer and muon spin relaxation ($\ensuremath{\mu}$SR) spectroscopies. The nonsuperconducting stoichiometric ($x=0$) $A=$ Sc system exhibits a static internal/hyperfine magnetic field both at the $^{57}\mathrm{Fe}$ and ${\ensuremath{\mu}}^{+}$ sites, indicating antiferromagnetic order of Fe moments below ${T}_{\mathrm{N}}$ $=$ 35 K with $~$0.1 bohr magneton per Fe at $T=$ 2 K. The superconducting and oxygen deficient ($x=0.4$) $A=$ V system exhibits a static internal field only at the ${\ensuremath{\mu}}^{+}$ site below ${T}_{\mathrm{N}}~40$ K, indicating static magnetic order of V moments coexisting with superconductivity without freezing of Fe moments. These results suggest that the 42622 FeAs systems belong to the same paradigm with the 1111 and 122 FeAs systems with respect to magnetic behavior of Fe moments.

(57)Fe Mössbauer spectroscopy has been used to investigate the magnetic order of non-superconducting NdFeAsO (T(N) = 140 K) and superconducting NdFeAsO(0.88)F(0.12) (T(c) = 45 K). A magnetic hyperfine field B(hf) was observed at the (57)Fe nucleus below T(N)∼140 K for NdFeAsO. Below ∼2 K an increase of B(hf) relative to the saturation value was attributed to the transferred B(hf) at the Fe site resulting from the collinear antiferromagnetic (AF) spin structure of the Nd moments. The analysis of the spectra is consistent with a commensurate AF order of Fe spins. No B(hf) is observed in superconducting NdFeAsO(0.88)F(0.12) down to 1.5 K.

We have studied the magnetism in superconducting single crystals of EuFe2As1.4P0.6 by using the local probe techniques of zero-field muon spin rotation/relaxation and 151Eu/57Fe Mossbauer spectroscopy. All of these measurements reveal magnetic hyperfine fields below the magnetic ordering temperature T M = 18 K of the Eu2+ moments. The analysis of the data shows that there is a coexistence of antiferromagnetism, resulting from Eu2+ moments ordered along the crystallographic c-axis, and superconductivity below T SC ≈ 10 K. We find indications for a change in the dynamics of the small Fe magnetic moments (~0.07 μ B ) at T ⁎ ≃ 15 K that may be triggering the onset of superconductivity: below T⁎ the Fe magnetic moments seem to be “frozen” within the ab-plane.

We report muon spin rotation ($\mu$SR) measurements of single crystal Ba(Fe$_{1-x}$Co$_x$)$_2$As$_2$ and Sr(Fe$_{1-x}$Co$_x$)$_2$As$_2$. From measurements of the magnetic field penetration depth $\lambda$ we find that for optimally- and over-doped samples, $1/\lambda(T\to 0)^2$ varies monotonically with the superconducting transition temperature T$_{\rm C}$. Within the superconducting state we observe a positive shift in the muon precession signal, likely indicating that the applied field induces an internal magnetic field. The size of the induced field decreases with increasing doping but is present for all Co concentrations studied.

A hybrid system which consists of a superconducting (SC) Pb film (100 nm thickness) containing $\ensuremath{\sim}1\text{ }\text{vol}\text{ }%$ single domain ferromagnetic (FM) Co particles of mean size $\ensuremath{\sim}4.5\text{ }\text{nm}$ reveal unusual magnetic properties: (i) a controlled switching between the usual diamagnetic and the unusual paramagnetic Meissner effect in field cooling as well as in zero-field-cooling experiments (ii) amplification of the positive magnetization when the sample enters the SC state below ${T}_{c}$. These experimental findings can be explained by the formation of spontaneous vortices and the possible alignment of these vortices due to the alignment of the Co particle FM moments by an external magnetic field.

We have performed transverse field muon spin rotation measurements of single crystals of $\text{Ba}{({\text{Fe}}_{0.926}{\text{Co}}_{0.074})}_{2}{\text{As}}_{2}$ with the applied magnetic field along the $\stackrel{\ifmmode \hat{}\else \^{}\fi{}}{c}$ direction. Fourier transforms of the measured spectra reveal an anisotropic line-shape characteristic of an Abrikosov vortex lattice. We have fit the $\ensuremath{\mu}\text{SR}$ spectra to a microscopic model in terms of the penetration depth $\ensuremath{\lambda}$ and the Ginzburg-Landau parameter $\ensuremath{\kappa}$. We find that as a function of temperature, the penetration depth varies more rapidly than in standard weak-coupled BCS theory. For this reason we first fit the temperature dependence to a power law where the power varies from 1.6 to 2.2 as the field changes from 0.02 to 0.1 T. Due to the surprisingly strong field dependence of the power and the superfluid density we proceeded to fit the temperature dependence to a two-gap model, where the size of the two gaps is field independent. From this model, we obtained gaps of $2{\ensuremath{\Delta}}_{1}=3.77{k}_{B}{T}_{C}$ and $2{\ensuremath{\Delta}}_{2}=1.57{k}_{B}{T}_{C}$, corresponding to roughly 6 and 3 meV, respectively.

By muon spin-relaxation measurements on single-crystal specimens, we show that superconductivity in the $A{\text{Fe}}_{2}{\text{As}}_{2}$ $(A=\text{Ca},\text{Ba},\text{Sr})$ systems, in both the cases of composition and pressure tunings, coexists with a strong static magnetic order in a partial volume fraction. The superfluid response from the remaining paramagnetic volume fraction of $({\text{Ba}}_{0.5}{\text{K}}_{0.5}){\text{Fe}}_{2}{\text{As}}_{2}$ exhibits a nearly linear variation in $T$ at low temperatures, suggesting an anisotropic energy gap with line nodes and/or multigap effects.

Recently, high-transition-temperature (high-T(c)) superconductivity was discovered in the iron pnictide RFeAsO(1-x)F(x) (R, rare-earth metal) family of materials. We use neutron scattering to study the structural and magnetic phase transitions in CeFeAsO(1-x)F(x) as the system is tuned from a semimetal to a high-T(c) superconductor through fluorine (F) doping, x. In the undoped state, CeFeAsO develops a structural lattice distortion followed by a collinear antiferromagnetic order with decreasing temperature. With increasing fluorine doping, the structural phase transition decreases gradually and vanishes within the superconductivity dome near x D 0 : 10, whereas the antiferromagnetic order is suppressed before the appearance of superconductivity for x > 0.06, resulting in an electronic phase diagram remarkably similar to that of the high-T(c) copper oxides. Comparison of the structural evolution of CeFeAsO(1-x)F(x) with other Fe-based superconductors suggests that the structural perfection of the Fe-As tetrahedron is important for the high-T(c) superconductivity in these Fe pnictides.

We report measurements which indicate the appearance of spontaneous vortices in lead superconducting films with embedded magnetic nanoparticles and a temperature-induced phase transition between different vortex phases. Unlike common vortices in superconductors, the vortex phase appears in the absence of applied magnetic field. The vortices nucleate exclusively due to the stray field of the magnetic nanoparticles, which serve the dual role of providing the internal field and simultaneously acting as pinning centers. Transport measurements reveal dynamical phase transitions that depend on temperature $(T)$ and applied field $(H)$ and support the obtained $H\text{\ensuremath{-}}T$ phase diagram.

X-ray diffraction, ac susceptibility and 57Fe Mössbauer spectroscopy measurements have been performed in the MgC(Ni1-xFe x )(0 ~ 7K. The Fe doping quickly depresses Tc, completely destroying the superconducting state for x ~ 0.04. The establishment of an ordered magnetic state is only observed for Fe concentration (x > or = 0.3) far above the concentration for which the superconducting state has completely disappeared. An expansion of the lattice with the increase of the Fe content was observed.

Samples with different fractions of structural modulation were synthesized in order to study the effect of a defective modulated structure on the critical temperature T c of Hg-1234. Electron microscopy revealed the existence of modulated and unmodulated Hg-1234 structures in these samples. Measurements of critical temperature T c on these samples revealed that the structural modulation significantly suppresses the T c . For the Hg-1234 sample without the structural modulation, the T c was found to be close to the highest T c for the Hg-1223 phase.

The superconducting and the magnetic phase diagram (in x-T plane) of the pseudo-quaternary (Y 1−x Gd x )Ni 2 B 2 C series is obtained. Superconductivity is observed to be montonically degraded with Gd-concentration for x