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We show that the zero-field normal-state resistivity of temperature-dependent resistivity ρ ( T ) of SrFe 2− x Ni x As 2 can be reproduced by the expression ρ ( T ) = ρ 0 + c T exp(−2 Δ / T ). ρ ( T ) can be scaled using both this expression where the energy scale Δ , c and the residual resistivity ρ 0 are scaling parameters and a recently proposed model-independent scaling method (H.G. Luo, Y.H. Su, T. Xiang, Phys. Rev. B 77 (2008) 014529). The scaling parameters have been calculated and the compositional variation of 2 Δ ( x ) has been determined. This dependence show almost a linear decreasing in the underdoped regime similar to that reported for cuprates. The existence of a universal metallic ρ ( T ) curve in a wide temperature range which, however, is restricted for the underdoped compounds to temperatures above a structural and anitiferromagnetic transition is interpreted as an indication of a single mechanism which dominates the scattering of the charge carriers in SrFe 2− x Ni x As 2 ( x = 0–0.3).

The critical current density and magnetic relaxation as a function of temperature and magnetic field are compared for the melt-processed YBCO with Ce or Pt additions. The pinning parameters estimated show an enhancement of current density, and consequently pinning force in the presence of Ce. The difference in pinning capabilities between the Ce- and Pt-added melt-processed YBCO samples is attributed to inherent differences in microstructure. It was observed that in the Ce-added sample the size of the 211 particles can be reduced to 200–500 nm, while in the melt-processed sample with Pt additions the size of the 211 precipitates is usually greater. The obtained results suggest that the benefits of melt-processed YBCO with Ce additions come from the increase of the effective pinning area of the refined 211 precipitates.

Scanning Hall probe round robin measurements of the trapped field of two CFRF-clad, epoxy resin-reinforced large grain samples of Sm–Ba–Cu–O fabricated at ISTEC-SRL by top seeded melt growth (TSMG) have been performed by five European laboratories over a two year period under the auspice of the Versailles Project on Advanced Materials and Standards (VAMAS) and Technical Committee 90 (TC90) of the International Electrotechnical Commission (IEC). This study completes the interim report of the round robin tests presented at the fourth PASREG Workshop, Jena, July 2003. The peak fields derived from flux profiles measured by the different groups are found to correlate to within 6% over the measurement period. The flux trapping ability of the samples, measured by the peak field in the upper surface, deteriorated over the period of measurement by around 2% and 1%, respectively. Overall the measurements suggest that the trapped flux profile is most sensitive to changes in magnetizing field, experimental geometry and the sample–Hall probe separation. As a result, variations in these parameters should be minimized during flux mapping. In general a magnetizing field of 1.75 times the maximum trapped field for field cooled (FC) samples is required to ensure complete magnetization of a homogeneous cylindrical sample with an aspect ratio of ≈2.5, corresponding to the geometry of the samples measured here. Finally, 20 min relaxation time following magnetization was observed to be sufficient to yield consistent measurement of the peak trapped field within the error of the measurement, which was typically less than 4%. Other than the recommended magnetizing field of 3 T, which should be determined by aspect ratio, temperature and maximum trapped field rather than pre-defined, the experimental conditions were confirmed to lie within the guidelines described in a draft International Electrotechnical Commission (IEC) Technical Committee 90 (TC90) standardization document on flux mapping. On the basis of the present results the proposed 5% spread in measured trapped field and a 20 min relaxation period appear to be reasonable criteria for trapped field measurements performed at different laboratories.

It is found that 0.5–0.7 wt% of Ir, Ru Hf, Ta added to pure Y123 phase strongly reduce the amount of secondary 211 phase. X-ray analysis of the produced samples showed no more than 3 wt% of 211, while the standard melt-processed YBCO usually contains up to 20 wt%. The microstructure of the obtained melt-processed samples reveals characteristic periodic morphology, when the trapped bands enriched in the corresponding chemical addition separate the regions of the Y123 phase. The reduction of 211 was found to be detrimental to the mechanical properties of the samples. The melt processing performed in oxygen flowing atmosphere shows promising results in improving mechanical strength of YBCO with periodic growth.

The pinning forces and trapped fields obtained in bulk YBa 2 Cu 3 O 7 based material doped with Li are compared with results reported for melt-processed samples doped with Zn. The increase of the volume pinning force of Li doped material is twice that of Zn doped YBCO at 77 K. This leads to 20% higher magnetic fields trapped in the Li-doped sample at 77 K. The measurements of the magnetic relaxation at a field of 1 T reveals that the activation energy above 65 K is higher for the Li-doped sample indicating that Li impurities are more effective in increasing the pinning energy.

Resistivity, ac susceptibility and magnetization measurements are reported for polycrystalline samples of the Gd x Y 1− x Ni 2 B 2 C series as a function of temperature and magnetic field. The magnetic Gd impurities cause an almost linear decrease of the superconducting transition temperature T c with increasing Gd content in the range of x T c have been observed. The effect of the 4f local moments manifests in a complete suppression of superconductivity for x ⩾0.3 and in antiferromagnetic ordering for x >0.3. In zero applied magnetic field, a distinct concentration region around x ∼0.3 has been revealed separating superconductivity and antiferromagneting ordering. A metamagnetic transition has been observed in the compound with x =0.5 at a magnetic field of 0.8 T.

We report studies of magnetic properties of bulk melt-processed YBa2Cu3O7−δ based material (YBCO), doped with Ca and Cd. The superconducting parameters such as critical temperature, irreversibility field and critical current are found to be strongly dependent both on Ca content and oxygen variation in the (Y,Ca)BCO samples. In contrast, no pronounced change of superconducting properties of Cd doped YBCO has been observed in the wide range of Cd substitution before and after annealing. No scaling of the normalized pinning forces at different temperatures has been found for a Cd content varying between 0.1 and 1 wt.% as well. It is suggested that only a small amount of Cd is incorporated into the YBa2Cu3O7−δ phase.

Magnetic properties of melt-processed YBCO doped with different content of Li, Ni and Pd impurities are reported. The increase of critical currents of doped materials has been observed in intermediate magnetic fields. The normalized pinning force is scaled into a single curve for all measured isotherms (60–77 K) for the samples with higher impurities level. The analysis of magnetic relaxation measurements at a field of 1 T and in the temperature interval between 10 and 70 K reveals a logarithmic dependence of the effective activation energy vs. current density. The observed time dependence of the magnetization in doped materials can be described in the framework of collective pinning theory.

The test results of electrical reluctance motors with superconducting YBCO bulk material operating at 77 K are compared to conventional machines. The design of a superconducting 150 kW reluctance motor is presented. We discuss the demands on YBCO bulk material which are necessary for a high performance of superconducting motors. Furthermore, we present the conceptual design of a superconducting linear motor. In the end we discuss future applications for superconducting motors of medium power range.

Large grain bulk superconductors with various compositions: SmBa 2 Cu 3 O 7-δ , (Sm 0.5 Eu 0.5 )Ba 2 Cu 3 O 7-δ , and Eu-Ba 2 Cu 3 O 7-δ have been successfully fabricated in air. The scaling studies of the volume pinning force evidenced two competitive pinning contributions at low and high fields for all compositions. The analyses of the scaling curves showed that the low field pinning kept identical for three compositions, while in high fields, the dominant pinning for Sm123 and Eu123 was the small-sized normal core pinning and the δT c pinning in SE123.

For a variety of high- T c superconducting systems with two-dimensional flux lattices the irreversibility lines can be described in a uniform manner by the theory of Matsushita which is based on a depinning mechanism caused by thermally activated flux creep. Two Bi-based and two Hg-based samples are selected for which in some cases the measurements are extended to magnetic fields up to 21 and 14 T, respectively. The theoretical description fits well in the whole temperature ranges between about 0.25 T c and T c . The activation energy for flux creep U 0 increases almost parabolically with decreasing temperature.

The suppression of the superconducting transition temperature T c of pseudo-quaternary R x Y 1− x Ni 2 B 2 C compounds due to paramagnetic impurities R=Tm, Er, Ho, Dy, Tb, Gd, Sm, Nd and Pr was investigated in the concentration range x T c was found to be mainly caused by magnetic pair-breaking in the case of the heavy rare-earth elements (R=Tm, Er, Ho, Dy, Tb, Gd) and by lattice distortions in the case of the light rare-earth elements (R=Sm, Pr, Nd) resulting from the large difference in the ionic radii between the yttrium host and the R 3+ ions.

The previously introduced modified melt crystallization process (MMCP) has been applied to prepare single grain YBCO bulk material with Zn partially substituting for Cu. Hole doping was controlled by an appropriate oxidizing treatment of the as-grown bulk. A field induced pinning was indicated by a well pronounced peak of the critical current density jc in the jc vs. H relationship for the maximal oxidized (overdoped) material containing Zn, whereas pure overdoped YBCO shows no peak effect. The peak effect for Zn-doped YBCO appearing for T=77 K at a relatively high field of about 3 T can be attributed to pair breaking by locally induced magnetic moments due to in plane Zn for Cu substitution. Besides high quality of the bulk YBCO, the peak effect is the reason for the trapped field as large as 1.12 T at 77 K in the cylinder of only 25 mm in diameter.

Superconducting properties and magnetic ordering of pseudoquaternary Tb x Er 1− x Ni 2 B 2 C intermetallic compounds have been investigated by susceptibility, resistivity and magnetization measurements on polycrystalline samples. Tb-rich compounds display an additional metamagnetic transition which can be suppressed by magnetic fields as low as 50 mT. A sizable reduction of the saturation magnetization of pseudoquaternary compounds is observed with a minimum located near x =0.5. T c decreases with increasing Tb fraction up to x ≈0.25, where superconductivity breaks down. The effect of composition fluctuations within the homogeneity range of the compound leads to two different types of materials with distinct T c for a given Tb/Er ratio. A depression of T N with decreasing Tb content is observed.

Superconducting and magnetic properties as well as the lattice structure of polycrystalline Ho x La 1− x Ni 2 B 2 C samples have been studied by susceptibility, X-ray, and neutron diffraction measurements, respectively. A miscibility gap has been found between x =0.4 and x =0.7 if the samples are prepared by standard arc-melting. Samples in this concentration range have been successfully prepared by melt-spinning. All samples exhibit the LuNi 2 B 2 C-type structure and the lattice parameters depend linearly on the La content. Already a low lanthanum concentration of about 10% leads to a rapid depression of T c from 8.5 K to 2 K. Furthermore this concentration leads to a complete depression of the a -axis modulated incommensurate antiferromagnetic structure. Two other magnetic structures, one with commensurate (0 0 1) and one with incommensurate wave vector (0 0 0.915) were found to be less sensitive to the La concentration. The neutron-diffraction peak intensities connected to these structures decrease with increasing La content and disappear at x =0.75 (commensurate structure) and x =0.55 (incommensurate structure). A comparison with Ho x R 1− x Ni 2 B 2 C compounds (R=Y, Lu) shows that these two magnetic structures are influenced in different way by the size of the nonmagnetic rare-earth ion R. No size effect was observed for the magnetic ordering temperature of the commensurate antiferromagnetic structure, whereas the incommensurate spiral structure along c -axis was found to be sensitive to the difference in the size of the R and the Ho ions.

The effect of alloying on magnetic ordering and superconducting properties of pseudoquaternary Tb x Y 1− x Ni 2 B 2 C intermetallic compounds has been investigated by susceptibility and resistivity measurements on polycrystalline samples. The single phase nature of the samples was proven by high-temperature DTA, X-ray and electron microprobe methods. Antiferromagnetic ordering and a depression of the Neel temperature with decreasing Tb content have been revealed for x >0.45. For lower Tb concentrations x T C increases with increasing Y fraction. The magnetic susceptibility χ ( T ) of TbNi 2 B 2 C shows a maximum at T N =15.5 K, passes through a minimum with decreasing temperature, and rises toward a second maximum near 5.5 K. This second maximum was ascribed to weak ferromagnetic ordering which has been confirmed by magnetic hysteresis effects. For decreasing Tb content the ferromagnetic ordering temperature was found to shift to lower temperatures and to disappear for x

Global transport measurements and local Hall-probe array measurements of a section of silver clad BSCCO tape, are reported. Transport measurements are in good agreement with the `brick wall' model. The induction profiles obtained from magnetic measurements agree with the assumption of a uniform bulk current at low (40–60 K) temperatures. At higher temperatures and relatively high fields (700 G and up) local measurements seem to reveal the existence of surface currents. These currents are the result of small current loops, flowing within the grains, which result in a net current only at the edges of the sample. Thus, the local magnetic measurements do not agree with global transport ones, and we contend that the `brick wall' model aptly describes the flow of transport currents, but fails to describe the flow of currents induced by an external magnetic field.

The superconducting transition temperature T c , the temperature dependence of the upper critical field H c2 ( T ), and the lattice structure of polycrystalline Y x Lu 1− x Ni 2 B 2 C samples have been studied by susceptibility, resistivity, and X-ray measurements, respectively. All samples exhibit the same LuNi 2 B 2 C-type structure and the lattice parameters depend linearly on the yttrium content. The residual resistivity ratio (RRR) decreases steeply first for slight deviations from both pure stoichiometric limiting contents and then saturates at a low level in a broad plateau in between 0.1 x T c ( x ) and H c2 ( x ) at fixed T change nearly parabolically. The H c2 ( T )-curves show a positive curvature for all samples. This dependence can be described within a broad temperature region 0.3 T c T ≤0.95 T c by a simple and convenient empirical expression H c2 ( T )∝(1− T / T c ) α . The parameter α >1 describing the positive curvature of H c2 ( T ) characterizes the sample quality and decreases with increasing disorder in the Y x Lu 1− x Ni 2 B 2 C samples. However, the case of complete disorder (dirty limit) where α =1 is not reached yet in the investigated samples. The nonmonotonous dependence of T c and H c2 upon x is attributed to partial disorder within the Y–Lu basis layers. Together with the RRR-saturation they reflect the complex multiband character of the materials under consideration.

Bulk melt textured YBCO samples in the shape of disks were developed for use as trapped field magnets. The trapped field in single grains with diameter of 24 mm reached maximum values up to 0.56 T at 77 K and a value of 5.5 T at 30 K. Degradation of the trapped field was observed in samples after the activation process at 20 K in a high applied magnetic field. This is interpreted in the framework of a model of magnetic stress induced damage.

Superconducting and magnetic properties of polycrystalline samples of the pseudoquarternary system (Ho x Y 1− x )Ni 2 B 2 C have been investigated by resistance and susceptibility measurements. A linear depression of the superconducting transition temperature with increasing magnetic ordering temperatures was found by variation of the Ho content providing evidence for magnetic pair breaking. This behaviour is analogous to the known scaling with the de Gennes factor of the rare earth elements in the family of quaternary R Ni 2 B 2 C compounds. Both cases are described by a common scaling behaviour including the superconducting and magnetic transition temperatures. A reetrrant behaviour observed for Ho contents x >0.5 results in maximum in the temperature dependence of the upper critical field H c2 ( T ). These results are compared with H c2 ( T ) data of the R Ni 2 B 2 C family ( R =Tm,Er).

The irreversibility line of silver-sheathed Bi-2223 tapes and of melt-processed Bi-2223 samples doped with Te has been investigated by AC susceptibility, DC magnetization and transport measurements. The temperature dependence of the irreversibility field B ∗ follows a relation B ∗ (T) ∝ (T c /T−1) 1 n with n = 0.6 which can be interpreted in the framework of a flux-creep model with an expression for the activation energy U ∝ (1− T / T c )/ B n .

Bulk textured MgB 2 material of nearly full density was obtained by hot deformation of a stoichiometric MgB 2 pellet prepared by a gas–solid reaction. The texture of the material was verified by comparing the X-ray diffraction patterns of the hot deformed material with isotropic MgB 2 powder. A small, but distinct anisotropy of the upper critical field H a , b c2 / H c c2 ∼1.2 was found by resistance and susceptibility measurements.

A variety of Hg-1223 high- T c superconducting systems, partly substituted by lead or fluorine, are investigated by means of magnetic susceptibility measurements. As already observed earlier, the double peak structure in the imaginary parts of the ac signals below about 1 T is confirmed indicating different pinning mechanisms. Irreversibility lines are determined from the upper peak values and analysed applying the theory of Matsushita which is based on a depinning mechanism caused by thermally activated flux creep. It leads to a suitable description in the whole temperature range investigated and allows a discussion of differences of the various samples.

The upper critical field H c2 ( T ) of sintered pellets of MgB 2 superconductor was investigated by transport, AC susceptibility and DC magnetization measurements in magnetic fields up to 16 T. The highest H c2 values obtained from magnetization and resistance measurements ( H c2 mag ≈ H c2 90 ) can be attributed to the grains oriented with their ab -planes parallel to the applied field. H c2 ( T ) shows a positive curvature near T c similar to that found for the borocarbides YNi 2 B 2 C and LuNi 2 B 2 C indicating that MgB 2 is in the clean limit. The irreversibility field consistently determined from DC magnetization and from the imaginary component of AC susceptibility was found to increase up to 8.5 T at 10 K.

Trapped fields up to 16 T were achieved at 24 K in superconducting mini-magnets consisting of two bulk YBCO disks. The YBCO disks were encapsulated in steel tubes in order to reinforce the material against the large tensile stress acting during the magnetizing process and to avoid cracking of the material. Additionally, Ag and Zn additions were used for improving the mechanical and the pinning properties of the superconductor, respectively.

Substitutional disorder in Y(Ni 1− x Pt x ) 2 B 2 C and Lu(Ni 1− x Pt x ) 2 B 2 C is found to reduce relevant quantities as T c , the upper critical field H c2 (0) at T =0 and a characterisitic positive curvature of H c2 ( T ) near T c observed for these compounds. Starting from the clean limit at x =0, a dirty-limit-like state is achieved near x =0.2 in qualitative accordance with a two-band model. The electronic specific heat c es ( T ) follows, for the clean limit superconductors YNi 2 B 2 C and LuNi 2 B 2 C, a power law whereas an exponential BCS-like temperature dependence is observed in the dirty limit. Nevertheless, the electronic specific heat parameter γ ( H ) in the mixed state shows significant deviations from the linear γ ( H ) law also in the dirty limit.

The temperature and magnetic field dependence of the specific heat c p ( T , H ) in the superconducting mixed state has been measured for polycrystalline Y x Lu 1− x Ni 2 B 2 C and Y(Ni 1− y Pt y ) 2 B 2 C samples. The deviations from the usual linear-in- H law of the linear-in- T electronic specific heat contribution γ ( H ) T can be possibly ascribed to unconventional pairing. The γ ( H ) dependence is discussed in the unitary d-wave scenario as well as in the intermediate transition region in between dirty and clean s-wave limits. From a consideration of γ ( H ) data, unconventional pairing cannot be ruled out.

Magnetostriction hysteresis data reveal the presence of the fishtail effect in isothermal scans between 72 and 80 K. Thermal cycles in a constant magnetic field show the behaviour of pinning of flux lines in the different phases. Ultrasonic attenuation and the relative change of the sound velocity for various magnetic fields were measured. The aim is to examine the elastic behaviour of the flux line lattice. In the framework of the thermally assisted flux flow model the depinning temperature T D ( B ) and the activation energy U ( B ) were been deduced. The measurements on YBCO crystals are discussed and compared with investigations on YBCO/Zn + Ag composites. The magnetostrictive effects are verified by supplementary DC-magnetization measurements.

AC susceptibility and dc magnetization have been studied for LuNi 2 B 2 C and YNi 2 B 2 C single crystals in magnetic fields H up to 50 kOe ( H ⊥[001], [100], [110]). A pronounced dip anomaly in the ac screening response in the mixed state has been found for both compounds which was found to be connected with the peak effect of the superconducting critical current.

Magnetic and superconducting properties were studied for high quality Al-free orthorhombic Gd 1− x Pr x Ba 2 Cu 3 O 7− y single crystals (0≤ x ≤ 1.0) grown by the flux method. An indication of the interaction between Pr and Cu(2) magnetic sublattices was found for nonsuperconducting Pr123. Different sign of magnetic anisotropy was established for the Pr and Gd ions at low T for crystals with x =0.6 and 0.44. It was also shown that the superconductivity reported by Zou et al. [Phys. Rev. Lett. 80, 1074 (1998)] for Pr123 single crystals grown by TSFZ method seems to be connected with partial substitution of Ba for the Pr-sites.

Bulk melt-textured YBCO materials in the shape of disks are being developed for use as trapped field magnets and in superconducting magnetic bearings. We report on the superconducting properties relevant for these applications. The axial trapped field distribution on the top of the disk and its temperature dependence is measured and discussed in terms of a distribution of critical currents over the bulk of the sample. The time dependence of the trapped field distribution is investigated for different external conditions, such as in the homogeneous field of a vibrating sample magnetometer and the field of a rare earth magnet. The materials are tested in a model of a motor with superconducting levitating bearings. Different possibilities to further improve the performance of the motor by raising the critical current density j c and optimizing the overall configuration are discussed.

The U ( J )-relations of a Bi2223 tape, determined from measurements of the time dependence of the magnetic moment at 0.4, 1 and 4 T perpendicular to the tape surface, are analysed within the collective pinning theory. At low temperatures the system can be described by the movement of 2D single vortices (pancakes), which at higher temperatures move collectively.

Bi-2223 tapes have been investigated by transport and magnetization measurements. At high temperatures, where the critical current j c is governed by thermally activated motion of pancake vortices a good agreement was found between the j c obtained by transport and magnetization measurements. Characteristic changes in the electric field-current characteristics and the j c (B) dependence at low temperatures are interpreted in the framework of the ‘brick wall’ model by c axis weak links between the grains. The transport j c at low temperatures is limited by the critical Josephson current of weakly coupled grains for which we determined values between 3000 and 6000 A/cm 2 . In the low temperature region the j c derived from magnetization loops is considerably lower than that of transport measurements.