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We employ a combination of pulsed- and continuous-wave polarized terahertz spectroscopy techniques to probe temperature-dependent spin waves in the antiferromagnet $\mathrm{Nd}\mathrm{Fe}{\mathrm{O}}_{3}$. Our optical data span 1.6--467 K and reveal a conspicuous spin reorientation between 110 and 170 K, during which the lower-energy mode softens completely. Complementary inelastic neutron scattering reveals that the frequencies of the optically excited spin waves are consistent with a temperature-variable spin gap in the low-energy spin-wave dispersion of $\mathrm{Nd}\mathrm{Fe}{\mathrm{O}}_{3}$. The result links the temperature dependence of the spin waves to a dynamic in-plane anisotropy. The magnetic anisotropy is calculated based on the results of the optical measurements. The change observed in the anisotropy energy along the $a$ and $c$ crystal axes suggests that the spin reorientation evident in $\mathrm{Nd}\mathrm{Fe}{\mathrm{O}}_{3}$ is driven by temperature-dependent in-plane anisotropy.

A new procedure for the preparation of amorphous Ni-Co-B nanoparticles is reported, with a detailed investigation of their morphology by X-ray diffraction and transmission electron microscopy, as well as their magnetic properties. Many factors, such as chemical composition, anisotropy, size and shape of the particles, were controlled through chemical synthesis, resulting in the control of morphological and magnetic properties of the nanoparticles. Controlling pH values with ethylenediamine and using sodium dodecyl sulfate surfactant lowered the size of the nanoparticles to below 10 nm. Such a small structure and chemical disorder in nanocrystalline materials lead to magnetic properties that are different from those in their bulk-sized counterparts. The obtained nanoparticles can be used for different purposes, from pharmaceutical applications to implementations in different materials technology. The focus of this research is the synthesis of Ni-Co-B nanoparticles in a new way and studying the reaction of Ni-Co-B nanoparticles with Mg and B precursors and their effect on MgB2 properties. New nanostructures are formed in the reaction of Ni-Co-B nanoparticles with Mg: Mg2Ni, Co2Mg and possibly Mg2Co.

The effects of cation substitution in the manganese site of charge-ordered Nd0.5Sr0.5MnO3 have been investigated to elucidate how the properties of this material change with chromium doping. Nd0.5Sr0.5Mn1−xCrxO3 (x=0, 0.05, 0.1, 0.15, 0.2, 0.25) compounds have been studied by X-ray diffraction and transport and magnetic measurements. All samples were determined to be single phase and crystallized in an orthorhombic structure. Magnetic measurements showed that chromium doping decreases the charge-ordering temperature Tco as x values are increased from 0 to 0.05. The charge ordering is completely destroyed for all the higher Cr doping levels (x>0.05), and the material changes from an antiferromagnetic insulator to the ferromagnetic metallic (FMM) state for 0.05 0.2. At the same time, the magnetization increases with increasing Cr content, reaches a maximum for x=0.15 and then decreases for x>0.15. It is proposed that Cr destroys the charge ordering and facilitates double exchange between Mn3+–O–Mn4+. It is thus responsible for the FMM state seen at low Cr doping levels, while an antiferromagnetic interaction between Mn4+–O–Cr3+ or Cr3+–O–Cr3+ is responsible for the insulator state at high Cr doping levels.

Magnetic hysteresis loops and magnetic relaxation have been measured for pure and heavily lead-doped Bi2-xPbxSr2CaCu2Oy (Bi2212) single crystals. From the hysteresis loop measurements, the normalized critical current density Jc/Jc0 has been calculated, and from the relaxation measurements the activation energy, current-voltage curves and normalized relaxation rate have been obtained. The field and temperature dependences of Jc/Jc0 for Pb-doped crystals were much weaker than for the pure Bi2212 crystals. A large upward shift in the crossover temperature TCR, a temperature separating two different pinning regimes, has been observed for the first time with heavy lead doping. This was interpreted by improved inter-layer Josephson coupling of two-dimensional vortices, as a result of Pb doping. Heavily Pb-doped crystal had TCR = 35 K for the field that was within the secondary peak in its magnetic hysteresis loop. For pure Bi2212, TCR = 19 K.

Dc magnetisation and ac susceptibility were measured on Gd2CoMnO6 perovskite manganite synthesised by solid state reaction in dc magnetic fields up to 5 T and an ac magnetic field of 1 Oe at frequencies of 21, 217 and 2000 Hz over a wide temperature range from 300 K down to 4.2 K. A spin glass transition with a very sharp transition width of 1 K at temperatures as high as 112 K was observed after the paramagnetic to ferromagnetic transition. An antiferromagnetic transition occurs at 43 K, far below the spin glass state. The spin glass transition temperature is totally suppressed at a field of 5 T.

The effect of grain connectivity on the magnetoresistance (MR) was studied on porous and highly dense polycrystalline bulk samples of La0.7Ca0.3MnO3 and La0.8Li0.2MnO3 synthesised by conventional solid state reaction and partial melting technique. SEM and XRD investigations demonstrated that the partial-melted samples were very dense with large grain sizes whereas the porous sample contained large numbers of small sized grains. The porous La0.7Ca0.3MnO3 and highly dense La0.8Li0.2MnO3 samples exhibited a very wide transition width in resistance vs. temperatureOR 2 TU plots taken in magnetic fields of 0.3 or 1 T over a wide temperature range from 77 to 235 K. In contrast, the partially melted La0.7Ca0.3MnO3 sample only showed a very sharp transition around the ferromagnetic transition at 235 K in the R 2 T curve. It is proposed that it is the grain connectivity and not the density of the samples that is responsible for the transition width in R 2 T. q 2000 Elsevier Science Ltd. All rights reserved.

LiNi x Co 1− x O 2 solid solutions are synthesised and their electrochemical performance as cathodes is examined in lithium cells. Through DTA/TG analysis and self-discharge tests, it is found that both the thermal stability at high temperature and the electrochemical stability in the fully-charged state for LiNi x Co 1− x O 2 increases with increasing content of cobalt in the compound. The average operating potentials for the solid solutions are lower than those for pure LiCoO 2 . This indicates that both cobalt and nickel take part in the redox reactions during lithium insertion and extraction. The substitution of Ni 3+ for Co 3+ in LiNi x Co 1− x O 2 can improve the specific capacity, but with a small sacrifice in cycleability.

Magnetic vortex pinning processes were studied for heavily Pb-doped Bi 2− x Pb x Sr 2 CaCu 2 O y crystals. The atomic ratio of Bi to Pb atoms was 1.66:0.34, as obtained by EDS. The hysteresis loops of the crystals exhibited a very pronounced secondary peak effect, in the temperature range 20 K T T c . J c0 at T =0.3 T c was 75 kA/cm 2 , several times higher than for a pure Bi2212. The current-dependence of the activation energy was obtained from the magnetic relaxation measurements at different temperatures. Temperature scaling U∼ (1−( T / T c ) 2 ) was employed. Fitting of the activation energy by the interpolation formula U ( J )= U c [( J 0 / J ) μ −1] gave μ =0.33 and 0.1 for the fields at the secondary peak and away from the peak, respectively. This shows that variable range hopping (VRH) processes define the magnetic relaxation at the secondary peak, whereas single vortex pinning was obtained at higher fields. It was discussed that increased c -axis coupling with Pb-doping contributed to the increased pinning in both field ranges. The nanometer-size laminae, observed in heavily Pb-doped Bi2212, were suggested to be responsible for the variable range hopping processes at the secondary peak. The nanometer-size amorphous regions were most probably responsible for the increased pinning at higher fields.

Measurements of the total a.c. losses in an applied parallel a.c. magnetic field have been measured for Bi2223/Ag tapes with twisted and untwisted filaments. The measurements were undertaken using suitable pickup loops and a lock-in amplifier. For all frequencies (37–200 Hz) and field amplitudes used (

The effect of deformation rate and silver hardness on J/sub c/ and microstructure of Ag/Bi-2223 tapes was studied under cryogenic processing conditions. It was found that cryogenic deformation improved the grain connectivity, alignment, Ag/core interface and critical current density. These benefits may be attributable to the increased extent of mechanical deformation. The hardness of the silver sheath during cryogenic mechanical deformation was significantly increased, allowing for a much larger load to be applied during the deformation process comparing to the mechanical deformation at room temperature. The maximum J/sub c/ appeared at a 20% reduction rate of the tape thickness for cryogenically processed Ag/Bi-2223 tape. Higher deformation rate decreases J/sub c/ for cryogenically processed Ag/Bi-2223 tape which can be explained by the results of TEM examination. TEM images show that the sample having low J/sub c/ had many small grains with large angle grain boundaries after recrystallisation when a high rate of thickness reduction (58%) was applied during the process, while the sample having high J/sub c/ had large grains with many small angle c-axis twist grain boundaries and small angle tilt grain boundaries when 19% of thickness reduction was applied during the process.

Phase transformation during the final stage of Ag/2223 tape processing has been investigated through quenching and normal cooling in a specially-designed two-step sintering process. It was found that the phase assemblage in the final tape was determined by the equilibrium composition at the sintering temperature and also from any phase transformation which occurred on cooling. A two-stage sintering procedure in the final thermal cycle was found to be effective in transforming the liquid (or amorphous phase) and residual 2201 into 2212 and 2223. However, the annealing temperature during the final step was critical to the final phase assemblage. The optimal annealing temperature of the second step in the two-step process was around 825/spl deg/C, where all the low T/sub c/ phases and impurities were at a minimum. Annealing at temperatures below 810/spl deg/C, resulted in a substantial increase in 3221, and a lower J/sub c/. Annealing above 825/spl deg/C led to a large 2212 fraction with a small amount of 2201 because of insufficient time needed to convert these phases into 2223 on normal cooling.

Bi-2223/Ag multifilamentary tapes were fabricated using a hot-pressing technique to investigate its effect on grain connectivity and flux pinning. The critical current density in tapes after hot-pressing was doubled. J/sub c/ increased from 26000A/cm/sup 2/ to 56100A/cm/sup 2/ in a 19-filament tape. However, the critical current density in an applied magnetic field was not improved by hot-pressing. The fraction of strong links (I/sub C0//sup s//I/sub C0/) was improved from 47% to 68% in a 19-filament tape due to hot pressing. Microstructural analysis showed that hot-pressing improved grain connectivity, increased core density and reduced secondary phases.

Mono and multifilamentary powder-in-tube tapes with sheaths of Ag, AgCu/sub 0.02/, Ag(AgCu/sub 0.02/), AgAl/sub 0.25/, Ag(AgAl/sub 0.25/), AgNi/sub 0.25/Mg/sub 0.25/, AgTi/sub 0.25/Mg/sub 0.25/, and AgTi/sub 0.25/ alloys have been fabricated and their physio-chemical properties and effect on the phase formation of Bi-2223 determined. Alloying was found to have a significant effect on phase formation of the Bi-2223 phase formation linked to the alloying elements reactive to oxygen, Cu

Magnetisation and a.c. susceptibility were measured for porous and high density polycrystalline La 0.7 Ca 0.3 MnO 3 bulk samples synthesised by a conventional solid state reaction and a partial melting technique, respectively. A large irreversibility in the zero-field cooled and field-cooled magnetization was observed between 4.2 K and Curie temperature 270 K, for fields smaller than 200 Oe. Magnetization and a.c. susceptibility measurements gave a cusp at T cusp . At T cusp , a.c. susceptibility χ′ decreases with increasing frequency for the porous sample. The high density bulks having a sharp transition, but low T c (250 K) exhibited a similar behaviour with a large irreversible magnetization at 50 Oe and a cusp in a.c. susceptibility for fields up to 1 Tesla. With increasing d.c. field, the irreversibility in magnetization was gradually suppressed. Large irreversibility in d.c. magnetization was explained by magnetic domain freezing instead of spin glass state. The small impurity particles embedded in large grains act as a stronger pining centre than weak-link grain boundaries for domains.

Ag-sheathed Bi-2223 multifilamentary tapes were prepared with a hot-pressing technique to study its effect on grain connectivity. The critical current density after hot-pressing was significantly enhanced with the maximum increase reached more than double that before. Hot pressing raised the from to for 81-filament tape. However, the critical current density in an applied magnetic field was not improved by hot-pressing. The in a magnetic field of 1 T decreased from 33% of for the original tape to 21% of for the hot-pressed tapes before and after hot pressing. The fraction of strong links was improved from 64% to 73% for the 81-filament tape as a result of hot pressing. Microstructural analysis showed that hot pressing chiefly improved grain connectivity, increased core density, recovered microcracks and reduced secondary phase impurities.

The effect of the phase compositions after the final stage of heat treatment on the critical current density in Bi:2223/Ag tapes has been investigated. A high volume fraction of Bi:2223 has been considered the most critical factor in achieving a high ; however, it is extremely difficult to attain pure Bi:2223 phase via routine thermomechanical processing. Although a higher volume fraction of Bi:2223 phase can be attained using a higher processing temperature, it is always found to co-exist with Bi:2201 which resides at Bi:2223 colony boundaries. As these possess a plate-like morphology, the Bi:2201 joins weak links in a conductible chain. Bi:2201 can be eliminated using an annealing step at the end of the last thermal cycle, resulting in a significant improvement in . At annealing temperatures below , Bi:2223 decomposes into Bi:3221 and other cuprates, which degrade further. Magnetization, AC susceptibility and resistivity measurement results confirm that the Bi:2201 and Bi:3221 act as weak links.

A hot-pressing technique was used to prepare Ag-sheathed Bi-2223 multifilamentary tapes in order to study its effect on grain connectivity and flux pinning. The self field critical current density (Jc) after hot-pressing was significantly enhanced, the maximum increase reached more than double than before. Hot pressing raised the Jc (B=0) from 22 000 A/cm2 (Ic=18 A) to 56 800 A/cm2 (Ic=36.5 A) for 81-filament tape and from 26 000 A/cm2 (Ic=36.2 A) to 56 000 A/cm2 (Ic=56.6 A) for 19-filament tape. But, the Jc in applied magnetic field was not improved or degraded by hot-pressing. The Jc retention in magnetic field of 1 T decreased from 33% for the original tape to 21% for the hot-pressed 81-filamentary tapes and remained the same (24%) for 19-filamentary tapes before and after hot-pressing. The effect of cooling rate on connectivity and flux pinning in hot-pressed Bi-2223 PIT tape has been investigated. The fraction of strong links (IC0s/IC0) was improved from 64% to 73% for 81-filament tape due to hot pressing and slow cooling. However it was reduced to 42% for hot-pressed and normally cooled tape. Microstructural analysis showed that hot-pressing chiefly improved grain connectivity, increased core density, improved crystal alignment, recovered micro-cracks and reduced secondary phase impurities. Hot-pressing technique reduced some defects, but at the same time, it created different type of defects, so the whole amount of defects was not much changed. Therefore, hot pressing caused different influence on flux pinning for different original tapes. These results suggest that limitation of Jc in Bi-2223 tapes is multiple and potential for further improvement of Jc is great.

Several recent developments in powder-in-tube (PIT) processing are presented. A cryogenic deformation process has been developed, involving rolling or pressing the wires and tapes in supercold conditions, such as in liquid nitrogen. Cryogenic deformation has been found to improve the density, grain alignment and Ag-oxide core interface and to increase dislocation density, thereby enhancing and flux pinning. By incorporating Pb into Bi-2212 phase the sintering temperature can be raised above , resulting in a significant reduction of total sintering time from several hundred hours to 100 h. Recently, a new process to eliminate the decomposition and recovery of Bi-2223 during cooling and heating has been developed that further reduces the heat treatment time for Ag/Bi-2223 tapes to 20-30 h, with and Bi-2223 volume fraction in the tapes comparable with those in tapes treated for 120 h. A two-stage annealing procedure in the final thermal cycle has been used to eliminate residual amorphous phase and Bi-2201, which has been identified to be one of the major causes of weak links in PIT tapes. By incorporating hot deformation in a two-step process not only can Bi-2201 be eliminated, but also texture and density are improved, resulting in a very high at 77 K in multifilamentary tapes.

A cryogenic deformation process has been developed and investigated for use in the processing of Ag/Bi-Pb-Sr-Ca-Cu-O superconducting wires. A typical cryogenic deformation process used comprised rolling or pressing the wires into tapes at 77 K and then heat treating the tapes at 838- for a period of 40-60 h. It was found that the cryogenic deformation improved the density, grain alignment, Ag/oxide core interface and critical current density. Critical current densities for cryogenically rolled and pressed Ag-clad mono- and multifilamentary tapes showed a 10 to 20% increase over those observed in normal processed tapes. In comparison with hot deformation and processed tapes, the cryogenic processed tapes showed improved flux pinning capability whereas the hot deformation was found to only improve weak links. During cryogenic deformation, the hardness of the silver sheath was significantly increased, which allowed a greater pressure to be applied during the deformation process. This large pressure was found to be the principal cause of the enhanced densification, texturing and increased dislocation densities.

X-ray diffraction and AC susceptibility measurements have been carried out on LaBaCaCu 3− x M x O 7 (RE:1113) system, where M=Co, Ni, Ga with x =0.0, 0.06, 0.08, 0.24 and 0.30. X-ray diffraction results show that these cations substitute isostructurally in the tetragonal La:1113 phase, with decreased c -parameter for Co and Ga and nearly unchanged for Ni, at least up to 10 at% of doping level. Superconducting transition temperature ( T c ) of the pristine sample, determined from the measurements of both real and imaginary parts of AC susceptibility, decreases with an increase of x for all the three cations. T c depression rate with doping concentration (d T c /d x ) follows the order Ga>Ni>Co. These results, in terms of T c depression due to these impurities in the parent La:1113 system are similar to those observed for RE:124 (REBa 2 Cu 4 O 8 ), 247 and Nd:214 (Nd 2− x Ce x CuO 4 ) compounds, but are different from RE:123 and La:214.

A controversial issue in the study of Bi-based superconductors is the role of Bi2Sr2CaCu2O8 in determining the properties of silver-sheathed (Bi,Pb)2Sr2Ca2Cu3O10 (Bi2223) tapes. In this study, we prepared a series of Bi2223/Ag tapes with different Bi2Sr2CaCu2O8 (Bi2212) and Bi2Sr2CuO6 (Bi2201) ratios and investigated its effect on electrical and magnetic properties. It was found that residual Bi2201, instead of Bi2212, was the limiting factor for the critical current (Ic). Elimination of Bi2201 using a two-step sintering process in the final thermal cycle, or by cooling the tapes slowly, increased Ic by a factor of 2. The flux pinning and irreversibility field behavior were also improved by removing Bi2201.

We have investigated the structural aspects and superconductivity of La 1− x RE x BaCaCu 3 O 7 system in detail, with the help of X-ray diffraction and magnetic susceptibility studies. While a complete substitution of Nd and Pr takes place isostructurally in La:1113 (LaBaCaCu 3 O 7 ) compound, for Dy and Sm, both are only partial, namely, it is 50 at.% for the former and 75 at.% for later. Unlike the RE:123 system, in the presently studied RE:1113 compound, the phase formation and onset T c are both strongly dependent on the chosen rare earth (RE). Pr has a less deleterious effect on the superconductivity of La:1113 system than that reported for La:123. These results of phase formation and the different T c values for various rare earths are discussed on the basis of interesting structural chemistry of this compound, which involves the intermixing of RE, Ba and Ca sites in RE:1113 compounds.

A new interpretation of the diamagnetic transition in Bi2223/Ag tapes is proposed. The main distinction from the previous models is the proper treatment of the role of low-number stacks of unit-cells. The new arguments to the existence of these stacks are given. The onset temperature of the real AC susceptibility χ ′ is interpreted as the melting temperature of the thickest stack. The middle temperature of χ ′( T ) transition and the peak temperature of the imaginary part define the melting temperature of dominant stacks. The pinning and the critical current behavior are re-analyzed in connection with the appearance of low-number stacks and the suppression of interstack Josephson interaction.

Transport critical current ( I c ) was measured as a function of magnetic field ( H ) at 77 K, for Bi-2223/Ag tapes which were progressively bent. The field was perpendicular to the tape plane. Proportion of critical current through strong links in total I c ( I c0 s / I c0 ) was used as a measure of grain connectivity by strong links. It was found that I c0 s / I c0 changes with bending, going through a peak as the tapes progressively bend. A change of flux pinning with bending was also observed. However, the effect of the change of flux pinning on I c0 s / I c0 was only a minor one. Therefore, change of I c0 s / I c0 with bending is defined by grain connectivity and it reflects qualitatively different electromagnetic and mechanical properties of weak and strong links. This is in agreement with previous studies of electromagnetic properties and formation of weak and strong links.

Large Y-123 single domain textured sample was produced by slowly cooling Y-123+15% Y-211 precursor seeded with Sm-123 through the peritectic point. The sample was annealed in flowing O 2 at 380°C for 72 h. An O 2 distribution gradient in the range of 6.64–6.84 was determined in the volume of the sample by measuring ` c ' parameter of unit cell for the small cleaved sections. The J c of those sections was derived from hysteresis loops and T c from a.c. susceptibility. T c is practically constant in the range=6.84> x O >6.71 and drops sharply for smaller x O . A maximum J c of 76,000 A/cm 2 (77 K, self field) and an irreversibility field H irr =7.2 T were obtained for x O =6.77, which is in the range of structural transformation of orthorhombic I→orthorhombic II phase of oxygen superlattice, where the clustering in the oxygen superlattice is maximal. This results in a strong flux pinning, giving large J c and H irr . Any-minute deviation from x O =6.77 lowers J c and H irr substantially.

Magnetisation and AC susceptibility were measured on perovskite Y0.7Ca0.3MnO3 at DC fields up to 5 T and AC field of 1 Oe with frequencies of 21, 217 and 2000 Hz. A large irreversibility was observed in the zero-field cooled and field-cooled magnetisation with a cusp at Tcusp in ZFC magnetisation. AC susceptibility also showed a cusp at the Tcusp which shifts to high temperature with increasing frequencies. This suggests a spin-glass state in Y0.7Ca0.3MnO3. The spin-glass state was totally suppressed at a high field of 5 T.

The phase evolution, critical current density ( J c ) and irreversibility field ( H irr ) for (Bi,Pb) 2 Sr 2 Ca 2 Cu 3 O 10 (2223) tapes have been studied through a two-step sintering process. The tapes sintered at 840°C without second-step annealing at 820°C contain a small fraction of (Bi,Pb) 2 Sr 2 CuO 6 (2201). 2201 phase vanishes when the tapes are annealed at 820°C for 30 h after sintering at 840°C. (Bi,Pb) 2 Sr 2 CuO 6 (2201) phase was found to be the predominant factor for the depression of J c of the tapes. The J c vs. magnetic field shows that the depression of J c occurs only in the low field region, indicating that 2201 phase act as weak links in the tapes.

High critical current density is essential for most large-scale applications of high-temperature superconductors (HTSs). In addition to the weak link problem, weak flux pinning is a major cause of the rapid decline of with magnetic field at high temperatures. Through intensive research in the past 10 years the weak pinning problem has been partially overcome and has reached a level approaching the requirement for some commercial applications. In this article, a number of techniques by which effective pinning centres can be introduced to improve flux pinning in Bi - (Pb) - Sr - Ca - Cu - O HTSs are reviewed. These include surface pinning through a spiral growth, fine precipitates produced through phase transformation or phase formation - decomposition, inclusions through chemical doping or addition, cascades and columnar defects created by fast-neutron and heavy ion beam irradiation and various defects induced by mechanical deformation. In particular, the results of Bi-2223 formation - decomposition and cold deformation versus hot deformation have a significant implication for HTS wire fabrication. Comparative studies with comprehensive electrical, magnetic and microstructural characterizations have been carried out to assess the effectiveness of these techniques.

Measurements of the transport critical current in a magnetic field (H), hysteresis loops and a.c. susceptibility were performed on Bi-2223/Ag tapes that were subjected to different degrees of mechanical deformation and subsequently sintered. against H for some of the tapes was also measured at different stages of thermo-mechanical treatment. All the measurements (the field of the peak of the pinning force, the irreversibility field obtained from hysteresis loops and the temperature of the peak of a.c. susceptibility for different d.c. fields) clearly show an increase of pinning with increase of the degree of mechanical deformation. Measurements of samples at different stages of thermo-mechanical treatment indicate that the pinning depends not only on the degree of mechanical deformation, but also on the conditions of the grain growth. Sintering can provide imperfect healing of micro- and nano-cracks, introduced upon mechanical deformation. It is proposed that the imperfections introduced by the healing are responsible for the improvement in flux pinning, and not the dislocations introduced upon mechanical deformation.

Comparative studies on the Ag-clad 27-filamentary Bi-2223 tapes deformed under different conditions have been carried out using ac susceptibility and critical current measurements. Hot pressing improves the grain alignment and grain connectivity, resulting in a noticeable improvement in the critical current density in the low field region. However, the irreversibility line for the cold pressed Ag/Bi-2223 tape is positioned at higher temperatures than that for the rolled tape which is in turn positioned at higher temperatures than that for the hot-pressed tape. These results indicate that the hot deformation is only beneficial to the improvement of weak links whereas the cold deformation induces more defects which act as pinning centres in these tapes, responsible for the improvement in flux pinning.

The nature of grain-boundary links in high-quality tapes is clarified. The investigations focus on the role that various types of links play in the restriction of the critical current. The multiple exponential function is found to be a good approximation for the magnetic-field dependence of critical current . The behaviour of at different temperatures gives clear evidence of the two-dimensional character of critical current. It is shown that at all temperatures higher than the Kosterlitz - Thouless transition temperature of isolated superconducting layers the critical current is totally defined by the Josephson interaction between the layers.

Silver-sheated (Bi,Pb) 2 Sr 2 Ca 2 Cu 3 O 10 (Bi2223) superconducting tapes with different Bi 2 Sr 2 CaCu 2 O 8 (Bi2212) and Bi 2 Sr 2 Cu 1 O 6 (Bi2201) concentrations, were prepared by using a two-step sintering processing and by varying cooling rates in the fabrication of the superconductors. The effect of residual Bi2212 and Bi2201 phases on weak links and critical currents of the Bi2223/Ag tapes was investigated. It was found that residual Bi2201 caused weak links at grain boundaries and limited the current-carrying capacity of the tapes. Comparatively, the residual Bi2212 phase had much less influence on both weak links and critical currents. Elimination of Bi2201 by sintering tapes at a low temperature in the final thermal cycle, or by cooling the tapes slowly, increased critical current by a factor of two. Flux pinning property was also improved by removing the residual Bi2201 phase.

Two new effects, the existence of the limiting temperature for constant AC losses with the increase of magnetic field and a node point of minimum AC losses, give a clear evidence of predominant two-dimensional nature of dissipation in Bi 2 Sr 2 Ca 2 Cu 3 O x tapes. The effects are explained in the assumption of exponential suppression of Josephson interation in two types of stacks containing two and three well-connected unit-cells. Superconducting layers of isolated unit-cells are intact in magnetic field which leads to saturation of the temperature of constant losses at non-zero value. The stacks with more than three unit-cells behave in the same way as the ones with three unit cells. Suprression of Josephson interaction in stacks up to separation of superconducting layers, and the increase of eddy current losses in Ag sheath give the node point of AC losses in Ag-sheathed tapes near the temoperature of Kosterlitz-Thouless transition of single superconducting layer.

Manganese-based layered coordination polymer ([Mn(tfbdc)(4,4′-bpy) (H2O)2], Mn-LCP) with microporous structure was synthesized by reaction of 2,3,5,6-tetrafluoroterephthalatic acid(H 2tfbdc) and 4,4′-bipyridine(4,4′-bpy) with manganese(II) acetate tetrahydrate in water solution. Mn-LCP was characterized by elemental analysis, IR spectra, thermogravimetric analysis, X-ray single-crystal structure analysis, and powder X-ray diffraction. Magnetic susceptibility data from 300 to 1.8K show that there is a weak antiferromagnetic exchange between Mn(II) ions in Mn-LCP. As anode material, the Mn-LCP electrode exhibits an irreversible high capacity in the first discharge process and a reversible lithium storage capacity of up to about 390 mA h/g from the fourth cycle. It might provide a new method for finding new electrode materials in lithium-ion batteries © 2013 American Chemical Society.

Ag-sheathed Bi-2223 tapes were prepared by the oxide-powder-in-tube method. Multifilamentary and monofilamentary tapes were sintered with intermediate uniaxial cold pressing. The critical current ( I c ) of the tapes was measured at each step of the sintering, before the pressing. Measurements of I c were performed with the four-probe dc method, with the 1 μV/cm criterion. The samples were submerged in liquid nitrogen and a magnetic field H applied perpendicular to the tape plane. The critical current through the strong links in zero field ( I c0 s ) was distinguished from the total I c using the field dependence of I c . I c0 s was used as a measure of the population of strong links in the samples. It was found that strong links are mainly formed in the late sintering stages. I c 0 s I c 0 increases with sintering time, showing that grain alignment and phase purity are more important for the formation of strong than weak links. Therefore, the preparation of the samples requiring a large I c0 s has to involve large sintering times. Mechanical deformation after Bi-2223 was fully formed is detrimental for the formation of strong links. An increase of I c0 s by a factor 2.6 was obtained with prolonged sintering if the samples are not pressed in the late sintering stages.

An additional broad peak in AC losses of Bi 2 Sr 2 Ca 1 Cu 2 O x tapes is observed. Using a new method based on the model of separated finite-size stacks of superconducting layers it is found that this peak reflects the presence of Bi 2 Sr 2 CuO x intergrowth in the sample. Kosterlitz-Thouless transition temperatures of pure Bi 2 Sr 2 Ca 1 Cu 2 O x and intergrowths of different thickness are defined. Our results are confirmed by the direct TEM measurements.

The critical current densities and microstructures of tapes prepared using various pre-sintering thermomechanical treatments were compared. The phase composition prior to sintering was found to have a strong effect on the rate of 2223 phase growth and texture and, consequently, final transport properties. The difference in optimum requisite tape sintering processes when using powder with the same stoichiometry from different powder processes or even using identical powder obtained from the same source is due to variations in the content and composition of the secondary phases in the calcinated powder and/or the different phase transformations that occur due to different thermomechanical treatments prior to the sintering procedure. The optimum phase composition before sintering is a mixture of the main phase (textured mature Pb-2212 and Pb free 2212) and a small amount of secondary phases consisting of SrCaCuO and with small particle size. BiSrCuO, CuO and CaCuO are all detrimental to final tape performance characteristics and should be minimized in precursor powder. The optimum phase composition can be prepared by appropriate pre-sintering preparation and thermomechanical procedures. The particle size of the oxide in the Ag sheath can be decreased to a large extent by the deformation process. A critical current density of and a high (1 T, 77 K)/ of 23.4% has been achieved reproducibly using short sintering times and one intermediate pressing procedure. The reproducibility is a direct consequence of controlling the phase composition of the oxide core prior to sintering.

MgB 2 polycrystalline bulk samples with addition of 5 and 10 wt.% nano-sized Al 2 O 3 powders were prepared by solid state reaction using pre-reacted MgB 2 . All the samples were sintered at 850 °C for 40 min in Ar. All the samples were characterized by X-ray diffraction, scanning electron microscopy and magnetic measurements. Results show that the critical current density and irreversibility fields decrease significantly with increasing Al 2 O 3 level. J c values decreased significantly by more than one order of magnitude. The T c drops slightly from 37.9 to 36.6 K, but acquires a very wide transition width of more than 20 K. Furthermore, the amount of MgO was found to increase with increasing Al 2 O 3 level, probably indicating that Mg was replaced by Al, with the excess Mg forming extra MgO. The field dependence of J c for all the samples is also presented.

Voltage-current characteristics were measured for Ag-clad (Bi, Pb)2Sr2Ca2Cu3O10 multifilamentary samples at 77 K, with magnetic field H parallel to the c-axis. The data were fitted by V=V0 exp[-(l*/l)mu ], where l is the critical current with no thermal excitations. The parameter mu was obtained from logd(In V)/d/3] versus log l plots, where the data give straight lines for each of the fields. The lines for the fields 30 mT

Polyhedral Fe3O4 nanocrystals with multiple facets are hydrothermally synthesized from an aqueous solution of FeSO4, Fe(NO3)3, and LiOH (autoclave, 180 °C, 2 h).

Smooth modulation structure of Mg–B alloy in the quenched reaction product of Mg and amorphous B was studied. It indicates that the MgB2 formed possibly in spinodal decomposition, thus resulting in MgB2 nanodomains. It was found that the nanodomains with small angle boundaries of atomic-scale width were distributed within the subgrains that constitute the clusters in MgB2 grains. This nanostructural characteristic may be intrinsic in the quenched reaction product of Mg and amorphous B. It makes the nanodomain boundaries not act as barriers to the current percolation path, thus exhibiting no weak-link problem in the MgB2.

Measurements of peak shifts in the derivative of magnetisation with time versus driving field are performed on Co74Fe6B20 amorphous ribbons, carrying an electrical current. Ribbons with a cross-section of 2 × 0.025 mm2 are produced by melt-spinning. Magnetic anisotropy is induced in them prior to the measurements, by annealing at 300 °C in a non-saturated state. Measurements of the peak shift give information on the degree of stabilisation of domain walls with depth in the ribbons. For short annealing times (≈1 min), the stabilisation is inhomogeneous through the thickness; for long annealing times (≈2 h), just a slight inhomogeneity is obtained. From these results it is deduced that relaxation frequencies are highest, but not the same, in the vicinity of the free and the contact ribbon surfaces. It is also shown that inhomogeneous stabilisation of domain walls is induced in samples upon their production.

Measurements of the derivative of magnetization with time (d M /d t ) versus the driving field ( H ) were performed on melt-spun Co 74 Fe 6 B 20 and Fe 20 Ni 40 B 18 Si 2 amorphous ribbons by the induction method. Measurements of the peak shifts in d M /d t versus H with an electrical current ( J ) flowing through the ribbon showed that domain walls in nonmagnetostrictive Co 74 Fe 6 B 20 samples were pinned mainly at the ribbon surfaces. The experimental methods employed can show us which surface contains stronger pinning centres. It was found that domain wall pinning at the ribbon free surface is stronger than the pinning at the contact surface. This excludes surface roughness as the origin of the pinning. Detachment of domain walls from pinning sites is triggered either from the free or from the contact surface, depending on the direction and magnitude of J . This further supports a new model proposed to explain the effects of J on magnetization processes in the ribbon, and shows that the domain walls in our samples are flexible. The triggering of domain walls plays an important role in magnetization processes for our samples. For magnetostrictive Fe 40 Ni 40 B 18 Si 2 samples, the strength of the volume pinning of domain walls is comparable to the strength of the surface pinning.

Studies of superconductivity and flux pinning were carried out on (Bi1.64Pb0.36)Sr2Ca1−xYxCu2O8+y (x=0, 0.05, 0.11, 0.33) single crystals grown by the self-flux method. X-ray diffraction, transport, and magnetic measurements were performed for purposes of characterization. X-ray analysis revealed that the c lattice parameter systemically decreases as the Y doping level increases. The superconducting transition temperature Tc decreases from 80 to 30 K as x increases. A strong annealing effect on Tc and superconducting volume has been observed. Resistance measurements show that x=0.33 samples are semiconductive over a wide temperature range between 4.2 and 300 K for the as-grown state, but become metallic with Tc of 65–70 K after air or oxygen annealing. Flux pinning was studied by measuring the hysteresis loop at different temperatures and different fields. A peak effect was observed in all the codoped samples. Results show that at low temperatures, the peak field is smaller than in solely Pb doped crystal...

Mesoporous α-Fe2O3 materials were prepared in large quantity by the soft template synthesis method using the triblock copolymer surfactant F127 as the template. Nitrogen adsorption-desorption isothermal measurements and transmission electron microscope observation revealed that the as-prepared mesoporous α-Fe2O3 nanostructures have large mesopores in a wide size range of 5-30 nm. It has been found that the Morin transition depends on thermal history of mesoporous α-Fe2O3, which is driven by surface anisotropy. Superparamagnetic behavior of mesoporous α-Fe2O3 is also associated with surface spins with blocking temperature around 50 K. When applied as gas sensors, mesoporous α-Fe2O3 nanostructures exhibited high gas sensitivity toward acetic acid and ethanol gas. As anodes in lithium ion cells, mesoporous α-Fe2O 3 materials show a high specific capacity of 1360 mAh/g with excellent cycling stability and high rate capacity. © 2010 American Chemical Society.

Different forms of biochar can have greatly different properties as fertilisers, yet there is no effective screening method to distinguish them. Terahertz Time Domain Spectroscopy has been used to distinguish various biochars, including Sawdust char, BMC5 with lime char and Saligna char.

Highly ordered mesoporous Co3O4 nanostructures were prepared using KIT-6 and SBA-15 silica as hard templates. The structures were confirmed by small angle X-ray diffraction, high resolution transmission electron microscopy, and N2 adsorption-desorption isotherm analysis. Both KIT-6 cubic and SBA-15 hexagonal mesoporous Co3O4 samples exhibited a low Néel temperature and bulk antiferromagnetic coupling due to geometric confinement of antiferromagnetic order within the nanoparticles. Mesoporous Co3O4 electrode materials have demonstrated the high lithium storage capacity of more than 1200 mAhg -1 with an excellent cycle life. They also exhibited a high specific capacitance of 370 Fg-1 as electrodes in supercapacitors. © 2010 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim.

Measurements of hysteresis loop are performed on straight amorphous ribbons carrying an electrical current (J). The measurements are performed by induction method on Fe60Ni20B18Si2, Fe40Ni40B18Si2, and Co74Fe6B20 alloys. Obtained shift of hysteresis loop with J is for as-quenched samples in opposite direction with opposite J. Initially the shift increases with J, whereas for high J hysteresis loop does not shift further with the current. For annealed samples no shift of hysteresis loop in opposite direction with opposite J is observed. This is in agreement with a new model proposed for explanation of change of hysteresis loop with J. Such shift of hysteresis loop is in the model explained as a consequence of difference in strength of pinning centres for domain walls at the two ribbon faces. These findings may be useful for further improvement of new composite magnetic materials. An geraden, amorphen Bandern der Legierungen Fe60Ni20B18Si2, Fe40Ni40B18Si2, und Co74Fe6B20, die einen elektrischen Strom J fuhren, werden Hystereseschleifen mit der Induktionsmethode gemessen. Bei abgeschreckten Proben verschiebt sich die Hystereseschleife in entgegengesetzte Richtungen, wenn die Stromrichtung umgekehrt wird. Die Verschiebung nimmt bei kleinen J-Werten zu, fur grosere Strome tritt eine Sattigung ein. Fur angelassene Proben wird diese Umkehrung der Verschiebung fur entgegengesetzte Stromrichtungen nicht beobachtet. Das wird durch ein neues Modell erklart, das die Verschiebung der Hystereseschleife aus den verschiedenen Starken der Anhaftzentren fur Domanenwande an den beiden Bandoberflachen erklart. Diese Ergebnisse konnen bei der Verbesserung neuer, zusammengesetzter, magnetischer Materialien von Nutzen sein.