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Magnetic dynamics of iron oxyhydroxide akaganeite nanoparticles, functional building blocks of the newest generation iron deficiency drug Ferinject, has been studied. While magnetism of the involved nanoparticles relies in general on bulk properties of akaganeite, the low temperature magnetic dynamics of Ferinject differs a lot from large number of other iron-oxides-based intravenous drugs in pharmaceutical practice. Clear separation of the high-temperature (Curie) and the low-temperature (spin-freezing) regimes has been identified. Instead of standard single superparamagnetic blocking temperature we show that the spin freezing relies on the two dominant and superimposed relaxations, characterizing the temperature ranges 50–100 K and 4.2–10 K, respectively. Complex magnetic dynamics is interpreted to rely on presence of the two assemblies of nanoparticles differing significantly in their average sizes. Bimodal distribution of particle sizes has been confirmed using atomic force microscopy. The knowledge of the drug’s physicochemical characteristics has been thus significantly improved.

In this work, we present frequency-dependent magnetic susceptibility and dc electric transport properties of three different compositions of hexaferrite Ba1−xPbxFe12−yAlyO19 single crystals. We found a correlation between the activation energies of the dc electric transport and the ac magnetic susceptibility which persists over the whole studied range of aluminum substitution y = 0–3.3. This result is discussed in the context of charged magnetic domain walls, the pinning of which is determined by charge carriers activated across the transport gap. Our work points toward a general relaxation mechanism in ferrimagnetic semiconductors that directly affects the dynamic magnetic properties via electrical transport.

Magnetic ordering of the first row transition metal intercalates of NbS$_2$ due to coupling between the conduction electrons and the intercalated ions has been explained in terms of Fermi surface nesting. We use angle-resolved photoelectron spectroscopy to investigate the Fermi surface topology and the valence band structure of the quasi-two-dimensional layer compounds Mn$_{1/3}$NbS$_2$ and Ni$_{1/3}$NbS$_2$. Charge transfer from the intercalant species to the host layer leads to non-uniform, pocket selective doping of the Fermi surface. The implication of our results on the nesting properties are discussed.

Pure paramelaconite Cu4O3−x has been prepared in the form of nanoparticles with 56 nm in diameter. This mixed valency oxide crystallizes in a tetragonal lattice with 4 unit formulae and forms a pyrochlore structure which manifests in two stoichiometric forms; Cu4O3 and Cu4O2.5, the latter form having two oxygen vacancies per unit cell. Magnetic lattice consists of Cu spin 1/2, and both stoichiometric forms obey transition to the antiferromagnetic state at TN=45–55 K. Defect free Cu4O3 is indicated by an inversion symmetry and exhibits both antiferromagnetic and ferromagnetic state, where the latter is supposedly due to the superexchange interaction in Cu–O(1n)–Cu bonds. An additional magnetic transition was observed in Cu4O3 at T=120 K, probably as a result of an incommensurate ordering. Absence of an inversion symmetry in the oxygen defect Cu4O2.5 results in a long range Dzyaloshinsky–Moriya interaction accompanied by the strong superparamagnetism.

Two new isostructural cobalt selenite halides Co{sub 5}(SeO{sub 3}){sub 4}Cl{sub 2} and Co{sub 5}(SeO{sub 3}){sub 4}Br{sub 2} have been synthesized. They crystallize in the triclinic system space group P-1 with the following lattice parameters for Co{sub 5}(SeO{sub 3}){sub 4}Cl{sub 2}: a=6.4935(8) A, b=7.7288(8) A, c=7.7443(10) A, {alpha}=66.051(11){sup o}, {beta}=73.610(11){sup o}, {gamma}=81.268(9){sup o}, and Z=1. The crystal structures were solved from single-crystal X-ray data, R1=3.73 and 4.03 for Co{sub 5}(SeO{sub 3}){sub 4}Cl{sub 2} and Co{sub 5}(SeO{sub 3}){sub 4}Br{sub 2}, respectively. The new compounds are isostructural to Ni{sub 5}(SeO{sub 3}){sub 4}Br{sub 2}. Magnetic susceptibility measurements on oriented single-crystalline samples show anisotropic response in a broad temperature range. The anisotropic susceptibility is quantitatively interpreted within the zero-field splitting schemes for Co{sup 2+} and Ni{sup 2+} ions. Sharp low-temperature susceptibility features, at T {sub N}=18 and 20 K for Co{sub 5}(SeO{sub 3}){sub 4}Cl{sub 2} and Co{sub 5}(SeO{sub 3}){sub 4}Br{sub 2}, respectively, are ascribed to antiferromagnetic ordering in a minority magnetic subsystem. In isostructural Ni{sub 5}(SeO{sub 3}){sub 4}Br{sub 2} magnetically ordered subsystem represents a majority fraction (T {sub N}=46 K). Nevertheless, anisotropic susceptibility of Ni{sub 5}(SeO{sub 3}){sub 4}Br{sub 2} is dominated at low temperatures by a minority fraction, subject to single-ion anisotropy effects and increasingmore » population of S{sub z} =0 (singlet) ground state of octahedrally coordinated Ni{sup 2+}. - Graphical abstract: Two new iso-structural cobalt selenite halides Co{sub 5}(SeO{sub 3}){sub 4}Cl{sub 2} and Co{sub 5}(SeO{sub 3}){sub 4}Br{sub 2} have been synthesized which are iso-structural to Ni{sub 5}(SeO{sub 3}){sub 4}Br{sub 2}. Magnetic susceptibility measurements on oriented single-crystalline samples show anisotropic response in a broad temperature range, revealing significant single-ion anisotropy effects.« less

The crystal structure of the new compound Co7(TeO3)4Br6 is described together with the magnetic properties. The new compound crystallizes in the monoclinic space group C2/c with the unit cell parameters a = 20.6532(5) &Aring ; ; ; , b = 8.6533(2) &Aring ; ; ; , c = 14.7262(5) &Aring ; ; ; , β = 124.897(3)◦ , Z = 4. The crystal structure was solved from single crystal data, R = 0.0242. The structure can be considered as layered and the building units are [CoO4Br2] and [CoO2Br4] octahedra and [TeO3E] tetrahedra, E being the 5s2 lone pair on the Te(IV) ions. The magnetic properties of the compound are characterised by antiferromagnetic correlations at high temperatures, strong deviation from the Curie– Weiss behaviour in a broad intermediate temperature range and long range magnetic ordering setting-in at low temperatures. Magnetic ordering takes place in stages, being represented by two susceptibility anomalies, at TN = 33 K and TC = 27 K. Themagnetic transition at TC is ferromagnetic (or ferrimagnetic) in character.

We report detailed studies of the ac susceptibility butterfly hysteresis on the RuSr 2 EuCeCu 2 O 10 (Ru1222) ruthenocuprate compound. Two separate contributions to these hysteresis have been identified and studied. One contribution is ferromagnetic-like and is characterized by the coercive field maximum. Another contribution, represented by the so called inverted maximum, is related to the unusual inverted loops, unique feature of Ru1222 butterfly hysteresis. The different nature of the two identified magnetic contributions is proved by the different temperature dependences involved. By lowering the temperature the inverted peak gradually disappears while the coercive field slowly raises. If the maximum dc field for the hysteresis is increased, the size of the inverted part of the butterfly hysteresis monotonously grows while the position of the peak saturates. In reaching saturation exponential field dependence has been demonstrated to take place. At T = 78 K the saturation field is 42 Oe.

We report on magnetic properties of cubic compound Cu3TeO6 studied by $ac$ and $dc$ susceptibility and neutron powder diffraction for the first time. A novel magnetic lattice, three dimensional spin web, composed of almost planar regular hexagons of Cu2+, S =1/2 spins, defines the properties of Cu3TeO6. The behavior of the magnetic susceptibility in the paramagnetic state at approximately 170 K is suggestive for a competition between local anisotropies of Cu2+ hexagons. The resulting frustration is weaker than the antiferromagnetic nearest-neighbor interaction which leads to a collinear (or slightly canted) spin arrangement (k=0,0,0) and formation of magnetic domains below TN=61 K.

We have measured two structurally similar superconducting systems, RuSr2GdCu2O8 and GdBa2Cu3O7 by means of high-resolution ac susceptibility. The real and the imaginary part of ac susceptibility of both bulk-ceramic and powdered samples have been studied down to the very small magnetic-field levels. We show that there are significant differences in the evolution of superconductivity in the two studied superconducting systems. In particular, we show that the superconducting transition in the grains in RuSr2GdCu2O8 system is masked with intrinsic magnetism of complex origin.

The present state of knowledge in the field of layered organic superconductors based on \kappa-(BEDT-TTF) is reviewed. Normal and ground states inside the temperature versus pressure phase diagram are discussed. The question of unconventional versus conventional uperconducting pairing is addressed.

The anisotropic magnetic penetration depth lambda /sub b/(t) and lambda /sub ac/(t), where t=T/T/sub c/, in the layered organic superconductor kappa -(BEDT-TTF)/sub 2/CuN(CN)/sub 2/Br (abbreviated as kappa -ET-Br) are obtained from AC susceptibility data. We find lambda /sub b/(0)=133 mu m, consistent with earlier results and Delta lambda /sub b/(t)= lambda /sub b/(t)- lambda /sub b/(0) varies as t/sup 2/, which is new. Also, assuming lambda /sub ac/(0) approximately=1 mu m, we obtain Delta lambda /sub ac/(t) varies as t. The observed power laws are consistent with the ones expected in d-wave superconductivity.

ac susceptibility measurements are reported for single crystals of the layered organic superconductor $\ensuremath{\kappa}{\ensuremath{-}(\mathrm{B}\mathrm{E}\mathrm{D}\mathrm{T}\ensuremath{-}\mathrm{T}\mathrm{T}\mathrm{F})}_{2}\mathrm{Cu}[\mathrm{N}(\mathrm{CN}{)}_{2}]\mathrm{Br}.$ The value of the out-of-plane penetration depth ${\ensuremath{\lambda}}_{b}(0)$ was found to be 133 $\ensuremath{\mu}\mathrm{m},$ consistent with earlier results. The temperature dependence of the in-plane superfluid density, with the value of in-plane component ${\ensuremath{\lambda}}_{\mathrm{ac}}(0)$ of the order of 1 $\ensuremath{\mu}\mathrm{m},$ is strongly linear in T, whereas the out-of-plane superfluid density varies as ${T}^{2}$ at low temperatures. The observed behavior is fully consistent with the one expected for d-wave superconductivity.

We report on our experimental evidence of a substantial geometrical ingredient characterizing the problem of incipient dissipation in high-${T}_{c}$ superconductors (HTS's): high-resolution studies of differential resistance-current characteristics in the absence of magnetic field enabled us to identify and quantify the fractal dissipative regime inside which the actual current-carrying medium is an object of fractal geometry. The discovery of a fractal regime proves the reality and consistency of a critical-phenomena scenario as a model for dissipation in inhomogeneous and disordered HTS's, gives the experimentally based value of the relevant finite-size scaling exponent, and offers some interesting new guidelines to the problem of pairing mechanisms in HTS's.

Various aspects of the problem of current transfer in high- superconductors (HTSs) are reviewed. The spatial inhomogeneities of various types are identified as a primary cause of non-uniformity of both normal currents and supercurrents in real samples of HTSs. The role these inhomogeneities play in transport features of the samples is discussed. The case of grain boundaries in polycrystalline samples is elaborated in detail. The local structural and transport properties of isolated grain boundaries are first reviewed and then integrated into the knowledge of global (macroscopic) charge transport. The paper emphasizes the common ingredients characterizing the transport in various forms and families of HTS samples in small magnetic fields. The phenomenon of percolation is identified as the most obvious one and is shown to dominate a large number of observations covered by this report. The experimental results focused on by this report elaborate primarily the problems of critical currents, initial dissipation and current-voltage characteristics, penetration depth, resistive and metal-insulator transition, resistance noise and magneto-optical studies of current paths. Various models for current transfer (disordered bonds, brick wall and railway switch) are also reviewed and discussed.

A detailed study of dissipation in weak link networks of high-{ital T}{sub {ital c}} superconductors has been performed. The model for dissipation has been elaborated assuming a general correspondence with the problems of inhomogeneous charge transport. The model treats the basic experimental ingredients, current-voltage ({ital I}-{ital V}) and current-differential resistance ({ital I}-{ital dV}/{ital dI}) characteristics, as a current-induced critical phenomenon. The validity of a simple relationship between non-Ohmic weak link network and Ohmic classical percolation networks has been proved, allowing a quantitative interpretation of {ital I}-{ital V} characteristics. These characteristics are primary determined by dynamical features of transport on percolation networks. The model focuses on both aspects of of the scaling-related power law, its dynamical exponent and its prefactor, and gives specific predictions for magnetic field and temperature dependencies. The experimental results are in full agreement with both predictions. In particular, the experimental values of dynamical scaling exponents ({ital t}=2, {ital s}=0.7) coincide with generally accepted literature values for three-dimensional systems. {copyright} {ital 1996 The American Physical Society.}

Broadening of the resistive transition of sintered ${\mathrm{GdBa}}_{2}$${\mathrm{Cu}}_{3}$${\mathrm{O}}_{7\mathrm{\ensuremath{-}}\mathit{x}}$ superconductor has been studied varying the measuring current over the range of five orders of magnitude. The observed well-defined branching points in the resistive transition coincide with the quasi-Ohmic saturation of V-I curves and scale with the average grain size. The results suggest the validity of a simple percolation model for dissipation in systems whose global superconductivity is realized by Josephson coupling of the superconducting grains. A common origin of structured resistive transition, observed in many superconducting systems, has been also discussed.

V-I characteristics of well characterised Ag-clad BPSCCO tape have been studied in the temperature (T) range 80–106 K and magnetic fields (B) up to 60 mT. The zero field critical current density at 77 K was about 10 000 A cm−2. The variations of Jc with B (B−0.5) and T[( 1 − T T c ) 1.3 ] probably arise from the intragranular flux creep. The magnitude and the variation of the differential resistance (Rƒ) of BPSCCO core with T are consistent with those arising from the flux flow in low B. Very wide distribution of critical currents (CCD) within the core is tentatively ascribed to a broad distribution of the flux pinning energies in BSCCO.

The time dependence of the dc voltage that remains on a sintered ${\mathrm{YBa}}_{2}$${\mathrm{Cu}}_{3}$${\mathrm{O}}_{7\mathrm{\ensuremath{-}}\mathit{x}}$ sample after the application of a magnetic field has been systematically studied in various magnetic fields and at three different temperatures. The obtained logarithmic time dependence has been related to the intragranular flux creep assuming that the dissipative part of the sample, the subsystem of weak links, is controlled by local magnetic fields. A one-dimensional model for the dissipation has been introduced and solved numerically, showing that the inverse of the relaxation rate approaches the intragranular pinning potential in the limit of high applied fields. The experimental values of appropriately defined pinning potentials increases with temperature.

The variation of critical currents and their distributions with thickness has been investigated for two high-quality ${\mathrm{YBa}}_{2}$${\mathrm{Cu}}_{3}$${\mathrm{O}}_{7\mathrm{\ensuremath{-}}\mathit{x}}$ samples with different average grain size (10 and 30 \ensuremath{\mu}m for samples ${\mathrm{S}}_{1}$ and ${\mathrm{S}}_{2}$, respectively) in the temperature range 78\char21{}90 K and in the applied magnetic field H50 Oe. The critical-current density (${\mathit{J}}_{\mathit{c}}$) for ${\mathrm{S}}_{1}$ initially increased but later on leveled off on reducing the thickness, whereas for ${\mathrm{S}}_{2}$ remained essentially unchanged even after threefold reduction in thickness. Since the other parameters related to macroscopic homogeneity have not changed on reducing the thickness of the samples, the variations of ${\mathit{J}}_{\mathit{c}}$ are interpreted in terms of thickness and grain-size-dependent self-field effects. The same model explains well the changes of critical-current distribution curves with thickness and may also explain the variation of ${\mathit{J}}_{\mathit{c}}$ with the grain size, as reported recently for ceramic Y-Ba-Cu-O samples.

s74.30C 74.70V74.70574.70Y Some diffusive properties of oxygenin Y-Ba-Cu-O systems D. Djurek(~), V. Manojlov16(~), Z.Medun16 (1), M. Prester(2), E. Bab16(3) and K.Zadro(3) (1) Volta Applied Ceramics, 41000 Zagreb, Croatia, Yugoslavia (2) Instituteof Physics oftile UniversityZagreb, 41000 Croatia, Yugoslavia (3) Department of Physics,Faculty Natural Sciences,41000 Zagreb,Croatia,Yugoslavia(Received 21 August 1991, accepted 4October I991) Abstract.Wehavestudiedtilediffusive properties ofoxygeninY-Ba-Cu-O systems at 940°C by measurements oftileresistiveresponsetime r the samplessubjected to step-lflce increaseoftileoxygenpressure Apo~~ I to 5 bars. The superconducting(SC) transitionwidtlis AT~ plotted versus inverse diffusion constantD r obey the universal linear dependence. Thecharacteristic protruded tailofthereal part of AC susceptibility whichexhibitsthefeature at 86Kisabsentin samples withthenominal compositionY~Ba~CU~O~(Y-336) the regime r < 3 sas resultof weakly connected

Pronounced anisotropy of magnetic properties and complex magnetic order of a new oxi-halide compound Co-7(TeO3)(4)Br-6 has been investigated by powder and single-crystal neutron-diffraction, magnetization, and ac susceptibility techniques. Anisotropy of susceptibility extends far into the paramagnetic temperature range. A principal source of anisotropy are anisotropic properties of the involved octahedrally coordinated single Co2+ ions, as confirmed by angular-overlap-model calculations presented in this work. Incommensurate antiferromagnetic order sets in at T-N=34 K. The propagation vector is strongly temperature dependent reaching k(1)=[0.9458(6),0,0.6026(5)] at 30 K. A transition to a ferrimagnetic structure with k(2)=0 takes place at T-C=27 K. The magnetically ordered phase is characterized by very unusual anisotropy as well: while M-H scans along the b axis reveal spectacularly rectangular but otherwise standard ferromagnetic hysteresis loops, M-H studies along other two principal axes are perfectly reversible, revealing very sharp spin-flop (or spin-flip) transitions, such as in a standard antiferromagnet (or metamagnet). Altogether, the observed magnetic phenomenology is interpreted as an evidence of competing magnetic interactions permeating the system, in particular of the single-ion anisotropy energy and the exchange interactions. Different coordinations of the Co2+ ions involved in the low-symmetry C2/c structure of Co-7(TeO3)(4)Br-6 render the exchange-interaction network very complex by itself. Temperature-dependent changes in the magnetic structure, together with an abrupt emergence of a ferromagnetic component, are ascribed to continual spin reorientations described by a multicomponent, but yet unknown, spin Hamiltonian.

An inelastic neutron scattering study of Cu2Te2O5X2 (X=Cl, Br) shows strong dispersive modes with large energy gaps persisting far above T-N, notably in Cu2Te2O5Br2. The anomalous features: a coexisting unusually weak Goldstone-like mode observed in Cu2Te2O5Cl2 and the size of the energy gaps cannot be explained by existing theories, such as our mean-field or random-phase approximation. We argue that our findings represent a new general type of behavior due to intercluster quantum fluctuations and call for development of a new theoretical approach.

V-I curves of ten polycrystalline YBa2Cu3O7−x samples have been measured over a broad current range (

{ital V}-{ital I} curves of several Y-Ba-Cu-O samples have been measured in the temperature range 77--91 K in weak magnetic fields ({ital H}{lt}150 Oe). For the well prepared low-resistivity samples {ital R}{sub {ital f}}=={ital dV}/{ital dI} reaches a value of about 0.1{ital R}{sub {ital n}} ({ital R}{sub {ital n}} is the normal resistance just above transition) in the linear {ital V}-{ital I} regime. This is consistent with a picture of decoupled superconducting grains and does not yield any direct information about the flux-flow effects. Because of self-field effects, critical-current density ({ital J}{sub {ital c}}) obtained from transport (and magnetization) measurements does not provide clear insight on the nature of intergrain coupling. Information on the distribution of critical currents, obtained from {ital d}{sup 2}{ital V}/{ital dI}{sup 2}, supports the tunneling and/or proximity-effect coupling between the grains and seems to rule out large enhancements of {ital J}{sub {ital c}} in sintered samples with nonoriented grains.

Two new isostructural cobalt selenite halides Co{sub 5}(SeO{sub 3}){sub 4}Cl{sub 2} and Co{sub 5}(SeO{sub 3}){sub 4}Br{sub 2} have been synthesized. They crystallize in the triclinic system space group P-1 with the following lattice parameters for Co{sub 5}(SeO{sub 3}){sub 4}Cl{sub 2}: a=6.4935(8) A, b=7.7288(8) A, c=7.7443(10) A, {alpha}=66.051(11){sup o}, {beta}=73.610(11){sup o}, {gamma}=81.268(9){sup o}, and Z=1. The crystal structures were solved from single-crystal X-ray data, R1=3.73 and 4.03 for Co{sub 5}(SeO{sub 3}){sub 4}Cl{sub 2} and Co{sub 5}(SeO{sub 3}){sub 4}Br{sub 2}, respectively. The new compounds are isostructural to Ni{sub 5}(SeO{sub 3}){sub 4}Br{sub 2}. Magnetic susceptibility measurements on oriented single-crystalline samples show anisotropic response in a broad temperature range. The anisotropic susceptibility is quantitatively interpreted within the zero-field splitting schemes for Co{sup 2+} and Ni{sup 2+} ions. Sharp low-temperature susceptibility features, at T {sub N}=18 and 20 K for Co{sub 5}(SeO{sub 3}){sub 4}Cl{sub 2} and Co{sub 5}(SeO{sub 3}){sub 4}Br{sub 2}, respectively, are ascribed to antiferromagnetic ordering in a minority magnetic subsystem. In isostructural Ni{sub 5}(SeO{sub 3}){sub 4}Br{sub 2} magnetically ordered subsystem represents a majority fraction (T {sub N}=46 K). Nevertheless, anisotropic susceptibility of Ni{sub 5}(SeO{sub 3}){sub 4}Br{sub 2} is dominated at low temperatures by a minority fraction, subject to single-ion anisotropy effects and increasingmore » population of S{sub z} =0 (singlet) ground state of octahedrally coordinated Ni{sup 2+}. - Graphical abstract: Two new iso-structural cobalt selenite halides Co{sub 5}(SeO{sub 3}){sub 4}Cl{sub 2} and Co{sub 5}(SeO{sub 3}){sub 4}Br{sub 2} have been synthesized which are iso-structural to Ni{sub 5}(SeO{sub 3}){sub 4}Br{sub 2}. Magnetic susceptibility measurements on oriented single-crystalline samples show anisotropic response in a broad temperature range, revealing significant single-ion anisotropy effects.« less

Two new cobalt tellurite halides Co5(TeO3)4Cl2 and Co5(TeO3)4Br2 have been synthesized and found to be iso-structural with Ni5(TeO3)4X2 (X=Cl, Br). Co5(TeO3)4X2 crystallizes in the monoclinic system space group C2/c, and the Br-phase has the lattice parameters a=20.440(1) A, b=5.2760(2) A, c=16.4710(7) A, beta=124.790(5) degrees, and Z=4. The crystal structures were solved from single-crystal Xray data, R1=1.90 and 1.77, respectively, for the Cl- and Br-phases. The crystal structure is layered with only weak van der Waals’ interactions in between the layers. The layers are built by large [Co5O16X2] groups consisting of five edge- and face-sharing Co-octahedra. Each group is connected to adjacent groups via corner sharing through common oxygen atoms as well as through [TeO3E] groups. Magnetic susceptibility measurements on oriented single crystals reveal pronounced anisotropy in a broad temperature range and clear signs of antiferromagnetic ordering at low temperatures. Anisotropic susceptibility of an iso-structural Ni-based compound was also studied and compared with the corresponding results of Co5(TeO3)4X2. Magnetic anisotropy is discussed in framework of single-ion anisotropy effects.

Polarized and unpolarized neutron-diffraction studies have been carried out on single crystals of the coupled spin tetrahedra systems Cu2Te2O5X2 (X=Cl, Br). A model of the magnetic structure associated with the propagation vectors k(') (Cl) approximate to (-0.150,0.422, (1)/(2)) and k'(Br) approximate to (-0.172,0.356, (1)/(2)) and stable below T-N = 18 K for X= Cl and T-N = 11 K for X = Br is proposed. A feature of the model, common to both the bromide and chloride, is a canted coplanar motif for the four Cu2+ spins on each tetrahedron which rotates on a helix from cell to cell following the propagation vector. The Cu2+ magnetic moment determined for X = Br, 0.395(5)mu(B), is significantly less than for X= Cl, 0.88 (1)mu(B) at 2 K. The magnetic structure of the chloride associated with the wave vector k' differs ;from that determined previously for the wave vector k approximate to (0.150,0.422, (1)/(2))

The crystal structures of the two new synthetic compounds Co{sub 2}TeO{sub 3}Cl{sub 2} and Co{sub 2}TeO{sub 3}Br{sub 2} are described together with their magnetic properties. Co{sub 2}TeO{sub 3}Cl{sub 2} crystallize in the monoclinic space group P2{sub 1}/m with unit cell parameters a=5.0472(6) A, b=6.6325(9) A, c=8.3452(10) A, {beta}=105.43(1){sup o}, Z=2. Co{sub 2}TeO{sub 3}Br{sub 2} crystallize in the orthorhombic space group Pccn with unit cell parameters a=10.5180(7) A, b=15.8629(9) A, c=7.7732(5) A, Z=8. The crystal structures were solved from single crystal data, R=0.0328 and 0.0412, respectively. Both compounds are layered with only weak interactions in between the layers. The compound Co{sub 2}TeO{sub 3}Cl{sub 2} has [CoO{sub 4}Cl{sub 2}] and [CoO{sub 3}Cl{sub 3}] octahedra while Co{sub 2}TeO{sub 3}Br{sub 2} has [CoO{sub 2}Br{sub 2}] tetrahedra and [CoO{sub 4}Br{sub 2}] octahedra. The Te(IV) atoms are tetrahedrally [TeO{sub 3}E] coordinated in both compounds taking the 5s {sup 2} lone electron pair E into account. The magnetic properties of the compounds are characterized predominantly by long-range antiferromagnetic ordering below 30 K.

High-resolution ac susceptibility and thermal conductivity measurement on ${\mathrm{Cu}}_{2}{\mathrm{Te}}_{2}{\mathrm{O}}_{5}{X}_{2}(X=\mathrm{Br},\mathrm{Cl})$ single crystals are reported. For Br-sample, sample dependence prevents one from distinguishing between possibilities of magnetically ordered and spin-singlet ground states. In Cl-sample a three-dimensional transition at 18.5 K is accompanied by almost isotropic behavior of susceptibility and almost switching behavior of thermal conductivity. Thermal conductivity studies suggest the presence of a tremendous spin-lattice coupling characterizing Cl- but not Br-sample. Below the transition Cl-sample is in a complex magnetic state involving AF order but also the elements consistent with the presence of a gap in the excitation spectrum.

We overview the state of knowledge regarding the symmetry of the superconducting order parameter in layered organic superconductors based on • -(BEDT-TTF). A remnant intrinsic disorder at low temperatures in nominally pure single crystals emerges as one of decisive key ingredients that influences the value and the temperature dependence of the penetration depth, while leaving the transition temperature unchanged. However, the microscopic origin of this influence remains still unclear.

We report high-sensitivity ac susceptibility measurements of the penetration depth in the Meissner state of the layered organic superconductor $\ensuremath{\kappa}\ensuremath{-}(\mathrm{B}\mathrm{E}\mathrm{D}\mathrm{T}\ensuremath{-}\mathrm{T}\mathrm{T}\mathrm{F}{)}_{2}\mathrm{Cu}[\mathrm{N}(\mathrm{CN}{)}_{2}]\mathrm{Br}.$ We have studied nominally pure single crystals from the two different syntheses and employed controlled cooling procedures in order to minimize intrinsic remnant disorder at low temperatures associated with the glass transition, caused by ordering of the ethylene moieties in BEDT-TTF molecule at ${T}_{\mathrm{G}}=75\mathrm{K}.$ We find that the optimal cooling procedures (slow cooling of $\ensuremath{-}0.2\mathrm{K}/\mathrm{h}$ or annealing for 3 days in the region of ${T}_{\mathrm{G}})$ needed to establish the ground state depend critically on the sample origin, indicating different relaxation times of terminal ethylene groups. We show that, in the ground state, the behavior observed for nominally pure single crystals from both syntheses is consistent with unconventional d-wave order parameter. The in-plane penetration depth ${\ensuremath{\lambda}}_{\mathrm{in}}(T)$ is strongly linear, whereas the out-of-plane component ${\ensuremath{\lambda}}_{\mathrm{out}}(T)$ varies as ${T}^{2}.$ In contrast, the behavior of single crystals with long relaxation times observed after slow $(\ensuremath{-}0.2\mathrm{K}/\mathrm{h})$ cooling is as expected for a d-wave superconductor with impurities [i.e., ${\ensuremath{\lambda}}_{\mathrm{in}}(T)\ensuremath{\propto}{\ensuremath{\lambda}}_{\mathrm{out}}(T)\ensuremath{\propto}{T}^{2}]$ or might be also reasonably well described by the s-wave model. Our results might reconcile the contradictory findings previously reported by different authors.

We report ac susceptibility data on RuSr2(Eu,Ce)(2)Cu2O10-y (Ru-1222, Ce content x=0.5 and 1.0), RuSr2GdCu2O8 (Ru-1212), and SrRuO3. Both Ru-1222 (x=0.5, 1.0) sample types exhibit unexpected magnetic dynamics in low magnetic fields: logarithmic time relaxation, switching behavior, and "inverted" hysteresis loops. Neither Ru-1212 nor SrRuO3 exhibit such magnetic dynamics. The results are interpreted as evidence of the complex magnetic order in Ru-1222. We propose a specific multilayer model to explain the data, and note that superconductivity in the ruthenocuprate is compatible with both the presence and absence of the magnetic dynamics.

We measured polycrystalline samples of SrRuO3 and both superconducting and nonsuperconducting (hydrogen loaded) ruthenocuprates-Eu1.5Ce0.5RuSr2Cu2O10-x (Ru-1222)-samples using ac susceptibility, Ru 3p x-ray photoemission and Ru 2p x-ray-absorption measurements. The superconducting-and metallic-ruthenocuprate samples exhibit significant ferromagnetic fluctuations above the highest magnetic ordering temperature. The (Ru 3p) core level of the SrRuO3, and superconducting ruthenocuprate samples indicate that the (Ru) layers are metallic.

Measurements of AC magnetic susceptibility in kappa -(BEDT-TTF)/sub 2/CuN(CN)/sub 2/Br show that the b-axis penetration depth below T/sub c/ exhibits behaviour different from that observed in the ac plane, which exhibits the T-linear dependence. In agreement with earlier measurements, a leading quadratic temperature dependence is found at low temperatures. The temperature dependence of the out-of-plane superfluid density is shown to best agree with a model of d-wave superconducting layers coupled by coherent Josephson tunneling.

We summarize the main features of the magnetic field-temperature phase diagram for the superconducting state of κ-(BEDTTTF) 2 Cu[N(CN) 2 ]Br. The initial temperature dependence of the real component of ac susceptibility for field parallel and perpendicular to the ac plane are markedly different. The out-of-plane penetration depth λ b and the related superfluid density vary at low tempeartures as T 1 . Slow flux dynamics below the irreversibility line for the ac field perpendicular to the BEDT-TTF planes might be ascribed to a vortex-pair creation process. The characteristic relaxation time τ >10 −4 s is thermally activated with an activation energy of about 300K.

The onset of dissipation in thin, tape shaped form of high-Tc superconductors has been studied by the use of differential resistance characteristics in absence or in small magnetic fields. The studies reveal two characteristic currents: the onset current defining a sharp onset of dissipation and the critical current, a parameter of power-law dependencies. A broad range of intermediate currents is characterized by an anomalous saturation at the very low level of dissipation. The observations are interpreted inside the percolation model for the onset of dissipation as a crossover from the range dominated by finiteness of the sample on the scale of current-dependent correlation length to the range of power laws that corresponds to scaling regime of critical phenomena.© (1998) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.

Method in which lock-in detectors process signals from standard ac susceptometers, particularly in the detection of higher harmonics, is analyzed in details. The exact formulas have been derived and checked experimentally by measurements on soft ferromagnetic sample, using several available lock-in amplifiers. The reasons why the proper phase adjustment has to be implemented in the protocol of higher harmonics measurements have been elaborated. The procedure of the lock-in phase adjustment is described, enabling separation of Fourier or Taylor components of hysteretic ac susceptibility into real and imaginary sectors.

Universal aspects of inhomogeneous transport was studied in natural Josephson junction networks. The weak link networks of polycrystalline high-Tc superconductors were found perfectly suitable for quantitative investigation of cluster growth phenomena in percolation (or fractal) networks. We report the experimental results of our studies of I-V characteristics of the two high-Tc families, YBCO and BSCCO. The onset of dissipation, represented by these characteristics, is interpreted as a critical phenomenon, i.e., as a current-induced phase transition. The cluster growth which underlies this phase transition is analyzed by the use of an appropriate model. The model links the non- Ohmic weak link network with classical Ohmic percolation networks (e.g., random resistor network). As a result, we were able to determine the values of appropriate dynamical exponents (exponent t with high precision, t equals 2 +/- 0.1, and exponent s approximately, s approximately equals 0.7), as well as to assign the dissipative ranges inside which the corresponding cluster dynamics takes place. It is therefore concluded that the high-Tc weak link networks represent a natural mesoscopic-scale system convenient for systematic investigation of the transport critical phenomena.© (1996) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.

Initial dissipation of weak-link-diminated high-T c superconductors was investigated by systematic studies of I-V characteristics at different temperatures and in small magnetic fields. We propose a model for dissipation which focuses the quasi-Ohmic local dissipation at grain boundary Josephson junctions as an elemental dissipative process. According to this model the overall dissipation is organized as a global percolation in close relation with the problem of conduction in electrically inhomogeneous networks. As an experimental support we present the measured I-V characteristics which strictly obey the power-law behaviour above well-defined critical current, i.e., percolation threshold concentration. The experimental values of temperature and magnetic field independent power-law exponent strongly support its relation to conductivity exponent of percolation theory.

V-I characteristics of granular YBa2Cu3O7−δ samples with very different grain size and thickness have been studied in the temperature range 65–85K and magnetic fields up to 35 Oe. The results present clear evidence of percolation effects in a 3D network of weakly linked superconducting grains. The onset of dissipation follows a V ∝ (I − Ic)n power law n assumes values between 2 and 2.6 and does not depend on microstructure or thickness. It shows a very weak dependence, if any, on the external magnetic field, H, and temperature, T. This suggests that the shape of the low current side of the distribution of critical currents of the links is also quite insensitive to H and T. It also shows that the transition to the coherent state takes place at T > 85K. Peculiar percolation effects have been observed when the thickness of the sample approaches the grain size. V ∝ (I − Ic)n onsets of disspation with exponent n practically the same as in other granular superconductorcs (both conventional and high temperature ones) present clear evidence for the percolation effects in 3D network of intergrain links.

The crystal structure of a new modification of NiSeO3 is described together with its magnetic properties. The new polymorph crystallizes in the monoclinic system, space group C 2/c, with unit cell parameters a = 15.4965(7) A, b = 9.8250(4) A, c = 14.7278(8) A, β = 110.610(4)° and Z = 8. The crystal structure was solved from single crystal data, R = 0.0252. The structure can be considered as being layered along the bc-plane with [NiO6] octahedra arranged into two sub-groups. The layers are further connected to adjacent layers via groups of two edge-sharing [NiO6] octahedra. The Ni octahedra are tilted and are either edge or corner sharing, giving rise to two different Ni–Ni interactions. The lone pairs of Se4+ occupy non-bonding regions of tunnels along [001] and [010]. The complexity of the crystal structure is also reflected in the complex low-temperature magnetic state. An antiferromagnetic-like transition was observed at Ttr = 14.4 K. A possible structural rearrangement closely above Ttr complicates the details of the low-temperature state. The antiferromagnetic-like transition disagrees with the features of a three-dimensional (3D) Neel antiferromagnetic ordering. This is mostly because of the fact that the assumed bulk easy-axis susceptibility at the lowest temperature remains at high paramagnetic value. The consequences of the complex structure on basic magnetic parameters, zero-field splitting energy D and exchange coupling J, and accordingly on the magnetic properties, are discussed.

We present an experimental method enabling measurements of time dependence of ac susceptibility down to 1 ms after a steplike change of the applied dc magnetic field. An analog-digital converter has been used to collect the transient of the current, signaling the change of the dc magnetic field. The time window for investigation of magnetic time relaxation phenomena, as studied by the ac susceptibility technique, thus has been expanded in the range of short time scales for three orders of magnitudes.

The onset of dissipation has been studied in sintered (BiPb) 2 Sr 2 Ca 2 Cu 3 O 10+x samples by investigating I–V characteristics in small applied magnetic fields. The observed universal features, similar to those of vortex glass model, have been attributed and quantitatively related to critical exponents of percolation theory.

V-I characteristics of Ag-clad BPSCCO tape have been studied in the temperature (T) range 80–106K and magnetic fields (B) up to 60 mT. The zero-field J c at 77K is 10100 A/cm 2 . J c showed power law variations with T and B, characteristics for the intragranular flux creep. The magnitude and variations of the slope resistance R f with T and B are consistent with the flux flow in low fields. Very broad and featureless critical current distribution function (CCD) is ascribed to the variation in the flux pinning energies within BPSCCO.

Note: Kursumovic, A Cirm,Energoinvest,Tvornicka 3,Sarajevo,Yugoslavia Fac Sci Zagreb,Dept Phys,Zagreb,Yugoslavia Univ Zagreb,Inst Phys,Yu-41001 Zagreb,YugoslaviaCd783Times Cited:4Cited References Count:13 Reference LC-ARTICLE-1989-001 Record created on 2006-08-21, modified on 2017-05-10

Microstructural behaviour and microhardness of the La 2− x Sr x CuO 4 alloy system was studied using microhardness testing and observations with light and scanning-electron microscopes. Two possible processes for the grain growth in the course of sintering were observed. Primary recrystallization is operative for strontium content x x > 0.10. Consequently, the threshold composition is about x = 0.10. For the latter case ( x > 0.10) unusually large grains appear, with a frequency distribution similar to the expected log-normal distribution. The microhardness shows a sharp minimum near x = 0.06. The influence of the microstructure on the measured transport properties is also discussed, particularly the treatment-induced superconductivity of the parent compound.

The AC susceptibility and magnetisation were used to study the inter- and intragrain critical currents in a dense, low resistivity YBa2Cu3O7 ceramic in the temperature range 75-90 K. It is found that the magnetic-field dependence of the peaks in the imaginary part of the AC susceptibility can be used to determine both the inter- and intragrain critical currents. The relation between the field dependence of the intergrain peak in the imaginary susceptibility and the intergrain critical current has been verified by measuring the critical transport currents for two samples at 77.3 K. The magnetic-field dependences of the critical currents, lower critical fields and the flux penetration are briefly discussed.

Dielectric function of quasi-one-dimensional conductor NbSe3 has been measued in the vicinity of Peierls transition near Tp=145K by use of novel technique. The low frequency noise in V/I/ characteristics is attributed to the Gaussian type of position fluctuations of pinned charge density wave (CDW) and fundamental frequency of the noise is found to be identical with spring frequency of restoring forces.