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The temperature dependent neutron powder diffraction (NPD) and magnetization measurements of vanadium based garnet Ca 5 Ni 4 V 6 O 24 (CNVO) have been performed to explore its crystal and magnetic structures. The magnetization results illustrate two magnetic anomalies at 7 and 4 K. The Rietveld analysis of room temperature x-ray diffraction and NPD data confirms its Ia -3 d crystal structure. The temperature dependent NPD shows the emergence of magnetic reflections below 7 K whose intensity keeps on increasing with decreasing temperature down to 1.5 K. The crystal structure remains cubic down to 1.5 K. Rietveld analysis reveals that below 7 K, the Ni moments in CNVO undergo a commensurate collinear A-type antiferromagnetic ordering with propagation vector k =(0,0,0) and the per site ordered moment of Ni 2+ is 1.69±0.05 μ B . Absence of any additional magnetic or nuclear reflections below 4 K confirms that the nuclear and magnetic structures remain invariant across the 4 K magnetic anomaly.

A three-directional configuration of columnar defects has been induced in a Bi2Sr2CaCu2O8 single crystal by irradiation with heavy ions of high energy. Persistent current densities have been extracted, using the Bean model, from hysteresis loops recorded in the orientation H||c. We have shown that improvements in pinning properties are larger in this three-directional splayed configuration than in the one obtained with columnar defects parallel to the c-axis. This effect exists only for H larger than HΦ, where HΦ is the matching field, and disappears as temperature is increased and vortices become less stiff. This is the first time that such a beneficial effect is reported for a compound of such a high electronic anisotropy.

ZusammenfassungRezidivvarizen an der saphenofemoralen Mün-dung (SFJ) nach Crossektomie und Stripping der Stammvene sind ein bekanntes Problem. Unter Rezidivvarikose werden Krampfadern verstan-den, die in einem zuvor behandelten Stromgebiet auftreten. Das Auftreten einer Rezidivvarikose der Stammvene nach Behandlung mit dem Radiofrequenzverfahren und deren his-topathologischen Muster sind jedoch unklar. Wir zeigen an einem Fallbericht das histopathologische Muster an einer exzidierten und reperfundierten Stammvene. Der 66-jährige Patient wurde 2007 mit Radiofrequenzablation (VNUS Closure-Plus Verfahren®) an der linken Stammvene (V. saphena magna) behandelt. Ein Jahr danach zeigten sich erneut Seitenastvarizen an der linken unteren Extremität. Duplexsonographisch zeigte sich die Vene an der SFJ rekanalisiert und dilatiert. Es erfolgte eine Crossektomie, Teilsaphenektomie und Seitenastexhairese. Dabei wurde der proximale Anteil der VSM histologisch beurteilt. Neben einer Hämatoxylin/Eosinund Elastica-van-Gieson-Färbung, wurden immunhistochemische Färbungen mit Antikörper gegen α-sma und CD 31 durchgeführt. Der Fall-bericht zeigt, dass im Anschluss einer Radiofrequenzablation ein multiluminaler Rekanalisierungsprozess mit ausgekleideten Endothel auftreten kann. Unklar ist jedoch, welcher Mechanismus und welche Faktoren zur Wiedereröffnung von behandelten Venen führen.

The effect of substitution at the Fe site in the CuFeO2 delafossite is known to induce a magnetic structure responsible for the appearance of electric polarization. A CuFe1−xRhxO2 series of ceramic samples is studied in order to determine the composition range exhibiting such a polar state. It is found that for the CuFe1−xRhxO2 (x≤0.15) solid solution, the Neel temperature TN2 decreases monotonously with x from 11.5 K to 5.9 K, for x=0.00 to 0.15, respectively, and that the dielectric peak and the polarization transition temperatures coincide with TN2. In contrast, the dielectric peak and polarization magnitudes go through an optimum for CuFe0.92Rh0.08O2 (x=0.08). These results demonstrate that, compared to other substituting elements, the Rh3+ for Fe3+ substitution in CuFeO2 allows to extend significantly the ferroelectric region in the (x, T) phase diagram in connection with the slower TN2 decrease.

We observe a seemingly complex magnetic field dependence of the dielectric constant of hexagonal YbMnO3 near the spin ordering temperature. After rescaling, the data taken at different temperatures and magnetic fields collapse on a single curve describing the sharp anomaly in nonlinear magnetoelectric response at the magnetic transition. We show that this anomaly is a result of the competition between two magnetic phases. The scaling and the shape of the anomaly are explained using the phenomenological Landau description of the competing phases in hexagonal manganites. AB - We observe a seemingly complex magnetic field dependence of the dielectric constant of hexagonal YbMnO3 near the spin ordering temperature. After rescaling, the data taken at different temperatures and magnetic fields collapse on a single curve describing the sharp anomaly in nonlinear magnetoelectric response at the magnetic transition. We show that this anomaly is a result of the competition between two magnetic phases. The scaling and the shape of the anomaly are explained using the phenomenological Landau description of the competing phases in hexagonal manganites.

Magnetoelectric coupling in the polycrystalline antiferromagnets CuFe0.95Rh0.05O2 and CuFeO2 has been investigated. For both samples, electric polarization was observed in the absence of an applied external magnetic field demonstrating that for multiferroic research ceramics are worth to be studied. The observed magnetodielectric effect for CuFe0.95Rh0.05O2 in the electrically polar phase supports the existence of a noncollinear antiferromagnetic state. Interestingly, the electric polarization of this sample can be suppressed by a magnetic field. The temperature dependence of the relative magnitude of the magnetodielectric effect shows a discontinuity, clearly indicating different mechanisms of the magnetodielectric couplings in polar and paraelectric antiferromagnetic states., Comment: 6 pages, 5 figures, journal paper, Solid State Sci.(2009)

The investigation of the ordered oxygen-deficient perovskites LnBaCo 2 O 5.50+δ with Ln = Eu and Sm shows that a small deviation from the "O 5.50 " stoichiometry influences dramatically the magnetic properties of this structural type. Three ferromagnetic (FM) states can be obtained, depending on the oxygen content, 6. For a cobalt valency close to +3 (δ ∼ 0) a ferromagnetic ordering FM1 takes place around 250 K, previously observed for Ln = Gd, Tb cobaltites. For 6 +0.10) a new ferromagnetic state FM3 appears, involving Co 3+ -O-Co 4+ superexchange interactions.

Multiferroics are materials which display simultaneously ferroelectricity and ferromagnetism and possess the multiferroic coupling. However, availability of these materials are rare. Thus, designing the materials with these extraordinary properties is a big challenge. In the present paper, we address the basic question as to how one can design the artificial multiferroics by an alternative technique, namely the superlattice approach. We show such a possibility by taking the examples of our work on the superlattices composed of the ultrathin layers of La 0.7 Ca 0.3 MnO 3 and BaTiO 3 . Films were grown on SrTiO 3 at 720 °C and 100 mTorr O 2 partial pressure by pulsed laser deposition technique. Films are characterized by the various techniques for their structural and physical properties. The origin of the multiferroism in these structures will also be described.

Miniaturization of the electronic devices has prompted the search for new materials with multifunctional properties. In this regard, multiferroics have distinct advantages as multifunctional materials because they possess for instance both ferroelectricity and ferromagnetism. Therefore, in addition to the fundamental interest, these “smart” materials, which are able to react to external stimuli or signals, are very attractive for a wide range of potential applications, e.g., in information storage or spintronics. However, very few multiferroics are known to the humankind. In this article, we report the synthesis of artificial multiferroics using the superlattices approach by pulsed laser deposition (PLD) technique. A series of films were grown by combining the magnetic layers, e.g., Pr0.85Ca0.15MnO3 or La0.7Ca0.3MnO3, and ferroelectric (FE) layers like (Ba1−x Sr x )TiO3 with different thicknesses. The magneto-electric and multiferroic (magnetic and dielectric) properties of these materials have been inves...

The present paper deals with the evidence of an unprecedented ordering in the Bi-rich part of the Bi Ca Mn O diagram, where the spin glass state is observed at low temperature. The χ −1 ( T ) curves of three compounds, corresponding to different Ca/Bi + Ca ratio varying between 0.37 and 0.25, have been characterized and discussed in correlation with the structural transitions as a function of T. Three different behaviours have been observed for Bi0.63Ca0.37MnO3, Bi0.67Ca0.33MnO3 and Bi0.71Ca0.24MnO3−δ. The anomalies in the χ −1 ( T ) curves of Bi0.71Ca0.24MnO3−δ and Bi0.63Ca0.37MnO3 appear to be associated to two different mechanisms, which are not observed in the intermediate Bi0.67Ca0.33MnO3. For the Bi-rich compound, an unprecedented [111]p-type ordering of the perovskite cell is reported, which is associated to a change of the slope in the magnetic curve. Bi0.71Ca0.24MnO3−δ appears at the convergence point of a complex competition between the complementary lone pair ordering/orbital ordering of the ferromagnetic BiMnO3 and the charge ordering/orbital ordering of the pseudo CE antiferro-magnetic Bi0.63Ca0.37MnO3.

We report measurements and analysis of the voltage noise due the to vortex motion, performed in superconducting Niobium micro-bridges. Noise in such small systems exhibits important changes from the behavior commonly reported in macroscopic samples. In the low biasing current regime, the voltage fluctuations are shown to deviate substantially from the Gaussian behavior which is systematically observed at higher currents in the so called flux-flow regime. The responsibility of the spatial inhomogeneities of the critical current in this deviation from Gaussian behavior is emphasized. We also report on the first investigation of the effect of an artificial pinning array on the voltage noise statistics. It is shown that the fluctuations can lose their stationarity, and exhibit a Levy flight-like behavior., accepted for publication in Fluctuations and noise letters

The study of the “112” ordered oxygen-deficient perovskites EuBaCo2O5.43, EuBaCo1.92Zn0.08O5.45, and EuBaCo1.92Cu0.08O5.40 evidences a magnetic transition at 40 K which may be due to the ordering of europium moments. Below 40 K, one observes phase separation that consists of superparamagnetic droplets and ferromagnetic (FM) regions embedded in an antiferromagnetic matrix. The existence of two sorts of crystallographic sites for cobalt, and its trivalent state, suggest that these phenomena correspond to ferromagnetism induced by the presence of intermediate spin Co3+ species. Moreover, the magnetization hysteresis loops of the Zn- and Cu-doped phases exhibit ultrasharp magnetization multisteps, which are interpreted by the pinning effect of Zn2+ and Cu2+ cations at the narrow domain walls of the FM phase.

Nanoparticles of the ferrimagnetic epsilon-Fe2O3 oxide have been synthesized by sol-gel method. Here, we report on the measurements of the dielectric permittivity as a function of temperature, frequency and magnetic field. It is found that, coinciding with the transition from collinear ferrimagnetic ordering to an incommensurate magnetic state occurring at about 100 K, there is an abrupt change (about 30 %) of permittivity suggesting the existence of a magnetoelectric coupling in this material. Indeed, magnetic field dependent measurements at 100 K have revealed an increase of the permittivity by about 0.3 % in 6 T. Prospective advantages of epsilon-Fe2O3 as multiferroic material are discussed.

We have investigated the effect of negative magnetisation in the YbFe 4 Al 8 compound by means of Mossbauer effect and magnetic measurements. Our conclusions disagree with earlier studies of the compound, in which the existence of a small amount of superconducting phase was reported. We explain the effect of the negative magnetisation in terms of antiferromagnetic interactions between the moment of Yb and the effective moment of the canted Fe spins. The system exhibits negative magnetisation when the effective moment, which is oriented along the external magnetic field, is dominated at low temperatures by the opposite rare earth moment.

Vortex depinning and vortex dynamics are investigated by magnetotransport measurements on two irradiated microbridge single-crystals of Bi-2212. At low temperatures, a crossover from an elastic dynamic to a plastic has been evidenced when the applied magnetic field B is ranging from B B Φ to B > B Φ (where B Φ is the matching field). In addition, the study of the vortex dynamics according to the temperature clearly highlights two transitions at high temperatures. The first one is associated with the limit of the elastic depinning, and the second one is a transition from a viscous-elastic behavior (with I C ≠0) to a purely viscous one (with I C =0).

We report a SANS study of the microphase separation texture of a Pr 0.66 Ca 0.33 MnO 3 crystal as a function of temperature. At T C , ferromagnetic domains display elongated shapes randomly oriented with respect to the crystalline structure, which transform, at lower temperatures, in a texture of randomly oriented and polydisperse 2D sheets of 2 nm average thickness.

We have investigated I–V characteristics at very low temperature on a micro-bridge in a Bi-2212 single crystal with columnar defects patterned by laser ablation. We have shown that the critical current IC presents a maximum when increasing the applied magnetic field. This maximum is located below the matching field at a filling fraction f=BMax/BΦ=0.8. The transverse Meissner effect (TME) has also been checked by tilting the magnetic field away from the tracks. Contrary to the low field regime (f

Superlattices having the general formula (PrMnO3)n(SrMnO3)n were grown from PrMnO3 and SrMnO3 ceramic targets by pulsed laser deposition (PLD) in low pressures (laser-MBE) and were compared to those grown in standard pressures (“classical”-PLD). For the laser-MBE grown films, the growth was monitored and the thickness of each deposited layer was controlled in situ at the level of one unit cell using reflection high energy electron diffraction. Perfectly ordered stacks of PrMnO3 and SrMnO3 layers were thusly grown. The transport and magnetic properties of the laser-MBE superlattices differ from similar superlattices grown in a “classical” PLD system. The different physical behaviors are explained by the combination of two factors, the roughness of the interfaces between the different layers and the oxygen deficiency, both of which are larger in the films grown using classical-PLD. X-ray and electron diffraction, as well as high-resolution electron microscopy, were used to characterize the structures of the...

International audience; We report a meticulous study of the ac-magnetization dynamics (χac(T)), relative dielectric permittivity εr(T), and magneto-dielectric (Δεr/εr(H)) response of various complex magnetic transitions that occur below the ferrimagnetic Néel temperature TN of Mn3O4. Besides the known sequence of transitions at TN ∼ 42.75 K, T1 ∼ 39 K, and T2 ∼ 34 K, the existence of a new anomaly reported recently at 38 K (T*) has been successfully probed by χac(T) and εr(T) measurements. The effect of external dc-bias fields (HDC) and driving frequency (f) on the above mentioned transitions has been investigated in consonance with the εr(T) and Δεr/εr(T,H) results. For the first time, we observed a clear hysteresis of about 5.15 K in the zero-field εr(T) across the incommensurate-to-commensurate transition T2 ∼ 34 K, which provides evidence to the first-order nature of this transition. The Arrott plot (H/M vs. M2) criterion has been used to distinguish the nature of all the sequential transitions that take place below TN.

The magnetic measurements of the Josephson critical field and intergranular critical currents are presented for the Bi2223 compound. The investigations prove that the Josephson critical field value is mainly dependent on the intergranular critical current density. The other factors like the grain dimensions and shapes, sample porosity and density are less important. The relation between the Josephson critical field and critical current HcJ(Jc) is well described within the single junction model, Hcj∝JcλJ. The temperature dependence of HcJ can also be well fitted according to the single junction model with a critical current dependence, Jc∝(1-T/TcJ)α. The critical current density versus temperature is almost linear indicating the presence of superconductor-insulator-superconductor (SIS) type junctions.

Dc magnetic relaxation measurements were performed on pristine and irradiated Hg-1201 single crystals. In relation to the doping level, we analyse the measurements on the pristine single crystals near the fishtail line. The fishtail effect originates from a competition between a critical current that increases against the magnetic field and the pinning energy which decreases against the magnetic field. Depending on the doping level of the single crystals, the fishtail effect may be observed or not, without any correlation with a transition of the vortex relaxation creep (plastic to elastic). The pinning properties have been modified by the introduction of columnar defects along the c-axis by irradiation with 6 GeV Pb ions. The columnar nature of the induced latent tracks is probed by high-resolution electron microscopy. It is shown that, in the presence of correlated disorder, Hg-1201 exhibits features similar to those previously reported in Y-123, despite a larger electronic anisotropy.

We have investigated the role of the stress-induced by the presence of the substrate in thin films of colossal magnetoresistive manganites on structural, resistive and magnetic properties. Because of the strong coupling between the small structural distortions related to the charge-ordering (CO) and the resistive properties, the presence of the substrate prevents the full developpement of the charge ordering in Pr$_{0.5}$Ca$_{0.5}$MnO$_{3}$, especially in the very thin films. For thicker films, the CO state exists, but is not fully developped. Correlatively, the magnetic field which is necessary to suppress the CO is decreased drastically from 25 Tesla to about 5 Tesla on SrTiO$_{3}$ substrates. We have also investigated the influence of the doping level by studying the case of Pr$_{0.6}$Ca$_{0.4}$MnO$_{3}$.

Thin films of the charge-ordered (CO) compound ${\mathrm{Pr}}_{0.5}{\mathrm{Ca}}_{0.5}{\mathrm{MnO}}_{3}$ have been deposited onto (100)-oriented ${\mathrm{SrTiO}}_{3}$ substrates using pulsed laser deposition. Magnetization and transport properties are measured when the thickness of the film is varied. While the thinner films do not exhibit any temperature-induced insulator-metal transition under an applied magnetic field up to 9 T, for thickness larger than 1100 \AA{} a 5 T magnetic field is sufficient to melt the CO state. We have established the temperature-field phase diagram and compared it to the bulk material. It indicates that the robustness of the CO state in thin films depends strongly on the strains and the thickness. We proposed an explanation based on the distortion of the cell of the film.

Besides the standard configuration with all the columnar defects parallel to the c→-axis, different splayed configurations with all the defects inclined at the same angle θi from the c→-axis were obtained by irradiation of single crystals of the most anisotropic high-Tc superconductor Bi2Sr2CaCu2O8 with 6 GeV Pb ions. The pinning abilities of these configurations were studied by magnetic measurements performed with H→ parallel to c→. We found that, in the case of this very anisotropic compound, it is not necessary to invoke a splay effect to account for the pinning properties of the splayed configurations of defects. The major role of two parameters has been seen. First, the role of the effective size of the projection of the defects in the (a→,b→) planes has been studied by introducing columnar defects of different diameters or by tilting the ion beam direction from the c→-axis. Second, the symmetry of the defect distribution with respect to the applied magnetic field has been shown to allow a better accommodation of the vortices to columnar defects. These two parameters can account for the observed crossover between a low-T regime and a high-T regime. This crossover is not predicted as a splay effect in the theoretical models.

We report a detailed neutron diffraction study of both pinned and moving magnetic Flux Line Lattice (FLL) in NbTa and PbIn samples. In NbTa, the FLL presents a hexagonal lattice even in the absence of current, meanwhile PbIn presents a strongly dislocated phase. In PbIn, dislocations are eliminated by the application of transport current in agreement with theoretical predictions. On the other hand, the absence of curvature of the flux lines for subcritical currents confirm the presence of surface pinning of the FLL.

Pr 0.7 Sr 0.3 MnO 3 thin films were deposited at different temperatures on [100]-LaAlO 3 substrates by the pulsed laser deposition (PLD) technique. X-ray diffraction, electron diffraction, and high-resolution electron microscopy (HREM) show that the deposition temperature influences dramatically the orientation of the films and their microstructure, within a very short range. The films grown at low temperature are [101]-oriented and exhibit a columnar growth with well-connected grains. One observes the coexistence of [010]- and [101]-oriented domains poorly connected platelet-like crystals, for films deposited at high temperature. The HREM observation of a cross section of the low-temperature thin film reveals the presence of an interfacial strained layer close to the substrate, allowing a growth mechanism to be proposed. The magnetic measurements show that the Curie temperature is independent of the growth temperature. In contrast, the magnetotransport properties are strongly influenced, due to the different nature of the microstructure and especially to the role of the grain boundaries that may exhibit a depressed T c .

The growth of [101] epitaxial films of Pr0.5Ca0.5MnO3 on LaAlO3 substrates has been achieved. The structure, nanostructure, and magnetotransport properties are reported and compared to those of the bulk material. The electron diffraction study evidences, at RT, a monoclinic cell with a≈c≈5.4 A, b≈7.6 A, and β≈91°. The monoclinic distortion is assumed to be due to strain effects. At 92 K, a system of extra reflections is observed which implies an incommensurate modulated structure. The component q of the modulation vector along a* ranges between 0.35 and 0.40. The lattice images show that the modulation is established throughout the whole film. The q value observed in the film is significantly lower than the one of the bulk material. This effect is correlated to strain effects of the substrate, limiting the cell distortion induced by charge ordering. An unusual insulating-ferromagnetic phase is observed with a critical temperature of 240 K, close to the TCO.

Both the in-plane and out-of-plane magnetoresistivities have been measured in the normal state of an optimally doped ${\mathrm{Bi}}_{2}{\mathrm{Sr}}_{2}{\mathrm{Ca}}_{0.8}{\mathrm{Y}}_{0.2}{\mathrm{Cu}}_{2}{\mathrm{O}}_{8+\ensuremath{\delta}}$ single crystal with a magnetic field applied parallel and perpendicular to the ${\mathrm{CuO}}_{2}$ planes. Whatever the magnetic field and the current directions are, a negative magnetoresistivity is obtained over a wide range of temperature above the critical temperature ${T}_{c}.$ For the in-plane and out-of-plane measurements, the nondominant orbital contribution to magnetoresistivity suggests the substantial role played by the spin degrees of freedom.

DC magnetic relaxation measurements in HgBa2CuO4 single crystals are analyzed nearby the fishtail line. It is found that in this case, it is not necessary to introduce any crossover from plastic creep to elastic creep models at the fishtail line. This type of fishtail effect comes only from a competition between a critical current at low temperature which increases versus field and the activation energy, which decreases versus field. According to the doping level of the compound, the fishtail effect can be observed or not, without any correlation with a vortex phase transition. Moreover, in this type of fishtail effect, there is no history effects as recently observed in YBaCu2O3 by the partial magnetization loop technique, suggesting that the transition from plastic to elastic flow is here hidden by the disorder of these materials.

To investigate the existence of a splay effect in Bi2Sr2CaCu2O8 (Bi-2212), vortex pinning has been studied in different configurations of strongly inclined columnar defects (75 ° from the c axis), installed by heavy-ion irradiation. It is shown that the symmetry of the track setting with respect to the field direction is a more influent parameter than the presence of a dispersion in the track directions. We claim that the enhanced pinning efficiency which is observed in some splayed configurations of columnar defects in Bi-2212 can be interpreted without invoking a splay effect.

A direct method of induction in a small pick up coil shows that in untwinned YBaCuO crystals, the subcritical current flows close to the surfaces of the sample. By studying the frequency dependence of the ac penetration depth, we have also shown that the two-modes through Mathieu–Simon (MS) model of pinning by surface irregularities is able to account for the observed spectrum. The Campbell model of weak bulk pinning can be completely ruled out. Above the first order transition line, the critical current is zero and the ac response is that of an ohmic system. This can be interpreted by the curvature of the vortices close to the surface. This shows the irrelevance of weak bulk pinning models for clean untwinned YBaCuO crystals that were often proposed as model systems for this case.

We report on the vortex transport in ${\mathrm{Bi}}_{2}{\mathrm{Sr}}_{2}{\mathrm{Ca}}_{1\ensuremath{-}x}{\mathrm{Y}}_{x}{\mathrm{Cu}}_{2}{\mathrm{O}}_{8}$ single crystals irradiated in a parallel direction with the c axis with 5.8-GeV Pb ions at fluences of $3.75\ifmmode\times\else\texttimes\fi{}{10}^{10}{\mathrm{cm}}^{\mathrm{\ensuremath{-}}2}$ and $7.5\ifmmode\times\else\texttimes\fi{}{10}^{10}{\mathrm{cm}}^{\mathrm{\ensuremath{-}}2}.$ A detailed investigation of the vortex transport using current-voltage measurements is carried out with the magnetic field applied along the c axis. First, we investigate the critical behavior of the linear and nonlinear conductivity near the Bose-glass melting line. We obtain field and sample-independent critical exponents ${z}^{\ensuremath{'}}$ and ${\ensuremath{\nu}}^{\ensuremath{'}}$ consistent with a compressible Bose glass (i.e., ${\ensuremath{\nu}}_{\ensuremath{\parallel}}=2{\ensuremath{\nu}}_{\ensuremath{\perp}}\ensuremath{\equiv}2{\ensuremath{\nu}}^{\ensuremath{'}}).$ Using the values ${z}^{\ensuremath{'}}=5.28\ifmmode\pm\else\textpm\fi{}0.05$ and ${\ensuremath{\nu}}^{\ensuremath{'}}=1.04\ifmmode\pm\else\textpm\fi{}0.06,$ the data collapse into two single-scaling functions. Second, we report on conductivity measurements over a wide filling factor ${(B/B}_{\ensuremath{\varphi}})$ range from 0.04 up to 0.9 within the Bose-glass phase. Our data provide support for a variable-range hopping mechanism for low current densities, in accordance with the ideas of Nelson and Vinokur [Phys. Rev. B 48, 13 060 (1993)]. We determine a glass exponent value of 1/3 for all the filling factors investigated at large ratios of the penetration depth to the average defect distance $({\ensuremath{\lambda}}_{\mathrm{ab}}/d\ensuremath{\sim}10).$ This finding implies that no Coulomb gap occurs in the pinning energy spectrum. Furthermore, our results show that for low filling factors (1/2) the on-site disorder plays a major role in the bandwidth of flux-binding energies. Finally, the appearance of a well-defined crossover near half filling is consistent with the field ${B}^{*}$ separating the strongly pinned Bose-glass regime from the weakly pinned Bose-glass regime.

Small-angle neutron scattering (SANS) is an extremely powerful probe of the vortex state in type II superconductors. The technique may be further enhanced by the use of polarised neutrons and the application of the neutron spin-echo method. We discuss some recent applications of these techniques to the study of both conventional and unconventional superconducting materials, and describe the unique information which SANS can provide on the vortex state. ( 2000 Elsevier Science B.V. All rights reserved.

The AgCrS2 layer sulfide exhibits ferroelectricity below TN = 42 K as its AgCrO2 oxide counterpart. By considering the different Ag and Cr environments, these results offer new opportunities to discover magnetoelectrics in triangular lattice antiferromagnets.

The vortex phase transition was systematically studied in twin-free ${\mathrm{YBa}}_{2}{\mathrm{Cu}}_{3}{\mathrm{O}}_{7\mathrm{\ensuremath{-}}\mathrm{\ensuremath{\delta}}}$ for various \ensuremath{\delta}. For $7\ensuremath{-}\ensuremath{\delta}g~6.94,$ the first order transition was evidenced by an abrupt jump and a hysteresis of magnetization in the field-cooled (FC) cooling-warming cycles, which indicates the coexistence of vortex crystallites and liquid. For $7\ensuremath{-}\ensuremath{\delta}l6.89$ the jump was not sharp and FC magnetization was reversible, suggesting a second order transition. The first and the second order transitions merge at a critical point (CP) which goes to zero field for $7\ensuremath{-}\ensuremath{\delta}l6.89.$ Three-dimensional XY scaling satisfactorily fits the data at 1--7 T for $7\ensuremath{-}\ensuremath{\delta}l6.94$ and for $7\ensuremath{-}\ensuremath{\delta}l6.89,$ but does not fit for $6.89l7\ensuremath{-}\ensuremath{\delta}l6.94,$ where the CP passes through this field range.

There are several ways of investigating#ux lines with polarised neutrons. We shall describe our recent work on IN15 at ILL, investigating spatially varying "elds perpendicular to the #uxoid axes, which are present with tilted "elds in anisotropic superconductors. These "eld components may be detected by polarisation analysis of neutrons di!racted by the #ux line lattice, and allow a detailed investigation of #ux structure in such materials as YBCO and NbSe 2 . A further application of polarised neutrons is the use of neutron spin-echo techniques to measure the speed of moving #ux lines. We have recently demonstrated this in an Nb}Ta alloy on instruments IN11 and IN15, by measuring the energy change of neutrons di!racted by #ux lines, which are moving under the in#uence of a transport current. We shall also comment on others’ work on the search for interference terms between #ux lattice and nuclear di!raction signals, and the use of neutron depolarisation for investigation of #ux distributions in superconductors. ( 1999 Elsevier Science B.V. All rights reserved.

Various configurations of columnar defects have been installed by heavy-ion irradiation in ${\mathrm{YBa}}_{2}{\mathrm{Cu}}_{3}{\mathrm{O}}_{7}$ crystals: a standard configuration of tracks parallel to c, a bimodal splay consisting of crossed tracks at $\ifmmode\pm\else\textpm\fi{}5\ifmmode^\circ\else\textdegree\fi{}$ to the c axis, and a ``fan'' configuration consisting of crossed tracks at $\ifmmode\pm\else\textpm\fi{}5\ifmmode^\circ\else\textdegree\fi{}$ and $\ifmmode\pm\else\textpm\fi{}45\ifmmode^\circ\else\textdegree\fi{},$ with a fraction of large angles equal to $1/5.$ The aim is to directly investigate the influence of a small proportion of tracks at large angle on the vortex pinning. This study leads us to conclude that the large-angle tail of a Gaussian angular distribution should not, in itself, impede the occurrence of a beneficial splay effect, i.e., a further pinning enhancement relative to a standard irradiation along c.

Different configurations of tilted columnar defects have been introduced in Bi2Sr2CaCu2O8 (Bi-2212) superconducting single crystals by irradiation with 6 GeV Pb ions. All the defects are tilted of the same value θi from the c → axis and are introduced along one (n=1), two (n=2) or three (n=3) directions, symmetrically distributed around the c → axis (for n>1). Several values of θi have been studied: 45°, 60° and 75°. Magnetic measurements were performed with H → // c → . A crossover is observed between a low temperature regime, where the pinning efficiency is the same whatever n, and a high temperature regime where the pinning efficiency is strongly reduced for n=1. This transition can be related to a difference in the accommodation abilities of vortices to defects. For symmetric distributions (n>1), accommodation persists up to high temperatures in a wide field range, contrary to n=1. Another configuration, with two directions non-symmetrically distributed around c → was also investigated in order to distinguish the influence of the symmetry from the influence of angular dispersion.

A second peak effect is observed for a ~Tl, Bi!-1212 single crystal using ac susceptibility measurements. The analysis of the frequency dependence of the second peak position shows that plasticity governs the vortex dynamics on both sides of the second peak line. This suggests that no particular change in the vortex dynamics occurs by crossing this line. We propose that this second peak effect is due to the temperature activated form of the characteristic relaxation times and to the fact that the characteristic activation energy Uc and the critical current density Jc have inverse variations with the magnetic field B ~when one increases with B the other decreases!. We also propose that the time dependence of the second peak field position should indicate the vortex dynamics behavior: an increasing second peak field position with time is a fingerprint of elastic behavior while a decreasing second peak field position with time is a fingerprint of plastic behavior. The latter case agrees well with our experimental results. @S0163-1829~98!04441-5#

An extensive study of small angle neutron scattering was performed in twinned YBa2Cu3O7 crystals in its superconducting state as a function of the angle \(\) between the c-axis of the crystal and the magnetic field. The half of the twin boundaries are oriented in the horizontal plane, which also contains the neutron beam and the magnetic field. Two different diffraction patterns are studied as a function of \(\) at 5 K and B = 0.5 T, one along the c-axis of the crystal, the other one along the applied field. These variations are interpreted in the model of accommodation of the vortices on the twin planes by zigzagging from these planes to the ab-planes of the crystal, in order to minimize their energy.

Bi-2212 thin films have been irradiated by 936 MeV Pb ions. An High Resolution Electron Microscopy (HREM) study revealed the formation of square based defects, the so-called pillar tracks, extending throughout the thickness of the film and coexisting with columnar defects of same size. The pinning efficiency of this mixed structure of defects has been investigated by magnetic measurements and compared to a structure containing only columnar defects of the same size. This comparison suggests that pillar defects act the same way as columnar defects of about the same size (125 A).