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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.

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 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.

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

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.

Single crystals of Bi 2 Sr 2 CaCu 2 O 8 have been irradiated by Pb ions at different angles, θ i , from the c axis. The pinning efficiency of these different configurations of columnar defects has been investigated by SQUID magnetometry with the magnetic field applied along the c axis. A regime was found, at high temperatures and low fields, where the critical current density is lower for θ i =0° than for very high angles, like θ i =75°. This enhancement can be related to an increase of the individual pinning energy resulting from the larger size of the track projection in the ab planes. Several track configurations are compared to take into account the additional influence of the variation of track density in the ab planes owing to the ion beam inclination.

Different configurations of columnar defects tilted with respect to c have been tailored in Bi{sub 2}Sr{sub 2}CaCu{sub 2}O{sub 8} single crystals by controlling the orientation of a heavy-ion beam: One single direction of tracks yielding parallel defects (n=1), two coplanar directions of tracks (n=2), and three directions of tracks symmetrically distributed around the c axis (n=3). The pinning efficiency has been systematically investigated by magnetic measurements with the field applied along c. We found evidence for a low-T regime where the pinning efficiency is nearly independent of n, and a high-T regime where it essentially increases with n. These features are discussed in the frame of two different but not incompatible approaches: (i) a static approach based on accommodation effects, and (ii) a dynamic approach based on elementary flux-creep processes. Finally, the further comparison of these results with angular measurements performed for n=1 strongly suggests that the symmetry of the track distribution relative to the orientation of the applied field might play a major role in the pinning efficiency of splayed configurations. {copyright} {ital 1998} {ital The American Physical Society}

Magnetic relaxation measurements in (Tl 2/3 ,Bi 1/3 )Sr 2 CaCu 2 O x (Tl-1212) crystals, with moderate anisotropy, are analyzed nearby the fishtail line. It is found that plastic vortex creep takes place in a large domain of the H – T diagram above the fishtail line, whereas the elastic creep theory well describes the experimental behavior below this line. The field dependence of the activation energy obeys a power law behavior i.e.: U ∝ B ν , with nearly the same values of the exponent ν as it was found in YBa 2 Cu 3 O 7 by Abulafia et al., in favor of some universal character of the creep in systems of moderate anisotropies.

A small deviation from the ratio Mn 3+ : Mn 4+ = 1 in the manganite Pr 0.5 + y Sr 0.5 − y MnO 3 leads to a disapperance of the AFMI-FMM transition, the AFMI state being replaced by a FMM state. The present study of the series Pr 0.52 Sr 0.48 − x Ca x MnO 3 shows for the first time that the AFMI-FMM transition can be restored for a Mn 3+ : Mn 4+ ratio different from 1 by calcium doping. This result is interpreted by the fact that the decrease of the average size of the A-site cation, 〈 r A 〉, and of the mismatch, σ 2 , favors the AFMI state at the expense of the FMM state. The deviation from the perfect charge ordering of the 1 : 1 compound is then compensated by these two effects. The application of a magnetic field of 7 T shows that all the compositions exhibit a CMR effect. The maximum R 0 R 7 T ratio is equal to 3500 at 127 K for the Pr 0.52 Sr 0.26 Ca 22 MnO 3 compound.

Thin films of YBa 2 Cu 3 O 7 , a -axis oriented, were grown by laser ablation. The angular dependence of the resistivity was studied in the superconducting phase. The I – V curves were also studied close to the vortex lattice melting transition. This transition line, the critical currents and the pinning energies are compared to that of single crystals and to c -axis oriented films. These values appear much lower in a -axis oriented films. This can be compared to the decrease of the resistive transition temperature in zero applied field, suggesting an important role of the carrier deficiency which exists in such films. The origin of this deficiency seems not to be in the grains themselves but confined in the grain boundaries. These is also a minimum of resistivity when the field is applied parallel to the substrate. This can be due to the vortex pinning by the surfaces of the film, or by the grain boundaries which exist in parallel to these surfaces. In these films, the presence of the grain boundaries induces a vortex pinning, but also a decrease of T c and a correlative decrease of the vortex pinning ability. This decrease dominates the critical current properties.

The current-voltage characteristics are investigated in a high quality c -axis Tl-2212 thin film in perpendicular fields. For weak magnetic fields ( B ≤ 1 tesla), we find evidence for a 3D vortex glass phase undergoing into a liquid through a continuous transition. Above B ∼1–2 tesla, we show the disappearance of the 3D vortex glass. This is interpreted on the basis of a 3D–2D crossover according to various theories. Both below and above the crossover field, we find a plastic 3D and 2D flux flow mechanism respectively.

An `exact' numerical solution of Laplace's equation is performed in order to extract the anisotropic resistivity in a single crystal of a superconducting oxide from a multi-terminal transport measurement. A phenomenological fit of the results is proposed to provide a very simple method of extraction of the anisotropic resistivity. This extraction is successfully applied in a crystal of at room temperature.

A former paper had shown that some Tl-2 2 1 2 thin films can be described as two-dimensional arrays of weakly coupled superconducting grains. We here report on a comparative study of the superconducting transition (R = 0) in a Tl-2 2 1 2 thin film and a Tl-2 2 1 2 polycrystal. We find in this way rather striking evidence for two kinds of voltage-current behaviours respectively which places emphasis on the dimensionality of samples. We show that both these dissipative behaviours can quantatively be explained in terms of phase-coherence transitions predicted in D-dimensional arrays of superconducting grains. In particular, universal scaling properties of the three-dimensional phase-coherence transition and Kosterlitz-Thouless-like phase transition are found for the polycrystal and the film respectively.

Columnar defects at 45° with respect to the ab planes have been installed in a Bi-2212 single crystal by 5.8 GeV Pb ion irradiation. Transport measurements have been carried out in the so-called “top” and “cross” configurations, as a function of the temperature T , the magnetic field B , the current i and θ, the angle between B and the ab planes. In a restricted area of the B - T plane, the R ( θ ) curves exhibit a marked dip centered along the direction of the columnnar defects, for B lower or close to the fluence equivalent field B φ t which corresponds to a vortex density equal to the planar density of the columnar defects. The directional features become less pronounced as the field or the temperature increase, and disappear completely above a crossover line which extrapolates at low fields to a temperature lower than T c . This crossover is discussed in terms of a decorrelation of line vortices induced by thermal fluctuations.

We have measured the transport properties of a heavy ion (lead) irradiated Bi2212 single crystal. Current-voltage measurements were recorded for magnetic fields below and above the dose equivalent field Bφ (fluence = 1010 cm−2, Bφ = 0.2 T). The extracted activation energies Uact(J, B, T) are analyzed using individual (B Bφ). The vortex-vortex interactions are described using the analogous Ekman superfluid layer. This leads to a crossover from a field independent activation energy below Bφ to a B−0.5 dependence (as observed in pristine samples) which is only observed in the high field regime.

Through magnetization and conductivity measurements, the effect of dimensionality on the superconducting fluctuations has been investigated for ${\mathrm{Tl}}_{2}$ ${\mathrm{Ba}}_{2}$ ${\mathrm{CaCu}}_{2}$ ${\mathrm{O}}_{8}$ (Tl-2212), ${\mathrm{YBa}}_{2}$ ${\mathrm{Cu}}_{3}$ ${\mathrm{O}}_{7}$ (YBCO), and (Tl,Bi)${\mathrm{Si}}_{2}$ ${\mathrm{CaCu}}_{2}$ ${\mathrm{O}}_{7}$ (Tl-1212). Our results show that the bilayered thallium compound (i.e., Tl-2212 which is the most anisotropic compound) is clearly two-dimensional (2D) while a 3D-2D crossover is evidenced for Tl-1212 and YBCO. This crossover occurs through an intermediate region between two asymptotic behaviors: 3D near the transition and 2D at higher temperatures.