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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.

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

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.

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.

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.

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 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.

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#

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.

The flux motion has been investigated in a Tl 2 Ba 2 CaCu 2 O 8 thin film by transport measurements with a weak applied magnetic field perpendicular to the ( a , b ) plane ranging from 20 to 300 mT. We have found a well defined line T ∗ (H) separating ohmic from non ohmic behaviour in the H-T ∗ diagram. This line is well described by 1 - t ∗ ≈ H 2 3 where t ∗ = T ∗ /T c and T c is the zero magnetic field transition temperature. Above T ∗ (H) , a thermally activated resistance is observed. Our experimental results clearly indicate that the activation energy is well approximated by U(T, H) ∼ [l-(T/T c ] 3 2 /H both below and above the T ∗ (H) line in the H-T ∗ diagram. The nonlinear current dependence of the activation energy U ( T , H , I ) is extracted from I – V characteristics, and it is found that U ( T , H , I )= U ( T , H ) ln( I 0 / I ). Satisfactory agreement is found between our results and recent models which take into account a periodic logarithmic-like pinning potential.

Below ${T}_{C}$, the single-crystal study of CaBaCo${}_{4}$O${}_{7}$, a noncollinear ferrimagnet (${T}_{C}=64$ K) with a polar orthorhombic space group (Pbn2${}_{1}$) between 4 and 293 K, shows a large change in the electric polarization along its $\stackrel{P\vec}{c}$ axis reaching $\ensuremath{\Delta}P=17$ mC/m${}^{2}$ at 10 K. At 62.5 K, a magnetic-field-driven giant variation in polarization $P$(9 T) \ensuremath{-} $P$(0 T) $=$ 8 mC/m${}^{2}$ is observed. Moreover, the present magnetoelectric measurements are fully consistent with the $m$\ensuremath{'}$m$2\ensuremath{'} magnetic point group. This ferrimagnetic oxide, which belongs to the ``114'' structural family, opens an avenue for the search for new magnetoelectrics.

The thermoelectric power (TEP) S versus temperature has been investigated for the manganite ${\mathrm{Pr}}_{0.7}$${\mathrm{Ca}}_{0.3\mathrm{\ensuremath{-}}\mathit{x}}$${\mathrm{Sr}}_{\mathit{x}}$${\mathrm{MnO}}_{3}$ series. Although these materials exhibit the same hole concentration considering their Mn valency of 3.3, their S(T) curves are dramatically affected when x varies from 0 to 0.3, i.e., when the mean size of the interpolated cation increases. Particularly the ${\mathit{S}}_{290\phantom{\rule{0ex}{0ex}}\mathrm{K}}$ values go from positive to negative when x increases. The remarkable feature is that TEP data for samples which exhibit colossal magnetoresistive (CMR) properties are found to be similar and can be well described by the paramagnetic (P)-antiferromagnetic (AF) and AF-ferromagnetic (F) transitions which exist in these samples. On the other hand, for the samples with higher x values, the CMR effect is smaller and the S(T) curves change shape. Clearly, the electronic properties of these materials are very sensitive to the atomic orbitals overlapping which can be varied via cationic size variations as demonstrated by the present TEP measurements. Moreover, the signatures of both P\ensuremath{\rightarrow}AF and AF\ensuremath{\rightarrow}F transitions on the TEP curves seem to be a good indicator to detect samples with CMR properties. \textcopyright{} 1996 The American Physical Society.

Powder-in-the Ag tube processed ${\mathrm{Bi}}_{2}$${\mathrm{Sr}}_{2}$${\mathrm{CaCu}}_{2}$${\mathrm{O}}_{8+\mathrm{\ensuremath{\delta}}}$ and (Bi,Pb${)}_{2}$${\mathrm{Sr}}_{2}$${\mathrm{Ca}}_{2}$${\mathrm{Cu}}_{3}$${\mathrm{O}}_{10}$ tapes were irradiated with 5.8-GeV Pb ions along the tape normal for the ion fluences of 0.68, 1.0, and 2.0\ifmmode\times\else\texttimes\fi{}${10}^{11}$ ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}2}$. The angular (\ensuremath{\theta}) dependence of critical current densities ${\mathit{J}}_{\mathit{c}}$(\ensuremath{\theta},B,T) for these tapes was measured at temperature T=27 K and 55--80 K. Cusplike sharp peaks in ${\mathit{J}}_{\mathit{c}}$(\ensuremath{\theta},B,T) were observed for T\ensuremath{\geqslant}55 K when the applied magnetic field B is oriented parallel to the direction of the irradiation (\ensuremath{\theta}=90\ifmmode^\circ\else\textdegree\fi{}), and ${\mathit{J}}_{\mathit{c}}$(\ensuremath{\theta}) did not scale with the perpendicular component of B, (${\mathit{B}}_{\mathrm{\ensuremath{\perp}}}$=B sin\ensuremath{\theta}). Based on this evidence, it was concluded that the magnetic vortices behave as linelike (three-dimensional) rather than the pancakelike [two-dimensional (2D)] objects in these irradiated superconductors. However, at 27 K, neither the peaks in ${\mathit{J}}_{\mathit{c}}$(\ensuremath{\theta}) nor the scaling of ${\mathit{J}}_{\mathit{c}}$(\ensuremath{\theta}) with ${\mathit{B}}_{\mathrm{\ensuremath{\perp}}}$ was observed. We attribute this to the increased strength of the isotropic pinning by the small irradiation-induced defect debris relative to that of the columns and due to the increased interlayer coupling strength at reduced temperature, but not as evidence for the pancakelike 2D vortices at low temperatures. Also, for T\ensuremath{\geqslant}55 K, the enhancement in the pinning strength qualitatively followed the theory by Nelson and Vinokur [Phys. Rev. Lett. 68, 2398 (1992); Phys. Rev. B 48, 13 060 (1993)]. \textcopyright{} 1996 The American Physical Society.

The bulk magnetic properties, including dc and ac susceptibilities and heat capacity, of the pyrochlore oxides ${\mathrm{Ho}}_{2}$${\mathrm{Mn}}_{2}$${\mathrm{O}}_{7}$ and ${\mathrm{Yb}}_{2}$${\mathrm{Mn}}_{2}$${\mathrm{O}}_{7}$ are reported and compared with those of the previously studied ${\mathrm{Y}}_{2}$${\mathrm{Mn}}_{2}$${\mathrm{O}}_{7}$. In the latter case the magnetic ${\mathrm{Mn}}^{4+}$ ions occupy the 16c sites in Fd3m which define a potentially frustrated three-dimensional array of corner sharing tetrahedra. For ${\mathrm{Ho}}_{2}$${\mathrm{Mn}}_{2}$${\mathrm{O}}_{7}$ and ${\mathrm{Yb}}_{2}$${\mathrm{Mn}}_{2}$${\mathrm{O}}_{7}$ magnetic rare earth ions occupy the 16d sites, as shown by powder neutron diffraction, which are topologically equivalent to the 16c sites but displaced by a vector (1/2 1/2 1/2). ${\mathrm{Ho}}_{2}$${\mathrm{Mn}}_{2}$${\mathrm{O}}_{7}$ and ${\mathrm{Yb}}_{2}$${\mathrm{Mn}}_{2}$${\mathrm{O}}_{7}$ display sharp increases in both dc and ac susceptibilities near 38 K. In addition field-cooled-zero-field-cooled irreversibilities appear, also at 38 K, followed by broad maxima centered near 30 K. The ac data are similar with frequency variability setting below 38 K and broad, frequency dependent maxima at somewhat lower temperatures. Heat capacity data show only broad maxima centered near 30--35 K with a high temperature tail extending up to 80 K. The bulk behavior of ${\mathrm{Ho}}_{2}$${\mathrm{Mn}}_{2}$${\mathrm{O}}_{7}$ and ${\mathrm{Yb}}_{2}$${\mathrm{Mn}}_{2}$${\mathrm{O}}_{7}$ just described parallels the spin-glass-like behavior of ${\mathrm{Y}}_{2}$${\mathrm{Mn}}_{2}$${\mathrm{O}}_{7}$ but with a doubling of the temperature scale. Surprisingly, neutron diffraction data for both ${\mathrm{Ho}}_{2}$${\mathrm{Mn}}_{2}$${\mathrm{O}}_{7}$ and ${\mathrm{Yb}}_{2}$${\mathrm{Mn}}_{2}$${\mathrm{O}}_{7}$ show resolution limited reflections of magnetic origin in contrast to the heat capacity results. The resolution of the neutron diffraction data places a minimum on the correlation length of 100 \AA{}. Small angle neutron scattering data for all three materials are reported for the Q range ${10}^{\mathrm{\ensuremath{-}}2}$ A${\mathrm{\r{}}}^{\mathrm{\ensuremath{-}}1}$ to 2\ifmmode\times\else\texttimes\fi{}${10}^{\mathrm{\ensuremath{-}}1}$ A${\mathrm{\r{}}}^{\mathrm{\ensuremath{-}}1}$ and the temperature range 6--100 K. Data for the full Q range can be fitted for all three materials to a model consisting of a Lorentzian and a Lorentzian-squared term, i.e., I(Q)=A/(${\mathit{Q}}^{2}$+1/${\ensuremath{\xi}}_{1}^{2}$)+B/(${\mathit{Q}}^{2}$+1/${\ensuremath{\xi}}_{2}^{2}$${)}^{2}$, a cross section commonly found in spin-glass-like materials. A surprising result is that the correlation lengths ${\ensuremath{\xi}}_{1}$ and ${\ensuremath{\xi}}_{2}$ are unequal and in general ${\ensuremath{\xi}}_{2}$\ensuremath{\gtrsim}${\ensuremath{\xi}}_{1}$. ${\ensuremath{\xi}}_{1}$ remains finite reaching maximum values which range from 10 to 20 \AA{} depending on the compound, while ${\ensuremath{\xi}}_{2}$ shows a very strong temperature dependence and reaches large values of \ensuremath{\gtrsim}500 \AA{} for Y and Ho and appears to saturate near 400 \AA{} for Yb. The temperature dependence of the product B${\ensuremath{\xi}}_{2}$ is order parameter like. The above behavior is compared to that of reentrant spin glasses but with ferromagnetic and spin glass temperatures nearly coincident. \textcopyright{} 1996 The American Physical Society.

Columnar defects (CD{close_quote}s) inclined at 45{degree} from the {ital c} axis have been introduced in single crystals of different compounds: Bi{sub 2}Sr{sub 2}CaCu{sub 2}O{sub 8}, Tl{sub 2}Ba{sub 2}CaCu{sub 2}O{sub 8}, Tl{sub 2}Ba{sub 2}Ca{sub 2}Cu{sub 3}O{sub 10}, Tl{sub 2/3}Bi{sub 1/3}Sr{sub 2}CaCu{sub 2}O{sub 7}, and YBa{sub 2}Cu{sub 3}O{sub 7}. In each case, hysteresis loops have been recorded with the magnetic field parallel to the columns ({ital H}{searrow}{sub {ital a}}{parallel}CD) and perpendicular to them ({ital H}{searrow}{sub {ital a}}{perpendicular}CD). Two regimes are evidenced: an isotropic pinning enhancement [{ital J}{sub {ital c}}({ital H}{searrow}{sub {ital a}}{perpendicular}CD){equivalent_to}{ital J}{sub {ital c}}({ital H}{searrow}{sub {ital a}}{parallel}CD)] at low temperatures and a directional effect [{ital J}{sub {ital c}}({ital H}{searrow}{sub {ital a}}{perpendicular}CD){lt}{ital J}{sub {ital c}}({ital H}{searrow}{sub {ital a}}{parallel}CD)] at higher temperatures. The isotropic regime is ascribed to the occurrence of a {open_quote}{open_quote}totally adjusted{close_quote}{close_quote} state where the vortices zigzag between the CD{close_quote}s and the {ital ab} planes, keeping their mean direction along the applied field. An analysis is proposed for the crossover between these two regimes, which accounts for the observed influences of the temperature and electronic anisotropy. The vanishing of the difference between the two configurations at zero field is discussed in terms of a complete reorientation of the vortices along the more » CD{close_quote}s in the case {ital H}{sub {ital a}}{perpendicular}CD. {copyright} {ital 1996 The American Physical Society.} « less

The spinel FeV${}_{2}$O${}_{4}$ is known to exhibit peculiar physical properties, which is generally ascribed to the unusual presence of two cations showing a pronounced interplay between spin, orbital, and lattice degrees of freedom (Fe${}^{2+}$ and V${}^{3+}$ on the tetrahedral and octahedral sites, respectively). The present work reports on an experimental reinvestigation of this material based on a broad combination of techniques, including x-ray diffraction, energy dispersive, and M\"ossbauer spectroscopies, as well as magnetization, heat capacity, dielectric, and polarization measurements. Special attention was first paid to establish the exact cationic composition of the investigated samples, which was found to be Fe${}_{1.18}$V${}_{1.82}$O${}_{4}$. All the physical properties were found to point out a complex ordering process with a structural transition at ${T}_{S}$ $=$ 138 K, followed by two successive magnetostructural transitions at ${T}_{N}$${}_{1}$ $=$ 111 K and ${T}_{N}$${}_{2}$ $=$ 56 K. This latter transition marked the appearance of electric polarization, and magnetization data were analyzed in detail to discuss the nature of the magnetic state at $Tl{T}_{N}$${}_{2}$. An overall interpretation of the sequence of transitions was proposed, taking into account two spin couplings, as well as the Jahn-Teller effects and the mechanism of spin-orbit stabilization. Finally, the origin of ferroelectricity in Fe${}_{1.18}$V${}_{1.82}$O${}_{4}$ is discussed on the basis of recent models.

The orthovanadate DyVO3 crystal, known to exhibit multiple structural, spin and orbital ordering transitions, is presently investigated on the basis of magnetization, heat capacity, resistivity, dielectric and polarization measurements. Our main result is experimental evidence for the existence of multiferroicity below a high TC of 108 K over a wide temperature range including different spin-orbital ordered states. The onset of ferroelectricity is found to coincide with the antiferromagnetic C-type spin ordering transition taking place at 108 K, which indicates that DyVO3 belongs to type II multiferroics exhibiting a coupling between magnetism and ferroelectricity. Some anomalies detected on the temperature dependence of electric polarization are discussed with respect to the nature of the spin-orbital ordered states of the V sublattice and the degree of spin alignment in the Dy sublattice. The orthovanadates RVO3 (R = rare earth or Y) form an important new category for searching for high-TC multiferroics., Comment: 25 pages, 7 figures, 68 references, one supplementary material, Physical Review B, Published 23 July 2014

Magnetoelectric multiferroics showing coupling between polarization and magnetic order are attracting much attention. For instance, they could be used in memory devices. Metal-transition oxides are provided several examples of inorganic magnetoelectric multiferroics. In the present short review, spinel and delafossite chromites are described. For the former, an electric polarization is evidenced in the ferrimagnetic state for ACr 2 O 4 polycrystalline samples (A=Ni, Fe, Co). The presence of a Jahn–Teller cation such as Ni 2+ at the A site is shown to yield larger polarization values. In the delafossites, substitution by V 3+ at the Cr or Fe site in CuCrO 2 (CuFeO 2 ) suppresses the complex antiferromagnetic structure at the benefit of a spin glass state. The presence of cation disorder, probed by transmission electron microscopy, favors relaxor-like ferroelectricity. The results on the ferroelectricity of ferrimagnets and insulating spin glasses demonstrate that, in this research field, transition-metal oxides are worth to be studied.

A power law, V∼Ia, between voltage and current is reported in a Tl-2212 thin film in a weak magnetic field (up to 300 mT) applied perpendicularly to the layers. We find experimental support for the existence of a phase transition depending on the magnetic field, below the zero magnetic field transition Tc0. We discuss these results in terms of a defect-mediated vortex lattice (VL) melting leading to a Berezinskii-Kosterlitz-Thouless transition type. Consistency is found with a logarithmic coupling energy U(r) = A ln (r/r0) between VL defects where the unrenormalized interaction constant A∼T(T c0 -T) 3 2 /H and the Abrikosov lattice parameter sets the cutt-off r0.

The solid solutions of ${\text{CaBaFe}}_{4\ensuremath{-}x}{\text{Li}}_{x}{\text{O}}_{7}$ $({\text{CBFLi}}_{x})$ with the ``114'' hexagonal symmetry $(P{6}_{3}mc)$ could be synthesized for $0\ensuremath{\le}x\ensuremath{\le}0.4$. The dc magnetization study of these oxides shows that they are ferrimagnetic (FiM). The FiM to paramagnetic transition temperature, ${T}_{C}$, decreases from 265.6 K for $x=0$ to 97.5 K for $x=0.4$ in agreement with the introduction of the diamagnetic ${\text{Li}}^{1+}$ cation at the Fe sublattice. No magnetic saturation is observed and the coercive field ${H}_{C}$ is very large, ranging from 0.7 to 0.9 T at 5 K. Importantly, the ac magnetic susceptibility, ${\ensuremath{\chi}}_{\text{ac}}(T)$, indicates a frequency-dependent freezing temperature, ${T}_{f}$, increasing with frequency and satisfying the conventional power law, $\ensuremath{\tau}/{\ensuremath{\tau}}_{0}={({T}_{f}/{T}_{\text{SG}}\ensuremath{-}1)}^{\ensuremath{-}z\ensuremath{\upsilon}}$. For the undoped ${\text{CaBaFe}}_{4}{\text{O}}_{7}$ (CBFO) $(x=0)$, the best fitting parameters are characteristic of a spin glass with a transition temperature ${T}_{\text{SG}}=176\text{ }\text{K}$, spin-relaxation time ${\ensuremath{\tau}}_{0}=4.9\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}12}\text{ }\text{s}$, and critical exponent $(z\ensuremath{\upsilon})$ of 6.4. In contrast, for ${\text{CBFLi}}_{x}$ $(0.1\ensuremath{\le}x\ensuremath{\le}0.4)$ series the ${\ensuremath{\tau}}_{0}$ increases from $1.3\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}14}$ to $8\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}16}\text{ }\text{s}$ as $x$ is increased; the values of $z\ensuremath{\upsilon}$ are in the range of 12--13, which lies much closer to that of cluster glasses (${\ensuremath{\tau}}_{0}={10}^{\ensuremath{-}14}--{10}^{\ensuremath{-}16}\text{ }\text{sec}$; $z\ensuremath{\upsilon}=12--14$). The resistivity behavior is consistent with small polaronic hopping which changes to variable range hopping at low temperatures. The measurements under $H\ensuremath{\sim}7\text{ }\text{T}$ show that only CBFO (undoped) exhibits magnetoresistance, with a maximum value of 4.3% at 264 K.

We have performed small-angle neutron scattering of the flux line lattice (FLL) in a Fe-doped ${\text{NbSe}}_{2}$ sample which presents a large peak effect in the critical current. The scattered intensity and the width of the Bragg peaks of the equilibrium FLL indicate an ordered structure in the peak effect zone. The history dependence in the FLL structure has been studied using field-cooled and zero\char21{}field-cooled procedures, and each state shows the same intensity of Bragg scattering and good orientational order. These results strongly suggest that the peak effect is unrelated to a bulk-disordering transition and confirm the role of a heterogeneous distribution of screening current.

Current--voltage characteristics have been performed on an irradiated microbridged ${\text{Bi}}_{2}{\text{Sr}}_{2}{\text{CaCu}}_{2}{\text{O}}_{8+\ensuremath{\delta}}$ single crystal at low temperature $(T=5\text{ }\text{K})$ in a magnetic field applied parallel to the $c$ axis. The evolution of the I--V characteristics that leads to a peak in the differential resistance ${r}_{d}(I)=dV/dR(I)$ in the vicinity of the ``peak effect'' in the critical current was investigated. The analysis of the current-dependent differential resistance curves shows that the dynamics of the vortices in the vicinity of ${B}_{\ensuremath{\phi}}$ is dominated by the plastic flow of interstitially pinned vortex around regions of flux lines strongly pinned at the columnar defects. The pinning is found to be individual at low magnetic fields and collective when the vortex--vortex interactions are involved.

We report small angle neutron scattering measurements of the flux lines lattice (FLL) in ${\mathrm{Bi}}_{2}{\mathrm{Sr}}_{2}\mathrm{Ca}{\mathrm{Cu}}_{2}{\mathrm{O}}_{8+\ensuremath{\delta}}$. As previously reported, the scattered intensity decreases strongly when the magnetic field is increased, but it remains measurable far above the second peak. The direct observation of Bragg peaks proves that the characteristics of a lattice are still present. No structural features related to a symmetry breaking, such as a liquidlike or an amorphous state, can be observed. However, the associated scattered intensity is very low and is difficult to explain. We discuss the coexistence between two FLL states as a possible interpretation.

The angular dependence of the resistivity \ensuremath{\rho}(\ensuremath{\theta}) has been measured in the superconducting transition region in ${(\mathrm{T}\mathrm{l}}_{2/3}{,\mathrm{B}\mathrm{i}}_{1/3}{)\mathrm{S}\mathrm{r}}_{2}{\mathrm{CaCu}}_{2}{\mathrm{O}}_{x}$ crystals. This resistivity has been successfully scaled using an anisotropic effective mass model with an anisotropy $\ensuremath{\gamma}{=(m}_{c}{/m}_{\mathrm{ab}}{)}^{1/2}$ of 9--12, which is larger than in ${\mathrm{YBa}}_{2}{\mathrm{Cu}}_{3}{\mathrm{O}}_{7},$ but much smaller than in Bi-2212 or Tl-2212 compounds. This study was performed for measurements with the current applied along the $\mathrm{ab}$ planes or along the $c$ axis.

We investigate both thermoelectric and thermodynamic properties of the misfit cobalt oxide [Bi$_{1.7}$Co$_{0.3}$Ca$_{2}$O$_{4}$]$^{RS}_{0.6}$CoO$_{2}$. A large negative magnetothermopower is found to scale with both magnetic field and temperature revealing a significant spin entropy contribution to thermoelectric properties giving rise to a constant S$_0\approx$ 60 $\mu$V K$^{-1}$ equal to the high temperature asymptotic value of the spin 1/2 entropy. Low temperature specific heat measurements allow us to determine an enhanced electronic part with $\gamma\approx$ 50 mJ (mol K$^{2}$)$^{-1}$ attesting of strong correlations. Thereby, a critical comparison between [Bi$_{1.7}$Co$_{0.3}$Ca$_{2}$O$_{4}$]$^{RS}_{0.6}$CoO$_{2}$, other cobaltites as well as other materials reveals a universal behavior of the thermopower low temperature slope as a function of $\gamma$ testifying thus a purely electronic origin. This potentially generic scaling behavior suggests here that the high room temperature value of the thermopower in misfit cobalt oxides results from the addition of a spin entropy contribution to an enlarged electronic one., Comment: 4 pages, 4 figures

We report the first measurements of the voltage noise in the surface superconductivity state of a type-II superconductor. We present strong evidences that surface vortices generates surface current fluctuations whose magnitude can be modified by the pinning ability of the surface. Simple two-stage mechanism governed by current conservation appears to describe the data. We conclude that large voltage fluctuations induced by surface vortices exist while the bulk is metallic. Furthermore, this experiment shows that sole surface current fluctuations can account for the noise observed even in the presence of vortices in the bulk., accepted for publication in Phys. Rev. B

We have performed both in-plane resistivity, Hall effect, and specific heat measurements on the thermoelectric cobalt oxide ${\mathrm{Ca}}_{3}{\mathrm{Co}}_{4}{\mathrm{O}}_{9}$. Four distinct transport regimes are found as a function of temperature, corresponding to a low temperature insulating one up to ${T}_{\mathit{min}}\ensuremath{\approx}63\phantom{\rule{0.3em}{0ex}}\mathrm{K}$, a strongly correlated Fermi liquid up to ${T}^{*}\ensuremath{\approx}140\phantom{\rule{0.3em}{0ex}}\mathrm{K}$, with $\ensuremath{\rho}={\ensuremath{\rho}}_{0}+A{T}^{2}$ and $A\ensuremath{\approx}3.63\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}2}\phantom{\rule{0.3em}{0ex}}\ensuremath{\mu}\ensuremath{\Omega}\phantom{\rule{0.2em}{0ex}}\mathrm{cm}∕{\mathrm{K}}^{2}$, followed by an incoherent metal with ${k}_{F}l\ensuremath{\leqslant}1$ and a high temperature insulator above ${T}^{**}\ensuremath{\approx}510\phantom{\rule{0.3em}{0ex}}\mathrm{K}$. The specific heat Sommerfeld coefficient $\ensuremath{\gamma}=93\phantom{\rule{0.3em}{0ex}}\mathrm{mJ}∕(\mathrm{mol}\phantom{\rule{0.2em}{0ex}}{\mathrm{K}}^{2})$ confirms a rather large value of the electronic effective mass and fulfils the Kadowaki-Woods ratio $A∕{\ensuremath{\gamma}}^{2}\ensuremath{\approx}0.45\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}5}\phantom{\rule{0.3em}{0ex}}\ensuremath{\mu}\ensuremath{\Omega}\phantom{\rule{0.2em}{0ex}}\mathrm{cm}\phantom{\rule{0.2em}{0ex}}{\mathrm{K}}^{2}∕({\mathrm{mJ}}^{2}\phantom{\rule{0.2em}{0ex}}{\mathrm{mol}}^{\ensuremath{-}2})$. Resistivity measurements under pressure reveal a decrease of the Fermi liquid transport coefficient A with an increase of ${T}^{*}$ as a function of pressure while the product $A{({T}^{*})}^{2}∕{b}_{2}$ remains constant and of order $h∕{e}^{2}$. Both thermodynamic and transport properties suggest a strong renormalization of the quasiparticles coherence scale of order ${T}^{*}$ that seems to govern also thermopower.

We have studied the longitudinal and the transverse velocity fluctuations of a moving vortex lattice (VL) driven by a transport current. They exhibit both the same broad spectrum and the same order of magnitude. These two components are insensitive to the velocity and to a small bulk perturbation. This means that no bulk averaging over the disorder and no VL crystallization are observed. This is consistently explained referring to a previously proposed noisy flow of surface current whose elementary fluctuator is measured isotropic., Comment: accepted for publication in Phys Rev B

In a recent paper, Matl et al present a high-frequency study of the complex resistivity of a pinned vortex lattice in YBaCuO . They focus on the inductive-to-resistive transition which is investigated as a function of temperature at a constant field $B_0=2$ T, so that the transition is associated with the vanishing of vortex pinning strength. To our view, their conclusions rely on a rather brittle experimental body and the collapse of C66 results from an involved analysis of the finite frequency corrections to $\rho (\omega)$. These corrections are not necessary since the complex frequency spectrum has been previously interpreted by the two modes model, first proposed for low Tc materials. We think that it is more adequate to interpret the present data and should be at least considered.

Substitutions at the Mn site of the charge-ordered ${\mathrm{Pr}}_{0.5}{\mathrm{Ca}}_{0.5}{\mathrm{MnO}}_{3}$ manganite is an effective way to induce abrupt jumps on the magnetic-field-driven magnetization curve. In order to get new insights into the origin of this remarkable feature, the ${\mathrm{Pr}}_{0.5}{\mathrm{Ca}}_{0.5}{\mathrm{Mn}}_{0.97}{\mathrm{Ga}}_{0.03}{\mathrm{O}}_{3}$ perovskite manganite has been studied by neutron diffraction versus temperature and at 2.5 K in an applied magnetic field up to 6 T. A weak and complex antiferromagnetic order is found for the low-temperature ground state. Magnetic transitions, associated with structural ones, are evidenced for certain strengths of magnetic field, which gives rise to the steplike behavior corresponding to the magnetization curve. Small-angle neutron scattering provides evidence for a nucleation process of micron-size ferromagnetic domains which follows the magnetization behavior.

We report complex impedance measurements in an untwinned YBaCuO crystal. Our broad frequency range covers both the quasi static response and the resistive response of the vortex lattice. It allow us to characterize the irreversibility line without the need of any frequency dependent pinning parameters. We confirm the validity of the two modes model of vortex dynamic, and extract both the surface critical current and the flux flow resistivity around the first order transition $T_{m}$. This latter is identified by the abrupt loss of pinning and by an unexpected step of $\rho_{ff}(T)$ at $T_{m}$., Comment: accepted for publication in EPJB

We have evidenced by small-angle neutron scattering at low temperature the coexistence of ferromagnetism (F) and antiferromagnetism (AF) in ${\mathrm{P}\mathrm{r}}_{0.67}\mathrm{C}{\mathrm{a}}_{\mathrm{0.33}}\mathrm{M}\mathrm{n}{\mathrm{O}}_{\mathrm{3}}$. The results are compared to those obtained in ${\mathrm{P}\mathrm{r}}_{0.80}{\mathrm{C}\mathrm{a}}_{0.20}{\mathrm{M}\mathrm{n}\mathrm{O}}_{3}$ and $\mathrm{P}{\mathrm{r}}_{\mathrm{0.63}}{\mathrm{C}\mathrm{a}}_{0.37}{\mathrm{M}\mathrm{n}\mathrm{O}}_{3}$, which are F and AF, respectively. Quantitative analysis shows that the small-angle scattering is not due to a mesoscopic mixing but to a nanoscopic electronic and magnetic ``red cabbage'' structure, in which the ferromagnetic phase exists in the form of thin layers in the AF matrix (stripes or 2D ``sheets'').

Up to now, electric-field-induced nonlinear conduction in the ${\mathrm{Pr}}_{1\ensuremath{-}x}{\mathrm{Ca}}_{x}{\mathrm{MnO}}_{3}$ system (PCMO) has been ascribed to a current-induced destabilization of the charge ordered phase. However, for $xl~0.25,$ a ferromagnetic insulator state is observed, and charge ordering (CO) is absent whatever the temperature. A systematic investigation of nonlinear transport in the ferromagnetic insulator ${\mathrm{Pr}}_{0.8}{\mathrm{Ca}}_{0.2}{\mathrm{MnO}}_{3}$ shows rather similar results to those obtained in charge-ordered systems. However, the experimental features observed in ${\mathrm{Pr}}_{0.8}{\mathrm{Ca}}_{0.2}{\mathrm{MnO}}_{3}$ are distinct in that the collapse of the CO energy gap cannot be invoked as is usually done in the other members of the PCMO system. We propose interpretations in which the effectiveness of the double exchange is restored upon application of an electric field.

We describe how the ferromagnetic spin echo technique may be used to make a direct and microscopic measurement of the motion of flux lines in a superconductor under the influence of an applied current. Our experimental results agree well with theory and indicate that both the value and distribution of velocities of flux lines may be measured.

We have used the neutron spin-echo technique to measure the small energy change of neutrons which are diffracted by a moving vortex lattice in a low-pinning Nb-Ta superconducting sample. A transport current was passed in the mixed state to cause flux line movement. In the case of uniform motion, the flux velocity ${\mathbf{v}}_{L}$ was given as expected by the values of electric and magnetic fields, via $\mathbf{E}\phantom{\rule{0ex}{0ex}}=\phantom{\rule{0ex}{0ex}}\ensuremath{-}{\mathbf{v}}_{L}\ensuremath{\wedge}\mathbf{B}$. We show that with a nonuniformly moving vortex lattice, one can measure the dispersion of the velocities, opening up new possibilities for investigating moving vortex lines.

International audience; no abstract

Using a one-dimensional model of Brownian motion of vortices over pinning wells, we derive a current-voltage equation assuming a distribution of the effective pinning length. The model describes linear and nonlinear regimes of the $E(J)$ curves in a single-particle description. Thermally activated flux flow and flux flow define the two limits of the dissipative process, respectively, at very low and high current. The pinning relief is described by pinning well depth and pinning well gradient, respectively, which can be checked by resistive and current-voltage measurements. The model is then applied to a $\mathrm{Y}{\mathrm{Ba}}_{2}{\mathrm{Cu}}_{3}{\mathrm{O}}_{7\ensuremath{-}\ensuremath{\delta}}$ thin film. It provides a phenomenological model of the dissipation induced by transport current.

We report an extensive investigation of the complex penetration depth λAC for a transverse field in the mixed state for an untwinned YBaCuO crystal and conventional superconductors (PbIn, Nb, V). A broad frequency range (Ω/2 π =1 kHz – 10 MHz ) is used to cover the crossover Ω p from pinned to viscous vortex line dynamics. Skin effect conditions are realized by using thick samples d∼1 mm allowing a direct comparison with theory. Bulk pinning theories rely on a local complex resistivity, which entails a relatively narrow pinning crossover λ AC ∝(1− i Ω/Ω p ) −1/2 . Recent 2-mode-theories, which treat pinning as a boundary condition problem, predict a broad crossover λ AC ∝(1+(− i Ω/Ω p ) 1/2 ) −1 . Our results are fully consistent with the 2-modes electrodynamics and show the irrelevance of bulk pinning in homogeneous samples.

Experimental data of the reversible magnetization of Tl 2 Ba 2 Ca 2 Cu 3 O 10 (Tl-2223) crystals on both sides of the superconducting transition temperature, T c0 , for magnetic field, H applied perpendicularly to the ab (CuO 2 ) planes are presented. The data are analyzed in terms of order parameter and vortex fluctuations in multilayered superconductors.

The current-voltage (I–V) characteristics are investigated in a high quality Tl-2212 thin film in magnetic fields (up to 8 Tesla) applied perpendiculary to the layers. We find experimental support for the existence of a crossover from (3D) to (2D) vortex fluctuations occuring at magnetic field B cr . For B ≪ B cr , the scaling behaviour of the vortex glass transition is observed with critical exponents z ≈ 3 and ν ≈ 1.9, which are field independent. At sufficiently large fields B ≫ B cr , we show that the thereshold measurable ohmic resistance can be attributed to melting of a vortex lattice triggered by (2D) vortex fluctuations.

A power law, V ∼ I a , between voltage and current is reported in a Tl-2212 thin film in a weak magnetic field (up to 300 mT) applied perpendicularly to the layers. We find experimental support for the existence of a phase transition depending on magnetic field, below the zero magnetic field transition T co . We discuss these results in terms of a defect-mediated vortex-lattice (VL) melting leading to a Berezinskii-Kosterlitz-Thouless transition type. Consistency is found with a logarithmic coupling energy U(r) = A ln(r/r o ) between VL defects where the unrenormalized interaction constant A ∼ T ( T co − T ) 3 2 / H and the Abrikosov lattice parameter sets the cut-off r o .

The addition of columnar defects in Bi-2212 crystals shifts the irreversibility line to a position which corresponds roughly to one vortex per defect in a broad range of intermediate temperatures. However, a precise analysis of this shift suggests that there is a continuous melting of the vortex lattice. An other transition can be seen on the activation energy measured from the temperature dependence of the resistivity. At the last transition, there is a crossover in the angular dependence of the persistent current from a 2D to a 3D behaviour.

We have used a variety of microscopic techniques to reveal the structure and motion of flux line arrangements, while the flux lines are caused to move by a transport current. Using small-angle neutron scattering, we are able to demonstrate direaly the alignment of the nearest-neighbour direction of flux lines while they are moving. The results are similar but not identical to those from two-dimensional simulations. We also see the destruction of the flux lattice by plastic flow and the bending of flux lines. We have also employed μSR, which measures the distribution of field values within the mixed state, to obtain information about the speed and degree of order of the motion of flux lines past the muons. Such measurements give complementary information to that obtained by neutron techniques. The general aspects of these measurements, of interest to those working in other areas, are the creation of order in non-equilibrium situations and the interplay of the ordering cccuning during dissipation and the ordering caused by the crystal structure of the superconductor which is host to the flux lines.