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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.

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.

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.

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.

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#

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}

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.

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

{ital c}-axis-oriented superconducting Bi{sub 2}Sr{sub 2}Ca{sub 2}Cu{sub 3}O{sub 10+{delta}} (Bi-2223) tapes have been irradiated with 5.8-GeV Pb ions to produce columnar defects along the {ital c} axis. The magnetization {ital M} as a function of field and temperature were measured for both the unirradiated and irradiated samples at matching field {ital B}{sub {phi}} values of 1.28, 2.0, and 4 T. In striking contrast to the conventional log({ital B}) dependence of the reversible {ital M} in the London regime, the field dependence of the reversible {ital M} for the irradiated samples exhibits some unusual characteristics. A large reduction in the value of {ital M} was observed at {ital B}{le}{ital B}{sub {Phi}}, while a maximum appears at {ital B}{approx_gt}{ital B}{sub {Phi}}. We explain this field dependence in terms of the London model modified by the localization of the vortex lines in the columnar defects. Based on this study, we obtained the single vortex pinning energy for Bi-2223 by these columnar defects. {copyright} {ital 1996 The American Physical Society.}

In order to get to the bottom of substitution effects reported in CuCrO2 delafossite, structural, magnetic, and electric properties of several CuCr1−xMxO2 series, where M3+ is a S = 0 cation (M3+ = Al3+, Ga3+, Sc3+, Rh3+), have been investigated. It is shown that the homogeneity of the substitution is element dependent and is particularly difficult to achieve for the smallest Al3+ cation. Ferroelectricity and dielectric properties are found to depend on the x value, but also on the nature of M: The largest substitution effects are observed for the larger elements, owing to the decrease of the Cr3+-Cr3+ interaction resulting from the enlargement of the in-plane lattice parameter. However, the magnetic structures of CuCr0.9Ga0.1O2 and CuCr0.9Rh0.1O2, revealed close to that of CuCrO2 by neutron diffraction, demonstrate the great stability of this antiferromagnetic incommensurate structure against the presence of {M3+, S = 0} cations. This emphasizes the very different nature of the magnetic ground state of CuCrO2, as compared to CuFeO2, despite their common structural characteristics.

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.

BiFe0.5Mn0.5O3 could be stabilized in the perovskite structure by preparing it under high pressure and temperature. It has an orthorhombic structure with a possible ordered arrangement of Fe and Mn double rows and shows a magnetic ordering at high temperature (270 K). Low-temperature isothermal-magnetization measurements indicate the ground state to be antiferromagnetic with a spin canting. Surprisingly, it exhibits magnetization reversal at low applied fields below a compensation temperature, T∗. Below T∗, the sign of the magnetization can be switched between negative and positive value reversibly by increasing and decreasing the field. Magnetization reversal in this oxide seems to result from a competition between single-ion magnetocrystalline anisotropy and antisymmetric Dzyaloshinsky-Moriya interactions.

We demonstrate that ethylammonium copper chloride, (C2H5NH3)2CuCl4, a member of the hybrid perovskite family is an electrically polar and magnetic compound with dielectric anomaly around the Curie point (247 K). We have found large spontaneous electric polarization below this point accompanied with a color change in the sample. The system is also ferroelectric, with large remnant polarization (37??C/cm2) that is comparable to classical ferroelectric compounds. The results are ascribed to hydrogen-bond ordering of the organic chains. The coexistence of ferroelectricity and dominant ferromagnetic interactions allows to relate the sample to a rare group of magnetic multiferroic compounds. In such hybrid perovskites the underlying hydrogen bonding of easily tunable organic building blocks in combination with the 3d transition-metal layers offers an emerging pathway to engineer multifuctional multiferroics.

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.

Small angle neutron scattering (SANS) magnetic and electrical transport measurements were performed to study a single crystal of Pr0.7Ca0.3MnO3, a colossal magnetoresistive (CMR) material. While the magnetic field induced transformation of this phase separated compound consisting of an antiferromagnetic insulating phase (AFI) and a ferromagnetic insulating phase (FI), is continuous at high temperature (above 5K), at lower temperature a step like transformation is observed (around 5T at 2K). Macroscopic magnetization measurements and SANS indicate that this transformation occurs by the formation of mesoscopic ferromagnetic metallic (FM) domains in the AFI phase, and, eventually, in the FI phase. Although above 5K this transformation is continuous, below 5K a magnetization step marks the abrupt transition from a large scale FI/AFI phase separation to a large scale phase separation between AFI, FI and FM phases. Our results suggest that relaxation of elastic strains inherent to the coexistence of these different phases plays a crucial role in the mechanism of these transformations. The occurrence of magnetization steps could result from an intrinsic behavior of the AFI phase at low temperature.

We study the magnetoelectric coupling in $\mathrm{Y}\mathrm{Mn}{\mathrm{O}}_{3}$ single crystal, in which a part of ${\mathrm{Mn}}^{3+}$ ions is substituted by nonmagnetic ${\mathrm{Ga}}^{3+}$ ions. While the antiferromagnetic ordering temperature is gradually suppressed by Ga doping, the magnetocapacitance is enhanced by two orders of magnitude, which we attribute to the lifting of frustration of interlayer spin interactions in doped samples. We also find that the dielectric constant anomaly below magnetic ordering temperature is strongly anisotropic, which we explain using a phenomenological Landau description of ferroelectric antiferromagnets.

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.

${\mathrm{V}}_{2}{\mathrm{O}}_{3}$ thin films about $1000\phantom{\rule{0.3em}{0ex}}\mathrm{\AA{}}$ thick were grown on ${\mathrm{Al}}_{2}{\mathrm{O}}_{3}$ (0001) by pulsed laser deposition. The x-ray diffraction analysis is in agreement with the $R\text{\ensuremath{-}}3c$ space group. Some of them exhibit the metal/insulator transition characteristic of ${\mathrm{V}}_{2}{\mathrm{O}}_{3}$ bulk material and other samples exhibit a metallic behavior. For the latter, the x-ray photoelectron spectroscopy analysis indicates an oxidation state of $+III$ for vanadium. There is no metal/insulator transition around $150\phantom{\rule{0.3em}{0ex}}\mathrm{K}$ in this sample and a strongly correlated Fermi liquid $\ensuremath{\rho}\ensuremath{\sim}\mathrm{A}\phantom{\rule{0.3em}{0ex}}{\mathrm{T}}^{2}$ behavior of the resistivity at low temperature is observed, with a value of A of 1.2 ${10}^{\ensuremath{-}4}\phantom{\rule{0.3em}{0ex}}\ensuremath{\Omega}\phantom{\rule{0.2em}{0ex}}\mathrm{cm}$, three times larger than the bulk value at $25\phantom{\rule{0.3em}{0ex}}\mathrm{kbar}$.

Transport properties of the good thermoelectric misfit oxide ${\mathrm{Ca}}_{3}{\mathrm{Co}}_{4}{\mathrm{O}}_{9}$ are examined. In-plane resistivity and Hall resistance measurements were made on epitaxial thin films which were grown on $c$-cut sapphire substrates using the pulsed laser deposition technique. Interpretation of the in-plane transport experiments relates the substrate-induced strain in the resulting film to single crystals under very high pressure $(\ensuremath{\sim}5.5\phantom{\rule{0.3em}{0ex}}\mathrm{GPa})$ consistent with a key role of strong electronic correlation. They are confirmed by the measured high temperature maxima in both resistivity and Hall resistance. While holelike charge carriers are inferred from the Hall effect measurements over the whole investigated temperature range, the Hall resistance reveals a nonmonotonic behavior at low temperatures that could be interpreted with an anomalous contribution. The resulting unconventional temperature dependence of the Hall resistance seems thus to combine high temperature strongly correlated features above $340\phantom{\rule{0.3em}{0ex}}\mathrm{K}$ and an anomalous Hall effect at low temperature, below $100\phantom{\rule{0.3em}{0ex}}\mathrm{K}$.

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.

Thin films of ${\mathrm{Pr}}_{0.5}{\mathrm{Ca}}_{0.5}{\mathrm{MnO}}_{3}$ manganites exhibiting charge∕orbital-ordered properties with colossal magnetoresistance have been synthesized by the pulsed laser deposition technique on both $(100)\text{\ensuremath{-}}{\mathrm{SrTiO}}_{3}$ and $(100)\text{\ensuremath{-}}{\mathrm{LaAlO}}_{3}$ substrates. The effects of current-induced metallic-behavior of the films are investigated as a function of the temperature and the magnetic field. Calculations based on a heat transfer model across the substrate, and our resistivity measurements reveal effects of Joule heating on charge transport over certain ranges of temperatures and magnetic fields. Our results also indicate that a nonlinear conduction, which cannot be explained by homogeneous Joule heating of the film, is observed when the material is less resistive $(l{10}^{\ensuremath{-}2}\phantom{\rule{0.3em}{0ex}}\ensuremath{\Omega}.\mathrm{cm})$. The origin of this behavior is explained with a model based on local thermal instabilities associated with phase-separation mechanism and a change in the long range charge-ordered state.

The influence of twin boundaries (TB) on the flux-line-lattice (FLL) structure was investigated by smallangle neutron scattering (SANS) .Y Ba 2Cu3O7˛d single crystals possessing different TB densities were studied. The SANS experiments show that the TB strongly modify the structure of the FLL. The flux lines meander as soon as the magnetic field makes an angle with the TB direction. According to the value of this angle but also to the ratio of the flux lines density over the TB density, one observes that the FLL exhibits two different unit cells in the plane perpendicular to the magnetic field. One is the classical hexagonal and anisotropic cell while the other is affected by an additional deformation induced by the TB. We discuss a possible relation between this deformation and the increase of the critical current usually observed in heavily twinned samples.

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.

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'').

We have investigated the magnetic-field-induced phase transition of charge-ordered (CO) ${\mathrm{Pr}}_{0.5}{\mathrm{Ca}}_{0.5}{\mathrm{MnO}}_{3}$ thin films, deposited onto (100)-oriented ${\mathrm{LaAlO}}_{3}$ and (100)-oriented ${\mathrm{SrTiO}}_{3}$ substrates using the pulsed laser deposition technique, by measuring the transport properties with magnetic fields up to 22 T. The transition to a metallic state is observed on both substrates by application of a critical magnetic field ${(H}_{C}g10 \mathrm{T}$ at 60 K). The value of the field required to destroy the charge-ordered insulating state, lower than the bulk compound, depends on both the substrate and the thickness of the film. The difference of the critical magnetic field between the films and the bulk material is explained by the difference of in-plane parameters at low temperature (below the CO transition). Finally, these results confirm that the robustness of the CO state, depends mainly on the stress induced by the difference in the thermal dilatations between the film and the substrate.

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.

The application of a sufficiently large magnetic field in ${\mathrm{Pr}}_{0.63}{\mathrm{Ca}}_{0.37}{\mathrm{MnO}}_{3}$ can enforce a ferromagnetic state, resulting in an ``insulator-metal'' transition owing to both actuation of double-exchange mechanism and charge-order melting. In the present study, the low-temperature magnetoelectronic state of this compound has been investigated by specific-heat measurements. It was shown that the metamagnetic transition has clear signatures in specific-heat data. Combination of specific heat, magnetization, and resistivity measurements allowed us to propose a comprehensive picture of the hysteretic features around this transition in the frame of a field-dependent phase-separation scenario. A more quantitative analysis also gave strong support to the percolative nature of the magnetoresistive transition in this compound.

Angle-resolved photoemission using synchrotron radiation from single crystals of natural graphite is reported for the first time. Direct as well as indirect transitions are observed. The $\ensuremath{\pi}$ bands and the top of the $\ensuremath{\sigma}$ bands are obtained with the assumption of a two-dimensional dispersion. A d\ifmmode\acute\else\textasciiacute\fi{}etailed discussion of the three-dimensional dispersion is presented for the $\ensuremath{\pi}$ bands. In the $\ensuremath{\Delta}$ direction, the normal dispersion is described and leads to an effective mass of ${m}_{\ensuremath{\perp}}^{*}=1.2m$. Near the line $P$, the $\ensuremath{\pi}$ bands are analyzed with use of the Slonczewski-Weiss-McClure model. Numerical values for the parameters within this model are derived. Some general features of the conduction bands at low energy are also given. Finally the experimental results are compared with the band-structure calculations.

We have studied the magnetic specific heat of first and second stages of both ${\mathrm{FeCl}}_{3}$ and ${\mathrm{CoCl}}_{2}$ intercalated in graphite. The behavior is very different on both sides of the transition: The broad high-temperature tails superimposable for both stages in both compounds indicates a two-dimensional (2D) behavior in this range of temperatures. For the first-stage compounds the low-temperature tail has a ${T}^{3}$ behavior, the maximum is stage dependent and is interpreted as a 2D to 3D crossover. We discuss the shape and the origin of these magnetic specific heats according to 2D models which should include in these compounds the islandic nature of the intercalated layers.