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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}

Pinning properties of (Bi/Pb)-2223 textured ceramics have been improved by 6 GeV Pb ions irradiation. Two samples were irradiated at fluencesφt=5 × 1010 ions/cm2 and 2 × 1011 ions/cm2, respectively. The magnetic irreversibility is clearly improved in the intermediate range of temperature: (30–35) K

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.

Thin films of manganite materials (Pr0.7Ca0.3-xSrxMnO3-δ) have been grown, from a sintered target with the same cationic composition, by rf reactive magnetron sputtering. The film and target compositions have been verified as identical, within experimental errors, by energy-dispersive scattering (EDS) analysis. It is shown for the first time, from X-ray and electron diffraction studies, that the films, grown on LaAlO3 or SrTiO3 substrates, are [101] oriented. Nanostructural studies show that the defects are the same as those observed in bulk materials but that the magnetic behavior is different (Curie temperature is decreased from 130 to 50K). From a structural point of view this difference in magnetic behavior is difficult to understand, but the role of the oxygen content, the small grain size observed in such films, and the strains induced by the substrate film interactions is discussed.

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.

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 role of rare earth ion R3+ in spin reorientation and magneto dielectric response is investigated by substitution of non-magnetic smaller ionic radii Lu3+ in multiferroic hexagonal HoMnO3. The XRD analysis suggests that the dopant may preferably goes to C3V site up to 1/3rd of the composition in order to reduce the lattice distortion. We suggest that the R3+ ion at C3 site could play a strong role in spin reorientation than the C3V site. The observation of TSR even in LuMnO3 precludes the role of rare earth magnetic moment in driving the spin reorientation. Surprisingly, the magneto dielectric response of HoMnO3 is dominated by the rare earth RO 8 dipoles. The oppositely oriented RO8 dipole at the C3V and C3 determines the magneto dielectric response in various magnetic phases reaffirming the site specific substitution. Thus, site specific doping could be a way to enhance the magnetoelectric coupling strength. � 2013 AIP Publishing LLC.

The title compounds are prepared by solid state reactions of Cu, CuO, Cr2O3, and V2O5 (1100 °C, 48 h).

The change from antiferromagnetism induced ferroelectricity to spin glass ferroelectric relaxor has been studied along the CuCr 1− x V x O 2 (0≤ x ≤0.5) solid solution of polycrystalline samples. As x increases from CuCrO 2 ( x =0) to CuCr 0.82 V 0.18 O 2 , it is found that the Neel temperature decreases from ∼24 K down to ∼13 K. This progressive weakening of the antiferromagnetism of CuCrO 2 induces a rapid decrease of the spin induced ferroelectricity with polarization values going from ∼44 μC/m 2 down to ∼1.5 μC/m 2 for x =0.04 and x =0.08, respectively. Beyond x =0.18 (0.20≤ x ≤0.50), ac-magnetic susceptibility and magnetization measurements evidence a spin glass state while dielectric permittivity and polarization measurements point towards a relaxor behaviour. This shows that competing magnetic interactions in delafossites are an efficient way to transform a spin induced magnetoelectric into a multiglass (spin and dipolar) state.

Current - voltage characteristics are reported for a thin film with a granular structure. Our data provide support for a Berezinskii - Kosterlitz - Thouless transition type with an unusual vortex - antivortex interaction . Dissipation is discussed in terms of large-scale pair excitations through the sample inhomogeneities. Consistency is found with the theory worked out by Lobb, Abraham and Tinkham to explain resistive transition data in arrays of superconducting junctions arranged two-dimensionally. On this basis, the temperature behaviour of A(T) accounts for a weakening of the pair potential at interfaces due to the short coherence length of high- oxides, as was pointed out by Deutscher and Muller. We investigate the spatial renormalization of the vortex - antivortex interaction over the length scale probed by the current in the same manner as was first demonstrated by Kadin, Epstein and Goldman. Then good agreement with the renormalized Kosterlitz - Thouless theory is found.

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.}

The systematic study of the solid solution Pr 1 2 − x Y x Sr 1 2 MnO 3 shows the coexistence of two types of CMR effect on the same sample associated with a transition from an antiferromagnetic semiconducting to a ferromagnetic metallic state (at T co ) and then to a paramagnetic semiconducting state (at T C ) as T increases. The competition between these two types of CMR effect is shown for the first time. For low x values, T co (for charge ordering) increases and T C decreases as the mean size of the interpolated cation decreases. Beyond a critical x value the two antagonist effects interpenetrate each other so that they tend to disappear. The ρ ( T ) curves, registered in the Earth's magnetic field ( B = 0) and in B = 5 T show a negative CMR effect similar to that observed for Pr 1 2 Sr 1 2 MnO 3 , i.e. a R 0 / R B = 5 T ≅ 10 2 at 120 K for x = 0.04. It is remarkable that the application of a magnetic field has the same effect as increasing the size of the interpolated cation: it decreases T co and increases T C . The electron microscopy study of these samples shows that they exhibit a ‘pure’ body centered cell (Imma) derived from the perovskite ( a ≈ c ≈ a p √2 and b ≈ 2 a ).

Calcium doping of the type II CMR manganites Pr 1/2 Sr 1/2 MnO 3 has allowed the transition temperature T N to be significantly increased up to 190 K. The systematic study of the solid solution Pr 1/2 Sr 1/2− x Ca x MnO 3 shows that the mean size of the interpolated cation plays a prominent role in the CMR properties of these oxides: T N increases significantly whereas T C decreases only slightly as the mean size of the interpolated cation decreases. This behavior is fundamentally different from that observed for Pr 1/2− x Y x Sr 1/2 MnO 3 for which T N increases slightly and T C decreases dramatically as the mean size of the interpolated cation decreases. The ρ( T ) curves registered in a magnetic field of 7 T show a negative CMR effect characterized by a dramatic decrease of T N and a complicated variation of the R 0 / R H ratio that reaches a maximum value of 3 × 10 3 at 150 K in a magnetic field of 7 T for x = 0.09.

The systematic study of the manganese perovskite Nd0.7Ca0.3−xSrxMnO3−δ has allowed colossal magnetoresistance effects (CMR) with resistance ratios (RR), RB=0/RB=5T, up to 104 at 60 K in a field of 5 tesla to be evidenced. The influence of the mean size of the interpolated cation (Nd, Ca, Sr), on the magnitude of the RR ratios, has been demonstrated for the first time: RR goes through a maximum for a particular value of the mean radius of the interpolated cation. This effect seems to be correlated with the evolution of the cell volume that exhibits a singular point around x=0.085. A second important feature deals with the irreversibility of the CMR effect at low temperature that decreases as T increases and disappears at Tmax. It is also remarkable that for ρ(T) curves characterized by a maximum, Tmax not only increases as the size of the interpolated cation increases, but also as the magnitude of the applied magnetic field increases.

Heavy ions irradiation of oxides can create columnar defects, i.e. amorphous tracks of about 100Ain diameter. These columnar defects are very efficient pinning centers for vortices in superconducting oxides: the critical current density is enhanced by a large amount and the irreversibility line which limits the possibility of any application is shifted to higher magnetic fields. In this paper, we review the creation of columnar defects by ion irradiation in YBaCuO, Bi-based and Tl-based superconducting oxides. We present the modification of the superconducting properties induced by the presence of such defects. Since the defects are very well controlled in shape and density, they provide a unique model system to investigate the pinning properties in high T c superconductors.

Thallium based single crystals (TlBa2Ca2Cu3O9 (Tl-1223), Tl2Ba2CaCu2O8 (Tl-2212) and Tl2Ba2Ca2Cu3O10 (Tl-2223)) have been exposed to Pb ions irradiation along the c-axis at fluences of 5 × 1010 and 2 × 1011ions/cm2. Reversible magnetization as a function of the field for different temperatures was measured for each compound with different track densities. Two regimes are observed depending on the relative values of B and the dose equivalent field BΦ. They are explained in the framework of the London model taking into account the modification of the interaction energy due to the presence of irradiation induced columnar defects.

The Pr0.7Ca0.25Sr0.05MnO3 polycrystalline sample, which exhibits a huge negative magnetoresistive effect of 2×107% at Tmax= 88 K with B = 5 T, has been studied by coupling resistive (R(T) and R(B)) and magnetic (M(T) and M(B)) measurements. At Tmax, both R(B) and M(B) loops show an hysteretic behavior for B > 1 T but R and M return to their initial values when B is returned to 0. Our results suggest that a phase transition between two different magnetic states occurs for this sample close to Tmax; this transition can be also induced by the application of a magnetic field of several tesla.

Splayed configurations of columnar defects with different characteristic angles θi relative to the c-axis have been achieved in single crystals of YBCO, (Tl, Bi)-1212 and Bi-2212. Their pinning efficiencies for Hc have been studied by magnetic measurements. An increased pinning efficiency consistent with the splayed model of Hwa et al. [Phys. Rev. Lett. 71 (1993) 3545] has been observed with θi=10° for YBCO and (Tl, Bi)-1212. On the other hand, the effect is deterimental for the more anisotropic compound Bi-2212. Moreover, higher angles up to θi=75° have yielded further pinning improvements in all the compounds for high temperatures and low fields, but these latter results are more delicate to interpret.

We found that the current–voltage characteristics measured on La0.66Sr0.33MnO3∕YBa2Cu3O7−δ thin film heterostructures patterned as microbridges are nonsymmetric if the magnetization direction of the La0.66Sr0.33MnO3 film is in plane and perpendicular to the current direction. This effect is attributed to the contribution of the magnetic moment of this film to the vertical component of the field along the edges of the bridge. We suggest that this effect could find an application in the reading process of magnetic memories.

Transition metal oxides belong to the class of iono-covalent materials in which electrons are, to first order, localized in tight metal-oxygen bonds. They appear to be quite different from classical metals in which electrons are assumed to be delocalized in a free electron gas. Nevertheless, due to exchange interactions and hybridization of atomic orbitals, oxides exhibit many interesting physical properties like magnetic ordering and various electron transport properties that extend from wide gap insulators to narrow band conductors and high-temperature superconducting (HTS) superconductors. This class of materials appears well-suited to the study of the damage induced by heavy ion irradiation and its consequence on physical properties like electrical resistivity.In this article, we will describe the results of heavy ion irradiations in magnetic oxides, exhibiting a wide palette of electron transport properties from insulators to metals through hopping semiconductors and in HTS copper oxides that are narrow-band metals aboveTc. We will show that heavy ion irradiation, in addition to disordering matter, can induce changes in some physical properties like magnetic ordering and orientation of magnetic fields, and can produce large increases of the critical current. To account for these results, the thermal spike model will be considered as a possible way to understand the damage creation in iono-covalent compounds.

The thermoelectric power (TEP) S versus temperature has been systematically investigated for several series of the superconducting cuprates Tl(Ba,Sr) 2 Ca m −1 Cu m O 2 m +3−δ ( m = 2, 3) and Tl 2 Ba 2 Ca m −1 Cu m O 2 m +4+δ ( m = 1, 2, 3). The consideration of the S ( T c ) curves allows two important points to be found evidence for. The first one deals with the fact that all these superconducting thallium cuprates are systematically overdoped whatever T c , and whatever the number of Cu or Tl layers; no underdoped superconducting cuprate could be obtained. The second point shows that there exist two classes of Tl cuprates: the weakly overdoped cuprates that exhibit a T c max ≥ 100 K (all the triple copper layer cuprates and the 2212 cuprates) and those which can be heavily doped that exhibit a T c max ≤ 90 K (the 2201 and the 1212 cuprates). The different behavior of thallium cuprates compared to YBa 2 Cu 3 O 7−δ and to bismuth cuprates is discussed.

Tl-based superconducting ceramics of different phases (Tl-2212, Tl-1223, Tl-2223) have been irradiated by heavy ions (6 GeV Pb and U). It is shown by high resolution electron microscopy that Pb irradiations induce columnar defects of about the same diameter (95 ± 5) A in Tl-2223 and Tl-1223 whereas a larger dispersion of the diameter around the value of 95 A is observed for Tl-2212. The U irradiation leads to diameters slightly more scattered than in the Pb irradiation. These samples have been studied by SQUID magnetometry before and after irradiation. Drastic increases of the critical current induced by the irradiation have been observed and the ‘hierarchy’ existing between phases about the location of their irreversibility lines remains the same after irradiation at the same fluence.

The magnetic behaviour of the 58 K superconductor oxycarbonate thin film, which consists of an intergrowth of several members of a family with agreement composition (CaCuO2)m(Ba2CuCO3O2)n has been studied. Its pinning properties in terms of critical current densities and irreversibility line have been established using their hysteresis cycles. Typical Jc was calculated to approximately 105 A cm-2 at 5 K in zero field. Finally, the irreversibility line is compared to those obtained by Kumakura et al. (1994) on bulk oxycarbonate superconductors synthesized at high pressure.

The exploration of the magnetic and transport properties of four series of manganese perovskites, Pr0.7Ca0.34−xAxMnO3−δ (A=Sr, Ba), Pr0.7−xLaxCa0.3 MnO3−δ and Pr0.66Ca0.34−xSrxMnO3−δ has allowed four phases with colossal magnetoresistive (CMR) properties to be isolated: Pr0.7Ca0.25Sr0.025MnO3−δ and Pr0.66Ca0.26Sr0.08MnO3−δ that exhibit a variation of resistance of 2.5. 107% and 109% at μ0 H=5 T for T=88 K and 50 K respectively, Pr0.58La0.12Ca0.3 MnO3−δ that exhibits a variation of 6.106% for μ0 H=5 T at T=80 K and Pr0.7Ba0.025Ca0.275MnO3−δ for which a resistance variation of 5.109%, at T=50 K, for μ0 H=5 T is evidenced. for each compound of this series except the barium phase, one observes that the temperature Tmax, which corresponds to the resistance maximum on the R(T) curves in zero magnetic field, increases dramatically as the mean size of the interpolated cations increases, and that the CMR effect correlatively decreases dramatically. The comparison of the two series Pr0.7Ca0.3−xSrxMnO3−δ and Pr0.66Ca0.34−xSrxMnO3−δ shows also the crucial role of the hole carrier density: for a same mean ionic radius of the interpolated cation Tmax is decreased of about 50 K by introducing 0.034 hole per Mn mole.

The influence of the doping state upon the magnetic properties of overdoped superconducting TlBa 0.8 Sr 1.2 Ca 2 Cu 3 O 9−δ single crystals has been investigated. The temperature dependence of the in-plane London penetration depth, of the irreversibility line and of the so-called fishtail effect for different annealing treatments in ArH 2 and in oxygen pressure is fundamentally different from that observed for other overdoped cuprates and especially from thallium bilayer cuprates. This remarkable difference of behavior between thallium monolayer and bilayer overdoped cuprates is interpreted in terms of crystallographic structure and hole-reservoir effects.

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.

International audience; The formation mechanism, phase stability, optical and magnetodielectric properties of new cubic MgMnO3 bulk and nano-crystals are reported. The diffuse reflectance spectroscopy of this compound shows a ligand-to-metal charge transfer transition p[O2−]→eg[Mg2+] at 4 eV and the lowest spin allowed metal-to-metal transition at 2.43 eV. In bulk MgMnO3 with the defect cubic structure [Mg2+][Mg2+1/3Mn4+4/3□1/3]O4 (Seehra et al. Appl. Phys. Lett. 97, 112507 (2010)) a clear change in the dielectric constant (ɛ′) is observed on cooling through its blocking temperature TB ∼17.5 K. This dielectric anomaly is magnetic field independent even at 14 kOe. A clear evidence of the magneto-dielectric effect is observed for T < TB. Possible origin of the magneto-dielectric coupling has been discussed.

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.

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.

The fishtail effect has been intensively investigated in single crystals of six Tl based compounds: Tl2Ba2CaCu2O8, Tl2Ba2Ca2Cu3O10, TlBa2CaCu2O7, TlBa2Ca2Cu3O9, (Tl, Bi) Sr2CaCu2O7 and (Tl, Pb) Sr2CaCu2O7. The location in the H-T plane of the line of the second peak has been determined in each case. This location depends strongly on the nature of the compound (structure and cationic composition) whereas it is essentially sample independent. No influence of the method of preparation, of the size of the sample and of annealing treatments (oxygen non-stoichiometry) has been observed. Relaxation studies have demonstrated that the appearance of the second peak cannot be accounted for simply by a decrease of the relaxation rate. In particular, the occurrence around the second peak of a crossover between two regimes of collective creep has been discarded. Otherwise, the transient increase of pinning efficiency which characterizes the fishtail effect could be induced by a softening of the vortex lattice allowing a better adaptation to the structure of defects to occur. Several scenario's can be invoked such as a 3D-2D transition or a 3D melting. On the basis of three of these models, values of the electronic anisotropy for each compound have been calculated.

Nous presentons une etude sur l'influence des defauts vis-a-vis du piegeage des lignes de flux magnetique dans des monocristaux de Bi2Sr2Ca1Cu2Ox. Ces monocristaux, non irradies ou irradies sur l'accelerateur du GANIL avec des ions (Pb56+) d'energie 6 GeV ont ete decores dans l'etat supraconducteur selon la technique de Bitter. Les reseaux de lignes de flux magnetique ont ete caracterises par microscopie electronique a balayage et avec un analyseur d'images. Les resultats montrent que les reseaux de lignes de flux magnetique sont fortement dependant de la densite de defauts resultant de l'irradiation. Pour caracteriser l'etat de desordre, nous avons applique une methode de calcul de termes d'energie elastique des reseaux de lignes de flux magnetique. Cette methode ne peut etre appliquee que si la transformee de Fourier des reseaux de vortex presente des reflexions discretes.

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.

High-quality millimeter size single crystals of Tl 0.5 Pb 0.5 Sr 2 CaCu 2 O 7 with T c = 82 K have been synthesized for the first time. The regularity of their structure has been checked by using X-ray diffraction techniques and electron microscopy. Magnetic measurements have been performed in the geometry B | c . Fitting the reversible magnetization versus B with the London model yields the temperature dependences of both λ ab and H c2 . From these results the following superconducting parameters have been obtained: (- d B c2 /d T ) T c = 1.71 T/K, ξ ab (0) = 1.92 nm and λ ab (0) = 173.9 nm. Furthermore, Tl 0.5 Pb 0.5 Sr 2 CaCu 2 O 7 single crystals exhibit the highest irreversibility line B ∗ ( T / T c ) among all the single crystals of thallium cuprates. Moreover a scaling of the hysteresis loops M ( B ) can be performed in a wide range of temperature. All these data are discussed in connection with the structural parameters of Tl 0.5 Pb 0.5 Sr 2 CaCu 2 O 7 .