Thermally activated flux dissipation in c-axis-oriented YBa2Cu3O7−δ ultrathin films

The resistive transitions of ultrathin YBa₂Cu₃O_7−δ (YBCO) films with thicknesses 75 and 200 Å were studied under magnetic fields. For the 75 Å film under a 5 T parallel magnetic field (Hbab-plane), no broadening of the resistive transition occurred. In the perpendicular magnetic field (H ab-plane), the broadening of the resistive transition of the 75 Å film is larger than that of the 200 Å thick film. The flux activation energy U was found to be linearly dependent on the temperature and logarithmically dependent on the magnetic field for both 75 and 200 Å films, which means the two samples have a two-dimensional vortex lattice. Furthermore, the activation energy U also increased with the film thickness, indicating that the magnetic correlation length in the c-axis direction l_c is larger than the 200 Å for bulk YBCO.     

Cracking and oxygenation of YBaCuO bulk superconductors: application to c-axis elements for current limitation

A process to elaborate crack free, oxygenated c-axis samples in YBa₂Cu₃O_x bulk superconductors is reported. This includes texturing of crack free pellets and development of an adequate oxygen annealing treatment to avoid cracking during oxygen uptake. For current limitation in c-direction such a process is needed as c-axis elements are particularly susceptible to ab-plane cracking during oxygenation. Mechanical properties have been compared for different annealing treatments by flexural strength measurements. With the load applied in ab-direction the mean flexural strength is 58 MPa in as-textured c-axis bars and 48 MPa in c-axis bars which were oxygenated using a customized annealing treatment. The conventional oxygen annealing treatments lead to macroscopic oxygenation cracks which reduced flexural strength to 10–20 MPa. Bars cut in ab-direction do not show significant variation in flexural strength (load applied in c-direction) as a function of the annealing condition with an average value of 145 MPa.     

Normal-state resistivity of superconducting Bi1.95Sr1.65La0.4CuO6+δ single crystals

The anisotropy of the normal-state resistivity of a number of superconducting Bi_1.95Sr_1.65La_0.4CuO_6+δ single crystals has been measured by using both a six- and a four-terminal technique. We find that the ab-plane resistivity _ab in general increases almost linearly with temperature (d_ab/dT> 0). the temperature variation of the c-axis resistivity _c, however, is strongly sample dependent. Both metallic-like (d_c/dT> 0) and mixed temperature dependences of _c, including d_c/dT < 0 are observed. The mixed _c(T) data can be well described by fitting to _c(T) = A + BT + C/T, indicating that the temperature dependence of _c(T) is the result of a competition between metallic and non-metallic terms. The temperature T_min at which _c reaches a minimum increases with increasing d_ab/dT, suggesting that the metallic term of _c arises from misaligned Cu---O planes. This is confirmed by an electron-microscope (HRTEM) analysis of the samples. The anisotropy ratio _c/_ab is of the order of 10³ but increases monotonically with decreasing temperature, indicating differences in the scattering processes parallel and perpendicular to the Cu---O plane.     

Crystal structure of YBa2Cu3O7−x thin films prepared by pulsed laser deposition with substrate bias voltage

The crystal structure of YBa₂Cu₃O_7−x thin films has been investigated by cross-section transmission electron microscopy. The samples were deposited on MgO (100) substrates at 670°C with substrate bias voltages of ±300 V. For the unbiased case, c-axis, a-axis and (103) oriented domains normal to the substrate surface were observed. In this film, the c-axis oriented domains are dominant, but the crystal often exhibits a longer c-lattice constant than that of the YBa₂Cu₃O_7−x system, so extra cationic layers are inserted in the YBa₂Cu₃O_7−x intrinsic stacking sequence. For the case of −300 V, rotated domains were dominant in the entire film; however, c-axis oriented domains also grow from the substrate surface. Small-angle semicoherent grain boundaries between them were observed. In the case of +300 V, all the grains show c-axis oriented YBa₂Cu₃O_7−x. The degree of preferential orientation of the grains is reduced at negative bias voltage of −300 V and the structure defects are reduced by applying a positive bias of +300 V.     

Temperature dependence of the resistivity and its anisotropy in n-type Nd1.85Ce0.15CuO4 single crystal

Measurements of the in-plane and c-axis normal-state resistivity in a superconducting Nd_1.85Ce_0.15CuO_4−δ single crystal are reported. The resistivity anisotropy of this n-type material is ≤250, much smaller than BiSCCO and comparable to YBaCuO. Both, _ab(T) and _c(T) displays a metallic-like positive temperature coefficient of resistivity with a basic T² dependence. We discuss some possible origins of this peculiar temperature dependence.     

Phase separation in YBa2Cu3O7−δ single crystals near δ = 0

X-ray diffraction, resistivity, AC and DC magnetization data on high-quality single crystals of YBa₂Cu₃O_7−δ are presented. We demonstrate that for δ<0.08, the (0 0 l) diffraction lines are split into two, indicating that at these high oxygen concentrations the crystals are no longer single phase but actually consist of two (or more) different phases with slightly different c-axis parameters. In the two-phase region, the electrical resistivity and the AC and DC magnetic susceptibilities show a broadening of the superconducting transition. This broadening is thought to be due to the proximity effect or strains in two finely dispersed phases with slightly different transition temperatures.     

Fabrication, microstructure and critical current density of screen-printed Ag-(Bi, Pb)2Sr2Ca2Cu3Ox superconducting tapes

The influence of the sintering conditions on the microstructure and critical current density J_c has been studied on screen-printed Ag-(Bi, Pb)₂Sr₂Ca₂Cu₃O_x tapes with a ceramics mono-layer core. Three kinds of fabrication processes, which consist of a combination of cold working (rolling and/or pressing) and sintering, are applied. Four times repetition of pressing and sintering after the pre-sintering produces the highest c-axis alignment and achieves J_c= 1.5 × 10⁴ A/cm² (77 K, 0 T). The J_c versus θ data with an angle θ between B and the c-axis elucidate the relation between the anisotropy ratio γ=J_c(Bc)/J_c(B|c and the half-height angular width Δθ of a peak for Bc. This is related to both grain alignment and the J_c value. An increase in J_c, which comes from an improvement for grain alignment, enhances γ and narrows Δθ. The J_c versus θ data are fitted to the expression J_c(B, θ)=J _c(B, 90°)/[(γ−1)|cos θ|ⁿ+1] by regarding both γ and n as adjustable parameters. Fabrication of screen-printed tapes with multilayers (1≤N≤5) is presented, where the critical current increases from 8.0 A to 30.2 A at 77 K and 0 T as N increases.     

Microstructure and transport properties of YBa2Cu3O7−δ films produced by laser ablation from a BaF2/Y2O3/CuO target

The effects of varying the temperature and duration of the post-deposition anneal in watersaturated oxygen were investigated for YBa₂Cu₃O_7−δ films of varying thickness. The films were produced by laser ablation from pressed powder targets consisting of BaF₂,Y₂O₃, and CuO mixtures. This technique produces superconducting films with a highly textured surface. The films were fabricated on SrTiO₃ substrates and were analyzed with X-ray diffraction, scanning electron microscopy, and temperature dependent resistivity. Critical current density (J_c) measurements were performed in magnetic fields up to 1 T. For film thickness on the order of 900 nm, completely c-axis oriented films were obtained with a 60 min anneal at 850°C. Thinner films required less annealing, either shorter times or lower temperatures, to achieve similar results, indicating that the optimal annealing conditions are dependent on film thickness.     

Low temperature magnetocaloric effects in Dy1Ba2Cu3O7−δ and Y1Ba2Cu3O7−δ high-Tc single crystals

The heat release has been measured in high temperature superconductors during and after a change of the external magnetic field μ₀H_a ≤ 0.5 T applied parallel to the c-axis at 0.1 K ≤ T ≤ 0.45 K. Two Y123 and one Dy123 twinned single crystals were investigated. An analysis of the heat release based on the Bean model and on a model for thin superconductors in perpendicular geometry recently proposed by Brandt et al. has been carried out. During field sweep we measured ∝ H_a²dH_a/dt. This dependence can be understood within the Bean model for longitudinal geometry. The critical current densities obtained with this method are in good agreement with those from magnetization measurements. We also present measurements, and a first quantitative analysis, of dissipation due to the time relaxation of the flux line lattice after a field sweep. For the heat release a t⁻¹ dependence has been found which corresponds to a logarithmic time dependence of the magnetization. The normalized relaxation rate we obtained is in agreement with literature values.     

Magnetic levitation of YBa2Cu3Oy single crystals

By using an ordinary electric balance, we are able to measure the magnetic levitation forces acting on a superconducting YBa₂Cu₃O_y (YBCO) single crystal. It is found that when the external magnetic field is parallel to the c-axis of the single crystal, the hysteretic levitation curve is similar to that of a melt-processed YBCO superconducting sample. However, when the external magnetic field is perpendicular to the c-axis of the YBCO single crystal, the levitation forces are too small to be measured by our equipment. Also, we have introduced a simple model with the Bean's approximations to explain the levitation forces. The critical current density derived from this model by fitting with experimental data is quite close to the value obtained from magnetization measurements.     

Enhanced diamagnetism in YBa2Cu3O7−δ by hydorgen-plasma treatment

We report that the diamagnetism of as-grown YBa₂Cu₃O_7−δ powders can be enhanced by treatment with RF-generated hydrogen plasma. The field-cooling susceptibility at 5 K is found to increase by up to 24%. The enhancement is reproducible for all five samples studied. The lattice parameter c decreases after hydrogenation. For a sample treated for the shortest period, its pinning potential,obtained from magnetic relaxation measurements, is reduced following H-treatment.     

Anomalous magnetotransport phenomena in θ-(BEDT-TTF)2I3

Anomalous magnetotransport phenomena have been observed in θ-(BEDT-TTF)₂I₃ crystals at temperatures below 15 K. The magnetoresistance M : (1) is a linear function of the magnetic field H, (2) is not affected by the angle between the electric current and the magnetic field, (3) but depends on the magnetic field orientation with respect to the crystal axis. Magnetoresistance is expressed as M = (aH²_a + bH²_b + cH²_c)₀^-3/2/H in terms of H = (H_a, H_b, H_c), the zero field resistivity ₀, and parameters a, b, and c which are independent of temperature and magnetic field. We have found that b a > c. Magnetoresistance up to 40 is observed for H = 7T along the b-axis at T = 1.5K.     

Magnetic properties of layer compound CsVF4

Magnetic properties of CsVF₄ have been studied by means of thermal diffusivity and neutron diffraction. The antiferromagnetic transition appearing at about 43 K is found to be of Ising spin order of V³⁺ magnetic moments which are oriented along the c-axis having the order of G-type. By applying a magnetic field vertically to the c-plane, it is also found that the magnetic moments of V³⁺ are re-oriented so as to be anti-parallel in each two-dimensional sheet of VF₆ octahedra and parallel between the network with the result that a ferromagnetic component appears.     

Neutron diffraction study of magnetic ordering in ErMn2Si2, ErMn2Ge2 and ErFe2Si2

Neutron diffraction study of polycrystalline compounds ErMn₂Si₂, ErMn₂Ge₂ and ErFe₂Si₂ was performed in the temperature range between 1.8 and 293 K. All compounds have tetragonal, ThCr₂Si₂-type crystal structure. The antiferromagnetic collinear structure of ErMn₂Si₂ and ErMn₂Ge₂ at both RT and LNT, consists of a sequence + - + - of ferromagnetic layers of Mn atoms. The magnetic moment of an Mn atom (≈2μ_B) is parallel to the c-axist. At low temperatures (LHT and lower), the ferromagnetic ordering within the Er sublattice is observed. The magnetic moment (μ_Er ≈ 9μ_B) is perpendicular to the c-axis. From the temperature dependence of the intensities of the magnetic peaks, the following values for the Curie temperatures were obtained: (10±5) K for ErMn₂Si₂ and (8.5±3) K for ErMn₂Ge₂. For ErFe₂Si₂ a collinear antiferromagnetic structure of the + - - + type was found, the magnetic unit cell consisting of the chemical one, doubled along the c-axis.     

Field-induced pinning effect of Nd(Ba1−xNdx)2Cu3O7−δ single crystals grown by the traveling-solvent floating-zone method

We have studied the superconducting properties of Nd(Ba_1−xNd_x)₂Cu₃O_7−δ (Nd123, x ≈ 0.1) single crystals grown by the traveling-solvent floating-zone method under 0.1% O₂ in Ar atmosphere. The enhancement of the magnetization with increasing field is observed in the hysteresis (M-H) loop in fields both parallel and perpendicular to the c-axis of the Nd123 single crystals as well as in the bulk crystals prepared by the oxygen-controlled-melt-growth (OCMG) method. The composition variation of Ba/Nd is observed in the matrix of Nd123 crystals by an analytical TEM equipped with a cold field-emission gun. It turns out that the enhancement is due to the field-induced pinning effect ascribed to the weak superconducting Nd---Ba substitution regions in the Nd123 matrix.     

Localization of charge carriers in the c-axis electronic transport of YBa2Cu3Oχ

Reflectance measurements with E c on a YBa₂Cu₃O_x single crystal where the oxygen content was varied between x = 6.8 and X = 6.98 are presented in the far infrared range (5 rmmeV < ω < 100 meV) at temperatures between 10 K and 300 K. Performing a Kramers-Kronig transformation we derived the optical conductivity δ _c ( ω). An analysis of the spectral weight shows that the main contribution of the electronic transport is due to localized carriers. With decreasing the oxygen content, the spectral weight of free carriers decreases whereas the spectral weight of localized carriers is enhanced.     

Electrical properties of the intermetallic compound Mn3Sn

Temperature variations of resistivities are measured on the single crystals of Mn₃Sn with a hexagonal crystal structure. Anomalies of resistivity along the c-axis are observed at 230 and 420 K which correspond to the transition temperature from a tringle-helix to triangular spin structure state and the Néel temperature, respectively.     

Magnetic properties of UFe5Sn single crystals

Millimetre-size UFe₅Sn single crystals were grown by the top seed solution growth method and characterized by magnetization, ⁵⁷Fe Mössbauer spectroscopy and specific heat measurements in order to study the magnetic transitions detected in powder samples at 248 and 178 K. The magnetization measurements show different behaviour along the three crystallographic directions but with similar values of spontaneous magnetization along a and c. The transition at 248 K is associated with ferromagnetic ordering of iron moments along the c-axis, while the transition at lower temperature is associated with a reorientation towards b. Mössbauer data show that this reorientation is concomitant to the ordering of the Fe2 sites, which in a large proportion remain paramagnetic between the two transition temperatures. Specific heat measurements are consistent with the establishment of magnetic ordering at 248 K, followed by a spin reorientation at 178 K, yielding γ(0 K)140 mJ/(mol K²) and θ290 K for UFe₅Sn.     

Electron concentration in YBa2Cu3O6.9 single crystals determined from field electron emission spectroscopic measurements

Results of field electron emission spectroscopic investigations of YBa₂Cu₃O_6.9 single crystal specimens oriented perpendicular to the c-axis are presented. It is shown that the shift of the total energy distribution of field-emitted electrons from YBa₂Cu₃O_6.9 along the energy scale with increasing electric-field strength at the surface of the specimen is due to the penetration of the electric field into the subsurface region of the YBa₂Cu₃O_6.9 specimen. The electron concentration in YBa₂Cu₃O_6.9 found from spectroscopic measurements is n=(2−4) × 10²¹ cm⁻³. Field electron emission spectroscopy and microscopy of YBa₂Cu₃O_6.9 allows one to draw the conclusion that the electron concentration in the bulk of the material does not change with cooling of specimens from 300 K to 115 K.     

X-ray diffraction characterization of MBE grown Pr1−xSrxMnO3 thin films on NGO(1 1 0)

Oxygen plasma-assisted molecular beam epitaxial (MBE) growth of Pr_1−xSr_xMnO₃ (PSMO) thin films has been carried out on NdGaO₃(1 1 0) (NGO) substrates. The growth parameters have been optimized to realize 2D layer-by-layer growth. XRD results of the epilayers show that the PSMO/NGO(1 1 0) thin films are of high crystal quality, as clear diffraction peaks can be observed belonging to the film and the substrate, respectively. Based on analysis of the peaks, it was concluded that epitaxial relation is PSMO(1 1 0)//NGO(1 1 0), i.e., the c-axis being parallel to the surface. Both single scans (ω scan, 2θ/ω scan) and 2-axis reciprocal space mapping (RSM) were performed in an effort to assess the crystal structure, crystalline quality, surface and interface properties of the epitaxial layers. High temperature annealing effects on lattice structure and crystal quality have been studied and discussed. Transport property measurement of the PSMO thin film samples has been carried out and main features discussed.