Effect of grain-boundary flux pinning in MgB2 with columnar structure

We studied the flux pinning properties by grain boundaries in MgB₂ films prepared by using a hybrid physical chemical vapor deposition method on the c-axis oriented sapphire substrates. All the films we report here had the columnar grains with the growth direction perpendicular to the substrates and the grain sizes in the range of a few hundred nanometers. At very low magnetic fields, no discernable grain-boundary (GB) pinning effect was observed in all measuring temperatures, but above those fields, the effect of GB flux pinning was observed as enhanced critical current densities (J_cs) and reduced resistances when an external magnetic field (B) was aligned parallel to the c-axis. We interpret the B dependence of J_c in the terms of flux line lattice shear inside the columnar grains activated by dislocations of Frank–Read source while the flux lines pinned by GB act as anchors for dislocations. Magnetic field dependence of flux pinning force density for B parallel to the c-axis was reasonably explained by the above model.     

Vortex dynamics in single crystal Hg-1201

Anisotropic superconducting materials often show an enhanced pinning along their crystallographic ab-planes. To obtain information about such a behavior of the high-T_c system Hg-1201 (HgBa₂CuO₄) magnetic investigations on a single crystal are performed for the two field orientations, parallel to the c-axis and parallel to the ab-planes. The dependence of the ac magnetization on temperature, magnetic field and frequency is determined. Compared to former results on powder samples of this system no indication of a second peak in the imaginary part of the susceptibility χ′′ is found. It seems to be shifted to higher temperatures overlapping now with the first peak. The corresponding irreversibility lines for both orientations, parallel to c and parallel to ab, are determined and discussed within the framework of a “diffusion” model.     

The anomalous anisotropy in the ac susceptibility of La1.45Nd0.4Sr0.15CuO4 single crystal

The dependencies of ac susceptibility on the superimposed dc magnetic fields for the stripe-ordered La_1.45Nd_0.4Sr_0.15CuO₄ single crystal with two superconducting transitions have been studied in the 3D and 2D superconducting ranges for the fields along different orientations of the crystal. The results show that with increasing fields the interlayer Josephson coupling and the in-plane superconductivity are suppressed orderly for the fields perpendicular to CuO₂ planes. The influences of the field parallel to CuO₂ planes on the 3D and 2D superconductivity are much weaker than those of the field parallel to the c-axis. The irreversibility lines for the 3D and 2D superconducting states are also studied.     

Non-collinear vortex states in twinned YBa2Cu3O7?δ single crystals

The magnetic moments parallel and perpendicular to thec-axis (measured simultaneously) have been studied as a function of direction of applied magnetic fieldH in twinned and de-twinned YBa₂Cu₃O_7−δ single crystals. They show pronounced anomalies when the field direction approaches thec-axis. These allow clear identification of the angle ϕ_L at which vortices are locked into twin planes. Complete shielding of theH _ab field component (transverse Meissner effect) was observed in the locked state. For larger angles, up to ϕ_T, the vortices continue to be non-collinear with the applied field, but their direction deviates from the trapping plane. ϕ_L exhibits a 1/H field-dependence, whereas ϕ_T shows only weak logarithmic variations withH.     

Irreversibility line and flux pinning properties in iron-based high-Tc superconductor SmFeAsO0.85

The irreversibility line and flux pinning properties of high-T_c superconductor SmFeAsO_0.85 were studied using DC magnetization data. Polycrystalline SmFeAsO_0.85 was prepared in a high pressure synthesis apparatus under the pressure of 6 GPa. The results of DC susceptibility showed the superconducting transition at about 55 K. A critical current density J_c(B) was calculated using Bean’s critical state model. At low temperatures (20 K), J_c(B) showed a relatively high value with weak dependence on an applied magnetic field. At higher temperatures, a stronger dependence of the magnetic field was observed, which resulted from decrease in a critical current density probably due to the flux creep effect. The irreversibility line (IL) agreed well with the flux creep theory of Matsushita et al. A comparison of normalized pinning force density with the theoretical models showed that the irreversible behavior in SmFeAsO_0.85 is dominated mainly by normal point pinning (δT_c) and surface pinning mechanisms.     

Mössbauer study of Lu2Fe3O7: A two-dimensional antiferromagnet on a triangular lattice

Mössbauer studies reveal that there are two kinds of Fe³⁺ spins with completely different characteristics in Lu₂Fe₃O₇: one is an Ising-like property and the other is a Heisenberg-like property in a two-dimensional antiferromagnet on a triangular lattice. The former spin orders ferrimagnetically along thec-axis at around 220 K. The latter spin is lying in thec-plane and a corresponding hyperfine magnetic field is observed at temperatures below 50 K. At very low temperatures, however, the latter spin has a component parallel to thec-axis and couples with the former spin. This finding is consistent with the modulated profile of the magnetic scattering in neutron diffraction and the result of a magnetization measurement.     

Vortex pinning properties of –Ba–Cu–O and –Ba–Cu–O superconducting bulks

We have studied the effect of a small amount of Y-site substitution by La or Pr ions on the vortex pinning in the Y–Ba–Cu–O system. (Y_1-xLa_x)–Ba–Cu–O and (Y_1-xPr_x)–Ba–Cu–O bulks were fabricated by the melt-textured growth, in which x was varied from 0 to 0.01. The critical current density J_c at 77 K is improved in magnetic fields parallel to the c-axis above 2–4.5 T and the corresponding irreversibility field, H_irr, shifts to the higher value in both bulks.     

Yba2Cu3O7-δ不可逆线与钉扎势的关联

本文分别测量了熔融织构YBa₂Cu₃O_7-δ高温超导体不可逆线与钉扎势U₀,发现该样品的不可逆线在温区72—85K范围内呈指数形式B^*-e^(-T*/T_c)函数关系,而且在温度77K,不可逆线并不能与磁测量临界电流密度J_c=0相对应,我们认为高温超导体的不可逆线的函数形式与样品的钉扎势紧密关联。 关键词：     

Synthesis and superconducting properties of bscco including precipitates with high density

Bi₂Sr₂Ca₁Cu₂O _y (2212) superconductors including normal particles with high density were synthesized by a melt process, and the temperature and magnetic field dependence of the critical current density was estimated from the magnetization hysteresis. The temperature dependence of the irreversibility field, B_r, was expressed as (1-T/T_c)^3/2 above 50 K, and became weak below 50 K as compared with that in the single-crystalline (2212). The B_r, in the magnetic field parallel to the c-axis was about one order of magnitude lower than in a magnetic field normal to the c-axis.     

Dynamics of flux lines studied by modulated microwave absorption

The solution of the coupled differential equations of motion of a flux line in the pinning potential is obtained under the condition of a swept de field, modulation field and microwave field. It can explain the hysteresis which is observed when lock-in detected signal is recorded in forward and reverse field sweeps. The width of the hysteresis is proportional to the pinning potential. From the measurements in laser grown the films of YBa₂Cu₃O_7-δ, we have deduced field and temperature dependences of the pinning potential. The irreversibility line fits to the formB∞ (T _c-T)^β with β=2.3±0.3.     

A thermodynamic description of the irreversibility line in strongly type-II superconductors

The one-component dense vortex plasma theory, which describes thermal motion of the magnetic vortices in strongly type-II superconductors in a magnetic field, is used to study the irreversibility line in high-T _c superconductors. We propose a possible universal form for the irreversibility line, and examine some recent experiments.     

高Tc氧化物超导体Bi(Pb)-Sr-Ca-Cu-O(F)的“不可逆线”

研究了高T_c氧化物超导体Bi(Pb)-Sr-Ca-Cu-O(F)体系的“不可逆线”。发现不可逆磁场H^*<120Oe时,H^*=1590(1—t)^3/2;当120Oe*<1000Oe时,H^*=35700(1—t)^3/2-2480。指出H^*(T)曲线是一个磁通格子熔化线,在曲线以下属于磁通蠕动区;在H^*(T)与H_c2(T)之间属于磁通格子液态区,即磁通流动区。 关键词：     

Dynamics of fluxons in Y1−Ba2−Cu3−O7 single crystals

Summary Evidence is given that in Y₁−Ba₂−Cu₃−O₇ single crystals a critical state is developed on increasing (or decreasing) the applied fieldH ₀, only whenH ₀ is parallel to the crystalc-axis. An ?irreversibility? line in the (H, T)-plane can be drawn where the irreversibility of the second-harmonic response sets in. In honour of Prof. Fausto Fumi on the occasion of his retirement from teaching.     

Pinning,thermally activated depinning and their importance for tuning the nanoprecipitate size and density in high Jc YBa2Cu3O7−x films

YBa₂Cu₃O_7?x (Y123) films with quantitatively controlled artificial nanoprecipitate pinning centers were grown by pulsed laser deposition (PLD) and characterized by transport over wide temperature (T) and magnetic field (H) ranges and by transmission electron microscopy (TEM). The critical current density J_c was found to be determined by the interplay of strong vortex pinning and thermally activated depinning (TAD), which together produced a non-monotonic dependence of J_c on c-axis pin spacing d_c. At low T and H, J_c increased with decreasing d_c, reaching the very high J_c ～ 48 MA/cm² ～20% of the depairing current density J_d at 10 K, self-field and d_c ～ 10 nm, but at higher T and H when TAD effects become significant, J_c was optimized at larger d_c because longer vortex segments confined between nanoprecipitates are less prone to thermal fluctuations. We conclude that precipitates should extend at least several coherence lengths along vortices in order to produce irreversibility fields H_irr(77 K) greater than 7 T and maximum bulk pinning forces F_p,max(77 K) greater than 7–8 GN/m³ (values appropriate for H parallel to the c-axis). Our results show that there is no universal pin array that optimizes J_c at all T and H.     

Effects of nanoscale defects on critical current density of (Y1−xEux)Ba2Cu3O7−δ thin films

In pulsed laser deposition of YBa₂Cu₃O_7?δ films, defect introduction into the films tends to anisotropically improve the pinning along the H||c direction due to the columnar growth mode of the process. In Eu-substituted samples, however, even though an increase in critical current density (J_c) in the H||c direction was observed for low fields (H = 0.2 T), the improvement was more notable for the H||ab-plane at both low and higher fields. Herein we present detailed TEM microstructural studies to understand these new trends in J_c(H), which are markedly different than flux pinning increases achieved with other methods, for example, with nanoparticle additions. Threading dislocations, observed in the Eu-substituted samples along the c-axis, account for J_c enhancement with H||c at low field. The enhanced ab-planar pinning in the Eu-substituted samples is attributed to the extensive bending of the {0 0 1} lattice planes throughout the film, and the crystal lattice defects with excess Cu–O planes, that were effective in increasing the J_c for H||ab at both low and high fields.     

The effect of SiC nanoparticle addition on the flux pinning properties of MgB2

The nanoparticle–MgB₂ composite superconducting sample, (SiC)_4wt.%(MgB₂)_96wt.% ((SiC)₄–MgB₂), was prepared, and the effect of nanoparticle additions on the magnetic flux pinning was investigated. The measurement and comparison of isothermal magnetization M(H), for pure-MgB₂ powder and sintered pellets of (SiC)₄–MgB₂ and pure-MgB₂ were carried out at temperatures between 5 and 50 K in fields up to 8.5 T. The magnetic irreversibility ΔM(H) curves of the (SiC)₄–MgB₂ follow almost identical lines of both pure-MgB₂ powder and sintered bulk MgB₂ in the region above a specific magnetic field (called a merged field), which gradually decreases as the temperature increases. The (SiC)₄–MgB₂ composite superconductor has exhibited the flux pinning effect which comes from both the grain boundaries and the point defect weak pinning centers in the region below the merged field line. This is different from the case of pure-MgB₂ powder and sintered bulk MgB₂ which showed mostly the grain boundaries pinning.     

Point-contact measurements of CeB6 and CeCu6 in high magnetic fields

Epitaxial YBa₂Cu₄O₇-thin films with thec-axis oriented perpendicular to the film plane were prepared bydc-sputtering from a single stoichiometric target on (100) SrTiO₃-substrates. Typical values of the inductively measured superconducting transitions were about 90 K with a width less than 0.5 K. Critical current densities were measured on 5 to 10 m wide strips as function of magnetic field and temperature. The temperature dependences ofj _c follow a universal functionj _c(B,T)=j _c ^* (T=0,B)·(1–T/T _c (B)) with =1.5±0.1. ForB=0 andT=77 K we obtainedj _c =4·10⁶ A/cm². The field dependence of the resistive transitions was measured with the magnetic field parallel to thec-axis. The slope of the upper critical fieldB _c2 (T) was determined for different criteria. The carrier concentration evaluated from Hall-effect measurements was found to decrease linearly from one per unit cell at 240 K with decreasing temperature extrapolating nearly through zero forT=0. Highly resolved angular dependent measurements of the critical current density withB perpendicular to the current but tilted from thec-axis show a very strong and sharp enhancement ofj _c for the magnetic field parallel to the (CuO₂)-layers (Bc). Additionally to this phenomenon, which is caused by an intrinsic pinning mechanism due to the layered structure of high-T _c -superconductors the influence of the anisotropy of the upper critical field onj _c (B, T, ) is evident nearT _c .     

Hysteretic magnetic moment of YBa2Cu3O7?δ: An angular study

The hysteretic magnetization of an YBa₂Cu₃O_7−δ crystal containing a high density of Y₂BaCuO_x precipitates was investigated by two-axis, quasistatic, de magnetometry. As the eff-axis applied magnetic fieldH was increased and the magnetization magnitude, decreased, its orientation approached a direction near the crystallinec axis. The bulk persistent currentsJ _p predominately flowed parallel to the Cu−O planes for a wide range of temperature and applied field. Even forH almost perpendicular toc, where the deduced irreversibility line maximized as commonly observed, the resulting magnetization was observed to be within 15° of [001]. Managed by Lockheed Martin Energy Research Corp, under contract AC05-96OR22464 with the U. S. Dept. of Energy.     

Effect of sorbic acid doping on flux pinning in bulk MgB2 with the percolation model

In this paper, we study the doping effect of sorbic acid (C₆H₈O₂), from 0 to 20 wt.% of the total MgB₂, on critical temperature (T_c), critical current density (J_c), irreversibility field (H_irr) and crystalline structure. The XRD patterns of samples show a slightly decrease in a-axis lattice parameter for doped samples, due to the partial substitution of carbon at boron site. On the other hand, we investigate the influence of doping on the behavior of flux pinning and J_c(B) in the framework of percolation theory and it is found that the J_c(B) behavior could be well fitted in high field region. The two key parameters, anisotropy and percolation threshold, play very important roles. It is believed that the enhancement of J_c is due to the reduction of anisotropy in high field region.     

Magnetic phases in UNi<Subscript>2</Subscript>Si<Subscript>2</Subscript>

The tetragonal compound UNi₂Si₂ exhibits in zero magnetic field three different antiferromagnetic phases belowT _N =124 K. They are formed by ferromagnetic basal planes, which are antiferromagnetically coupled along thec-axis with the propagation vectorq=(0, 0, q _z ). Two additional order-order magnetic phase transitions are observed below T _N , namely atT ₁=108 K and T ₂=40 K in zero magnetic field. All three phases exhibit strong uniaxial anisotropy confining the U moments to a direction parallel to the c-axis. UNi₂Si₂ single crystals were studied in detail by measuring bulk thermodynamic properties, such as thermal expansion, resistivity, susceptibility, and specific heat. A microscopic study using neutron diffraction was performed in magnetic fields up to 14.5 T parallel to the c-axis, and a complex magnetic phase diagram has been determined. Here, we present the analysis of specific-heat data measured in magnetic fields up to 14 T compared with the results of the neutron-diffraction study and with other thermodynamic properties of UNi₂Si₂.