Magnetic Irreversibility Line Research Articles

Strong pinning in the hole-doped pnictide superconductor La0.34Na0.66Fe2As2

We present magnetization studies as a function of time, temperature, and magnetic field for H ∥ c-axis, in a hole-doped pnictide superconductor, La0.34Na0.66Fe2As2, with Tc≈27K. The obtained vortex phase-diagram shows that the magnetic irreversibility line is very close to the mean-field superconducting transition line, similar to the low Tc superconductors, evidencing a strong pinning behavior. The irreversibility line does not follow a power law behavior with (Tc−T); however, it is well described using an expression developed in the literature, considering the effect of disorder in the system. The critical current density estimated using the Bean critical-state model is found to be of the order of 105A/cm2 below 12 K in the limit of zero magnetic field. A plot of the normalized pinning force density as a function of the reduced magnetic field at different temperatures shows good scaling, and the analysis suggests that the vortex pinning is due to normal point-like pinning centers. The temperature dependence of the critical current density suggests that the pinning due to the variation in the charge carrier mean free path alone is not sufficient to explain the experimental data. The magnetic relaxation rate as a function of temperature and magnetic field is also studied.

Magnetic irreversibility and zero resistance in granular Y358 superconductor

We report on magnetization and magnetoresistance measurements of polycrystalline Y3Ba5Cu8O18 superconductor (Y358), prepared by solid state reaction in order to study the correlation between the magnetic irreversibility line and the zero electric resistance as a function of applied field. The magnetization measurements were performed using a MPMS-XL SQUID magnetometer and the magnetoresistance measurements were made using a PPMS, both from Quantum Design, up to 1 T. The granular microstructure was confirmed by scanning electron microscopy. In this case the grain junctions are weaker than in the Y123 system and the effect of the applied field on the grain couplings is much stronger. Consequently, in our sample the zero resistance line shifts away from the irreversibility line and is lower by more than 15 K at 1 T. We explain our results in terms of the superconducting glass model and in comparison with results obtained in other materials from the YBaCuO family.

Correlation Between Enhanced ${\rm T}_{\rm c}$, AC Magnetic Irreversibility Line and Heat Treatment in High ${\rm T}_{\rm c}$ Superconductors

We report here on the preparation, resistivity, AC magnetic susceptibility measurements and effect of heat treatments in (Y <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">1-x</sub> Sm <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">x</sub> )(SrBa)Cu <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> O <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">6+z</sub> . Each sample was subject to two type of heat treatment: oxygen annealing [O] and argon annealing followed by oxygen annealing [AO]. An enhancement of T <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">c</sub> [AO] was observed for <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">x</i> ≥ 0.4. In the normal state, the [AO] treatment reduced the linear resistivity parameters indicating a diminution of the interaction of carrier charges with phonons. <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">Tc</i> (χ <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">'</sup> ) and <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">Tc</i> (ρ = 0) were in good agreement. An enhancement of the AC irreversibility line <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">H</i> ( <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">t</i> = <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">Tp</i> / <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">Tc</i> ) was observed due to [AO] treatment for <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">x</i> >; 0.5 indicating an improvement in the pinning properties. The data were analysed with the relation <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">H</i> = <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">K</i> <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">'</sup> (1- <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">t</i> ) <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">n</i> . Remarkable correlations were observed between δ <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">Tc</i> ( <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">x</i> )= <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">Tc</i> [ <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">AO</i> ]- <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">Tc</i> [ <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">O</i> ] and δ <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">K</i> <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">'</sup> ( <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">x</i> ); and between <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">Tc</i> ( <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">x</i> ) , <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">d</i> [ <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">Cu</i> (2)- <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">Y</i> / <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">Sm</i> ]( <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">x</i> ) distance and the number <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">psh</i> ( <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">x</i> ) of holes by Cu(2)-O <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> superconducting planes. A combination of several factors such as decrease in <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">d</i> [ <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">Cu</i> (1)-( <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">Sr</i> / <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">Ba</i> )] for x >; 0.4; increase in cationic and chain oxygen ordering; p <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">sh</sub> and in-phase purity for the [AO] samples may account for the observed data.

Granular superconductivity in YBa 2Cu 3O 7−δ single crystals by divalent impurities doping: Possible role of valence and orbital symmetry

We have investigated the effect of divalent Ca, Sr and Zn atoms at the Y, Ba and Cu sites, respectively, of the YBa 2Cu 3O 7− δ single crystal on the magnetoconductivity and magnetic irreversibility properties for H|| c axis and J|| ab plane. The resistive transition is a clear two-step process. While the upper temperature step is only weakly affected by low applied fields, the lower temperature step is visibly broadened and shifted down to lower temperatures. Between upper and lower temperature steps lays the magnetic irreversibility line, T irr( H) which follows the power law predicted by the flux creep theories in most of the high field range. However, in a low field region, the T irr( H) exhibits two different regimes dominated by disorder and frustration and characterized by the de Almeida–Thouless and Gabay–Toulouse power law behaviors. We attribute the origin of the observed stepwise resistive transition and low field magnetic irreversibility regimes mainly to the granularity of the superconducting state of our doped samples and contrast our magnetic irreversibility present results with those of a pure YBa 2Cu 3O 7− δ single crystal. We suggest that the valance as well as the local breaking of the orbital symmetry by impurities plays an important role in the induction of this state. On the other hand, our results show that granular superconductivity can be induced in the YBa 2Cu 3O 7− δ system by divalent impurities doping in whatever site Y, Ba or Cu.

Thermodynamics of the vortex liquid in heavy-ion-irradiated superconductors

It is shown that the large effect of heavy ion irradiation on the thermodynamical properties of the anisotropic superconductor ${\mathrm{YBa}}_{2}{\mathrm{Cu}}_{3}{\mathrm{O}}_{7\ensuremath{-}\ensuremath{\delta}}$ extends well into the superconducting fluctuation regime. The presence of the induced amorphous columnar defects shifts the specific-heat maximum at the normal-to-superconducting transition. This effect is similar to that recently put into evidence in cubic ${\mathrm{K}}_{x}{\mathrm{Ba}}_{1\ensuremath{-}x}{\mathrm{BiO}}_{3}$ $(x\ensuremath{\simeq}0.35)$. In both compounds, vortex pinning manifests itself as a sharp angular dependence of the equilibrium torque. In ${\mathrm{YBa}}_{2}{\mathrm{Cu}}_{3}{\mathrm{O}}_{7\ensuremath{-}\ensuremath{\delta}}$, pinning by the defects appears at the temperature ${T}_{{C}_{p}}^{max}$ of the specific-heat maximum, well above the magnetic irreversibility line ${T}_{irr}(H)$. In isotropic ${\mathrm{K}}_{x}{\mathrm{Ba}}_{1\ensuremath{-}x}{\mathrm{BiO}}_{3}$, the onset of the pinning-related torque anomaly tracks the onset of the specific-heat anomaly and the irreversibility line. In ${\mathrm{YBa}}_{2}{\mathrm{Cu}}_{3}{\mathrm{O}}_{7\ensuremath{-}\ensuremath{\delta}}$, fluctuations of the amplitude of the order parameter (and not vortex line wandering) are ultimately responsible for the vanishing of pinning. In ${\mathrm{K}}_{x}{\mathrm{Ba}}_{1\ensuremath{-}x}{\mathrm{BiO}}_{3}$, vortex pinning disappears only at the superconducting-to-normal transition. The results indicate that in both compounds, the pinning energy at the ``Bose-glass'' transition is large with respect to the total free energy gain in the superconducting state. By implication, the mechanism of this latter transition should be reconsidered.

Vortex Phase Diagram in Bi2Sr2CaCu2O8+δwith Damage Tracks Created by 30 MeV Fullerene Irradiation

Using 30 MeV C 60 fullerene irradiation, we have produced latent tracks of diameter 20 nm and length 200 nm, near the surface of single crystalline Bi 2 Sr 2 CaCu 2 O 8+δ . A preliminary transmission electron microscopy study shows evidence for a very high density of deposited energy, and the ejection of material from the track core in very thin specimens. The latent tracks reveal themselves to be exceptionally strong pinning centers for vortices in the superconducting mixed state. Both the critical current density and magnetic irreversibility line are significantly enhanced. The irradiated crystals present salient features of the ( B , T ) phase diagram of vortex matter both of pristine crystals, such as the first order vortex phase transition, and the exponential Bose-glass line characteristic of heavy ion-irradiated crystals. We show that the latter is manifestly independent of the pinning potential.

Influence of Sr doping on the second magnetization peak and the critical current density of YBa 2Cu 3O 7− δ single crystals

Magnetic hystheresis loops of a series of YBa 2− x Sr x Cu 3O 7− δ ( x=0.1, 0.25, 0.37 and 0.5) single crystals reveal a huge “peak effect” over a wide range of low temperatures. Previous magnetoresistivity and magnetic irreversibility measurements already have evidenced that the superconducting state of these samples has a granular character and that their magnetic irreversibility line crosses over from the high field flux-creep (fc) regime to low field regimes, characterized by the intergrain Josephson flux-dynamics in a frustrated superconducting-grain aggregate. The present work adds to this that the peak effect vanishes at the temperature limit where the crossover from the fc to the low field regimes takes place. This is a clear indication that the peak effect is entirely related to the intragrain Abrikosov flux-dynamics.

Critical currents of Bi:2212 doped by Fe and Ni

Measurements of the irreversible magnetization, ac susceptibility and electrical resistivity of Bi2Sr2Ca1−xErx(Cu1−yMy)2O8+d bulk are reported. Low concentrations of the doping elements increase the pinning force density and shift the magnetic irreversibility line towards higher fields. The intergranular critical current density was determined from ac susceptibility data by varying the field amplitude Hac in the range from 0.4 to 1000 A/m and from the irreversible part of magnetization using Bean’s model. The results were discussed in terms of SIS- and SNS-type models for granular superconductors.

Vortex- and Bragg-glass phases in bulk MgB 2

A more complete magnetic field–temperature ( H– T) phase diagram, containing the upper critical field H c2( T), the vortex-glass melting line H g( T), and the magnetic irreversibility line H irr( T) has been established for bulk MgB 2 ( T c=38.5 K), synthesized under 200 MPa pressure by hot isostatic pressing (HIPing). Scaling analysis of resistivity data, J c data, and electric field vs current density ( E– J) isotherms in the mixed-state suggest that a Bragg-glass state exists for H< 8 kOe. The critical scaling exponents are dependent upon field and change from ν≈0.6, z≈3.1 in low fields ( H<8 kOe) to ν≈0.9, z≈3.4 in high fields. Magnetization data reveal distinctly different critical current density ( J c) behaviors in high and low magnetic field critical scaling regions.

Bose-glass, vortex-glass and superconducting-glass properties in Sr doped YBaCuO single crystals

Very detailed magnetoresistivity and magnetic irreversibility data for fields applied along the ab-plane of the heavily twinned YBa 2− x Sr x Cu 3O 7− δ ( x=0.1 and 0.37) single crystals are reported. While the resistive transition evidences a granular superconducting character, the magnetic irreversibility line ( T irr( H)) exhibits several different regimes and crossovers. For fields higher than 10 kOe and parallel to the twinning planes (TPs) the flux-dynamics conforms to the standards of the conventional flux-creep theory. However, under fields above 20 kOe, the flux-dynamics displays anisotropic properties in connection with the TPs, probably the signature of a Bose-flux-glass phase. In an intermediary field region the flux-dynamics is isotropic about the c-axis of the single crystal, characterizing a vortex-glass phase. In a low field region ( H<5 kOe) the irreversibility data follow the de Almeida–Thouless (AT) and Gabay–Toulouse (GT) like power law behaviors, usually observed in granular superconductors. We appoint frustration of the superconducting grain aggregate as responsible for the AT and GT like power law behaviors below 5 kOe, while the pinning of the TPs is responsible for the anisotropy in the highest fields that possibly is a signature of the Bose-flux-glass.

Effects of Zn and Mg in Cu sites ofYBa2Cu3O7−δsingle crystals on the resistive transition, fluctuation conductivity, and magnetic irreversibilities

We report on the resistive transition and fluctuation conductivity of an ${\mathrm{YBa}}_{2}{\mathrm{Cu}}_{2.97}{\mathrm{Zn}}_{0.03}{\mathrm{O}}_{7\ensuremath{-}\ensuremath{\delta}}$ single crystal and on the magnetic irreversibilities of this compound in two different oxygen states and another ${\mathrm{YBa}}_{2}{\mathrm{Cu}}_{3}{\mathrm{O}}_{7\ensuremath{-}\ensuremath{\delta}}$ single crystal in which (1 at. %) of Cu was substituted by Mg. Our measurements show a very drastic decrease of the superconducting transition temperature and the growth of granularity effects already in this low dopand concentration. In the temperature region immediately above ${T}_{c}$ our results of fluctuation conductivity reveal the occurrence of critical and Gaussian regimes. The resistive transition of the ${\mathrm{YBa}}_{2}{\mathrm{Cu}}_{2.97}{\mathrm{Zn}}_{0.03}{\mathrm{O}}_{7\ensuremath{-}\ensuremath{\delta}}$ single crystal occurs in two steps. First superconductivity sets in within the grains and at somewhat lower temperature long-range superconducting order takes place over the whole sample. Between the zero resistance temperature ${T}_{c0}(H)$ and the pairing transition temperature ${T}_{c}(H)$ lays the magnetic irreversibility line ${T}_{\mathrm{irr}}(H),$ which follows the power law predicted by the flux creep theories in most of the high field range. However, in a low field region, dominated by Josephson flux dynamics, the irreversibility line exhibits two different regimes dominated by disorder and frustration and characterized by AT (de Almeida-Thouless) and GT (Gabay-Toulouse) power law behaviors. This Josephson flux dynamics is squeezed into a considerably smaller field region than in other granular ${\mathrm{YBa}}_{2}{\mathrm{Cu}}_{3}{\mathrm{O}}_{7\ensuremath{-}\ensuremath{\delta}}$ systems. We attribute the origin of the observed stepwise resistive transition and low field magnetic irreversibility regimes mainly to the granularity of the superconducting state, which is induced by the impurities at the Cu sites and increased by shortage of oxygen.

Mixed-state flux dynamics in bulk MgB 2

Electric field vs. current density ( E– J) isotherms in the mixed-state of a bulk sample of the high-temperature superconductor MgB 2 ( T c=38.5 K), synthesized under 200 MPa pressure by hot isostatic pressing (HIPing), have been measured and analyzed in terms of the critical scaling model. Magnetization data reveal distinctly different critical current density ( J c) behaviors in high and low magnetic field critical scaling regions. E– J isotherm sets at fields ranging from 2 to 90 kOe conform to the vortex-glass (VG) scaling anzatz. Scaling analysis, resistivity data and J c data suggest that a Bragg-glass state may exist for H<8 kOe. The critical scaling exponents depend on field and range from ν∼0.6, z∼3.1 in low fields ( H<8 kOe) to ν∼0.9, z∼3.4 in high fields. A more complete magnetic field–temperature ( H– T) phase diagram, containing the upper critical field H c2( T), the VG transition line H g( T), and the magnetic irreversibility line H irr( T) has been established for bulk MgB 2.

Transport and magnetic properties of superconducting MgB 2

We have investigated the transport and magnetic properties of the recently discovered superconducting materials MgB 2. We obtained irreversibility temperature T r( H) and irreversibility field H r( T) from magnetic measurements. The irreversibility temperature T r( H) was determined from the merging temperature of the magnetic moment observed in the zero field cooling process in to the one in field cooling process. We also defined the transport irreversibility temperature B c2 from resistivity measurements at zero-resistivity in the presence of magnetic field. Those results of the irreversibility lines are plotted in H– T plane. The magnetic irreversibility lines are consistent with each other, while the transport one locates at the higher temperature side.

Critical scaling and flux dynamics in bulk MgB 2 and high-purity YBa 2Cu 3O 7− δ single crystals

A dense sample of MgB 2 ( T c=38.5 K) was synthesized under 200 MPa pressure using a novel hot isostatic pressing (HIPing) technique, and a high-purity single crystal of YBa 2Cu 3O 7− δ ( T c=93.5 K) was grown in a home-made BaZrO 3 crucible. Electric field vs current density ( E–J) isotherms of the MgB 2 and YBa 2Cu 3O 7− δ high-temperature superconductors were measured and analyzed in terms of the critical scaling model. E–J isotherm sets were taken at fields ranging from 2 to 90 kOe. Scaling and resistivity data reveal a smooth transition from a vortex glass to a vortex liquid state for the bulk MgB 2 sample and the existence of both a vortex lattice melting line and a vortex glass to vortex liquid line in the YBa 2Cu 3O 7− δ single crystal. The critical scaling exponents for both systems are presented. A more complete ( H–T) phase diagram, containing the upper critical field H c2( T), the vortex-glass transition line H g( T), and the magnetic irreversibility line H irr( T) was established for bulk MgB 2.

MgB&lt;SUB&gt;2&lt;/SUB&gt;の磁化緩和

We have investigated the transport and magnetic properties of the recently discovered superconducting MgB2. We obtained irreversibility temperature Tr(H) and irreversibility field Hr(T) from magnetic measurement and irreversibility field Bc2 from resistivity measurement. The irreversibility temperature Tr(H), was determined from the merging temperature of the magnetic moment observed in the zero field cooling (ZFC) process in to the one in field cooling (FC) process. We define the transport irreversibility line from the temperature at zero-resistivity in the presence of magnetic field. We also measured the magnetic relaxation of the remanent. The sample was cooled in zero field and the field above H*, where the flux front penetrated at the center of the sample. The time decay of the remanent moment was measured for about 105 s. The relaxation of the remanent moment showed the linear ln (t) dependence in the whole observed temperature range. Those results of the irreversibility lines are plotted in H-T plane. The magnetic irreversibility lines are consistent with each other, while the transport one locates at the higher temperature side.

Current-enhanced anisotropy of Bi 2Sr 2CaCu 2O 8 in the mixed state

Transport and local magnetization measurements carried out simultaneously in Bi 2Sr 2CaCu 2O 8 (BSCCO) crystals reveal finite resistivity well below the magnetic irreversibility line, where the vortices should be pinned. This resistivity is non-monotonic with temperature and extremely non-linear. We describe the new observation in terms of a shear-induced decoupling, in which the pancake vortices flow only in the top few layers and are decoupled from the pinned vortices in the rest of the crystal.

Current-induced decoupling of vortices in Bi 2Sr 2CaCu 2O 8

Simultaneous transport and magnetization measurements in Bi 2Sr 2CaCu 2O 8 crystals are reported. The transport current is injected at the top surface, while Hall sensor arrays measure the local magnetic field across the crystals at the bottom. Surprisingly, at high-currents we find finite resistivity well below the magnetic irreversibility line, where the vortices are pinned. This resistivity is non-monotonic with temperature and highly non-linear. We interpret the new observation as a shear-induced decoupling, in which the vortices flow only in the top few layers and are decoupled from the pinned vortices in the rest of the crystal.

Shear-induced vortex decoupling inBi2Sr2CaCu2O8crystals

Simultaneous transport and magnetization studies in Bi2Sr2CaCu2O8 crystals at elevated currents reveal large discrepancies, including finite resistivity at temperatures of 40K below the magnetic irreversibility line. This resistivity, measured at the top surface, is non-monotonic in temperature and extremely non-linear. The vortex velocity derived from magnetization is six orders of magnitude lower than the velocity derived from simultaneous transport measurements. The new findings are ascribed to a shear-induced decoupling, in which the pancake vortices flow only in the top few CuO2 planes, and are decoupled from the pinned vortices in the rest of the crystal.

Magnetic irreversibility and relaxation in assembly of ferromagnetic nanoparticles

Measurements of the magnetic irreversibility line and time-logarithmic decay of the magnetization are described for three $Fe_{2}O_{3}$ samples composed of regular amorphous, acicular amorphous and crystalline nanoparticles. The relaxation rate is the largest and the irreversibility temperature is the lowest for the regular amorphous nanoparticles. The crystalline material exhibits the lowest relaxation rate and the largest irreversibility temperature. We develop a phenomenological model to explain the details of the experimental results. The main new aspect of the model is the dependence of the barrier for magnetic relaxation on the instantaneous magnetization and therefore on time. The time dependent barrier yields a natural explanation to the time-logarithmic decay of the magnetization. Interactions between particles as well as shape and crystalline magnetic anisotropies define a new energy scale that controls the magnetic irreversibility. Introducing this energy scale yields a self-consistent explanation of the experimental data.

Growth of high quality YBCO single crystals using BaZrO 3 crucibles

High density BaZrO 3 ceramic crucibles have been successfully fabricated and used to grow high quality YBa 2Cu 3O 7− x (YBCO) single crystals by the BaO–CuO self flux method. The ceramic crucibles are inert and insoluble in the Y 2O 3–BaO–CuO melt, have a long lifetime, and can be used repeatedly for crystal growth runs. The crystals grown using the crucibles have not only high chemical purity (99.99–99.995%), but also a high degree of crystalline perfection as characterized by an extremely sharp X-ray rocking curve (FWHM=0.007°), very weak magnetic flux pinning and a magnetic irreversibility line that lies about 20 K below the line for the best YSZ crucible grown crystals.