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A pristine ${\mathrm{Rb}}_{3}$${\mathrm{C}}_{60}$ single crystal with larger than millimeter dimensions was prepared by reaction of rubidium vapor with a seed-grown ${\mathrm{C}}_{60}$ single crystal (which exhibited remarkably sharp x-ray-diffraction peaks). A narrow superconducting transition width \ensuremath{\Delta}${\mathrm{T}}_{\mathrm{c}}$ (5--85% diamagnetic shielding) of the ${\mathrm{Rb}}_{3}$${\mathrm{C}}_{60}$ single crystal is less than 1 K. The onset of the transition was at a temperature, ${\mathrm{T}}_{\mathrm{c}}$=30.0tK. Measurements of temperature dependence of the zero-field-cooled and field-cooled dc magnetization (using ten applied magnetic fields up to 5.0 T) are reported here. These and magnetization vs field data at different temperatures (ranging from 2 to 29 K) have been used to determine thermodynamic superconducting parameters, including the lower [${\mathrm{H}}_{\mathrm{c}1}$(T)] and upper [${\mathrm{H}}_{\mathrm{c}2}$(T)] critical fields, the Ginsburg-Landau coherence length \ensuremath{\xi}, and the London or Ginzburg-Landau penetration depth, \ensuremath{\lambda}. These are significantly different from those extracted from similar data in the literature for powder specimens. Hysteresis and magnetic relaxation measurements have been used to determine the nominal critical current density ${\mathrm{J}}_{\mathrm{c}}$(H,T) and the effective-pinning potential ${\mathrm{U}}_{\mathrm{eff}}$(J,H), respectively.

Magnetic hysteresis, flux pinning, and flux creep in melt-powder-melt-growth processed ${\mathrm{YBa}}_{2}$${\mathrm{Cu}}_{3}$${\mathrm{O}}_{7}$ (Y 1:2:3) containing nominal 0, 25, and 40 mol % concentration of ${\mathrm{Y}}_{2}$${\mathrm{BaCuO}}_{5}$ (Y 2:1:1) inclusions were investigated. The strong pinning due to 2:1:1-phase precipitates in these samples allows for characterization of the hysteretic response as a function of pinning-site concentration over a large portion of magnetic-field\char21{}temperature space. We have found the following: (i) The curves of effective pinning energy ${\mathit{U}}_{\mathrm{eff}}$ versus current density J reveal a diverging behavior of ${\mathit{U}}_{\mathrm{eff}}$(J) in the low-J regime. This supports the existence of a vortex-glass state, and is a signature of a vanishing resistance as the current density approaches zero. (ii) Both the ${\mathit{U}}_{\mathrm{eff}}$ and the J values obtained from magnetic hysteresis loops were observed to increase with Y 2:1:1 concentration. The appearance of the butterfly-shaped (or ``fishtail'') hysteresis loops indicates a ${\mathit{J}}_{\mathit{c}}$ that is an increasing function of H (or B). Moreover, it has been demonstrated that the additional pinning leads to an increase in ${\mathit{U}}_{\mathrm{eff}}$ in an H-T region in which the butterfly is developed.

Magnetic hysteresis and flux creep characteristics in melt-powder-melt-growth YBa 2 Cu 3 O 7 with Y 2 BaCuO 5 inclusions were measured between 5 and 80 K for magnetic fields up to 5 T. The critical magnetization current densities which were calculated using the Bean model and the sample dimension show a weak dependence on magnetic field. The J c values are of the order of 10 4 A/cm 2 at 60 K over the investigated field range

We have measured the dependence of the transport critical current density on magnetic field and temperature of a high-{ital J}{sub {ital c}} Bi-Sr-Ca-Cu-O/Ag tape sample. At low temperature, {ital J}{sub {ital c}}{gt}10{sup 4} A/cm{sup 2} extending to high fields as previously reported by several groups. For {ital T}{gt}20 K and with the applied field parallel to the {ital c} axis {ital J}{sub {ital c}} declines precipitously with increasing field and temperature, reflecting the properties of intragranular pinning. The variation of {ital J}{sub {ital c}} with the angle between {ital B} and the tape normal is consistent with a two-dimensional model of the vortex lattice. Independent determination of {ital J}{sub {ital c}} by magnetic hysteresis measurement shows rough agreement with transport {ital J}{sub {ital c}}'s at low fields but falls below by a factor that increases with increasing field and temperature. This can be accounted for by a model for thermally activated flux motion.

Magnetic relaxation and hysteresis studies have been performed for a c-axis-oriented single crystal of ${\mathrm{La}}_{1.86}$${\mathrm{Sr}}_{0.14}$${\mathrm{CuO}}_{4}$. We demonstrate that the effective activation energy for flux creep, ${\mathit{U}}_{\mathrm{eff}}$, is a strongly nonlinear function of the current density J (as given by the irreversible magnetization). For fixed fields of 0.5, 1.0, 2.0, and 3.0 T and temperatures between 4 and 15 K (as consistent with full field penetration of the sample), ${\mathit{U}}_{\mathrm{eff}}$(J) is seen to vary approximately logarithmically with J. A scaling relationship, ${\mathit{U}}_{\mathrm{eff}}$(J)=[-${\mathit{U}}_{1}$g(T)/${\mathit{H}}^{\mathit{n}}$]ln(J/${\mathit{J}}_{\mathit{c}}$), is shown to be obeyed for this crystal with n\ensuremath{\simeq}0.9 (H in T), ${\mathit{U}}_{1}$\ensuremath{\simeq}180${\mathit{k}}_{\mathit{B}}$, and ${\mathit{J}}_{\mathit{c}}$(1 T)\ensuremath{\simeq}3.2\ifmmode\times\else\texttimes\fi{}${10}^{5}$ A/${\mathrm{cm}}^{2}$. Several functional forms of g(T) have been explored and shown to give reasonable fits if monotonically decreasing in temperature over the range where flux creep is observed. We also explore the ramifications of this functional dependence in explaining an observed exponential temperature dependence of the hysterically determined critical current density. Finally, we show that comparison of the temperature dependence of logarithmic relaxation rate A=dM/d ln(t) and the derivative of the magnetization with respect to ln(T) at fixed time, ${\mathit{dM}}_{0}$/d ln(T), can be used to set a scale of the attempt frequency for flux motion consistent with the other measurements.

The electronic and magnetic structure of layered Fe/(Au)„Fe/Ag„, Cr/Au„, and Cr/Ag„ (n=1, 3, 5, 7, 9) multilayers and interface (sandwich) geometries (n =~) have been investigated using the layer Korringa-Kohn-Rostoker method. Enhanced magnetism for Fe and Cr (T) monolayers is observed in all the layered configurations examined. These results are consistent with weak coupling between the Fe or Cr d bands and those of the noble-metal (N) host lead to localization of d states orthogonal to the T/N interface. Fe and Cr moments are shown to vary systematically with the number of mediating Ag or Au planes as well as with the Fermi energy of the system. Interface and multilayer geometries show similar magnetic properties, suggesting that exchange coupling between the Fe or Cr planes in the multilayer geometry is short ranged and not responsible for the observed variations of the moment. A model, based on the compositional modulation of the Fermi energy coupled with the constraint of charge neutrality, solved in the limit of strong ferromagnetism is shown to explain the systematics of the moment variation for diff'erent layered configurations. There has been considerable recent growth in the field of interfacial, surface, and multilayer magnetism. ' Much of the fundamental interest in these materials stems from predictions of enhanced local moments ' and interesting phenomena associated with predictions of two-dimensional (2D) magnetism. The increasing ability to fabricate (i.e., with techniques such as molecular-beam epitaxy) a seemingly infinite number of possible configurations involvirig layers of magnetic species coupled with other magnetic or nonmagnetic layers has fed a productive interplay between theoretical and experimental endeavors in this field. Technological interest in these structures lies in their potential importance in thin-film recording and magneto-optic devices. Properties such as perpendicular magnetic anisotropy and square hysteresis loops, enhanced local moments, novel exchangecoupling effects, large magnetoresistive response, and sizable Kerr rotation angles (without rare-earth components)

We present an ab initio theoretical investigation of the magnetization and phase stability of two different complex cubic structures of prototype Cr23C6 and Mn23Th6, with an emphasis on the Fe,Co,Ni23B6 and Fe,Co,Ni23Zr6 compositions. These phases have recently been observed as secondary or even primary crystallization products of Fe,Co,Ni-Zr-B and related metallic glasses that have been studied for applications as soft magnets with nanocrystalline grain size. We first demonstrate the validity of the theoretical technique employed through a detailed comparison between the predictions of the calculations for the Co-Zr binary system and the experimentally stable phases. We then investigate the magnetization and stability of the binary phases. While the Fe-based binary Fe23Zr6 and Fe23B6 phases are expected to have the highest magnetization, the Co-based binary Co23Zr6 and Co23B6 structures are predicted to be the most stable of each prototype. The Co23Zr6 structure is the only binary 23:6 structure predicted to be a stable phase for the Fe,Co,Ni23B6 and Fe,Co,Ni23Zr6 systems investigated here. Small additions of Zr atoms to the Fe,Co,Ni23B6 phases tend to substitutionally occupy the 8c Wykoff site and stabilize these structures. In contrast, small additions of B to the Fe,Co,Ni23Zr6 phases have a much weaker site preference and tend to destabilize these structures. As a result, Fe,Co,Ni23B6 structures are stabilized in Fe,Co,Ni-Zr-B systems relative to the binary Fe,Co,Ni23B6 systems while the Fe,Co,Ni23Zr6 phases are not. The results presented in this work are in good qualitative agreement with experimental observations of the compositional modifications tending to promote formation of the 23:6 phases in Fe-Co-Zr-B and related metallic glasses.

We have investigated magnetic relaxation on a grain-aligned sample of YBa{sub 2}Cu{sub 3}O{sub 7{minus}{ital x}} powder embedded in epoxy with the magnetic field along the {ital c} axis. Through a thermal-cycling procedure and through variations in the measuring temperature over the range 10--30 K, we manage to commence magnetic relaxation from a range of values of the magnetization, {ital M}, or current density {ital J}. Analysis of relaxation measurements at a magnetic field of 1.0 T over the temperature range 10--30 K reveals a nonlinear dependence of the effective activation energy {ital U}{sub {ital e}} on {ital M} or {ital J} that is consistent with the logarithmic dependence obtained by Zeldov {ital et} {ital al}. from transport measurements. This demonstration of a nonlinear dependence of {ital U}{sub {ital e}}({ital M}) is capable of resolving anomalies in the magnitudes and temperature dependence of flux-creep activation energies obtained from magnetic-relaxation studies.

Electronic-structure calculations of icosahedral Al-Mn clusters suggest well-defined quasiatomic Mn moments. The driving force for moment formation is a large Mn d density of states at the Fermi energy in a non-spin-polarized calculation. The onset of Mn magnetism results directly from the local coordination in the icosahedral environment, which does not split the Mn 3d levels and inhibits Al-Mn sp-d hybridization.

Calculs d'onde diffusee, de champ auto-coherent et de spin polarise sur des amas de FeM 14 , FeM 26 et FeCa 18 (M=Li, Na, K, Rb), modelisant les environnements locaux d'une impurete isolee de Fe dans des hotes (M)bcc et (Ca)fcc. Explication des resultats en termes d'hybridation

Alloys of ${\mathrm{Al}}_{50}$${\mathrm{Mn}}_{20}$${\mathrm{Si}}_{30}$ and ${\mathrm{Al}}_{55}$${\mathrm{Mn}}_{20}$${\mathrm{Si}}_{25}$ have been prepared by rapid quenching from the melt onto a single Cu roller. As-quenched alloys have been found to be fully amorphous. This amorphous phase crystallizes at approximately 600 K. Alloys annealed below the crystallization temperature transform into single-phase icosahedral quasicrystals. Both the amorphous and quasicrystalline phases are weakly ferromagnetic with Curie temperatures around 120 K and saturation magnetization in the order of 0.1 emu/g.

It is shown that Slater's transition-state technique often used with X\ensuremath{\alpha} calculations generalizes trivially to any local density functional. This technique is shown to yield excellent electron-removal energies for several popular local density functionals. More importantly we interpret density-functional-theory eigenvalues as orbital electronegativities, demonstrating that they quantitatively agree with Mulliken's definition of electronegativity to better than 1% on average for atoms.

The time-dependent decay of the diamagnetic magnetization was measured after zero-field cooling (ZFC) over a broad range of fields and temperatures. The time dependence was found to be linear for short times and logarithmic thereafter within the experimental time intervals for all fields and temperatures. The relaxation rates /ital A/(/ital H//sub 0/,/ital T/) and /ital A/(/ital H/,/ital T//sub 0/) show maxima at /ital T//sub /ital m// and /ital H//sub /ital m//, respectively (where /ital T//sub 0/ and /ital H//sub 0/ are constant values). The shape of /ital A/(/ital H/,/ital T//sub 0/) is more complicated than that of /ital A/(/ital H//sub 0/,/ital T/), which is nearly symmetrical around /ital T//sub /ital m//. Increasing /ital H//sub 0/ or /ital T//sub 0/ decreases /ital H//sub /ital m// or /ital T//sub /ital m//. The activation parameters were found to be strongly field dependent, whereas they agree well for both investigated compositions. The activation energy decreases with increasing field, following the empirical law: /ital Q/=/ital a/+/ital b///ital H//sup 2/. The frequency factor distribution broadens with increasing /ital H/, which can be described by the following empirical expression: ln(/tau//sub 02///tau//sub 01/)=d/minus/g/H. Using the activation parameters as determined from /ital A/(/ital H//sub 0/,/ital T/) and interpolating bymore » the above expressions, the relaxation rate /ital A/(/ital H/,/ital T//sub 0/), can be calculated and agrees well within the experimental data within the experimental error. The data are interpreted using the theoretical model of Beasley, Labusch, and Webb on thermally activated flux creep over pinning centers assisted by the field-dependent driving force.« less

A large difference in the rate of flux flow in YBa2Cu307 — ~ superconductors is observed between flux expulsion (field cooling then removing the field) and fiux penetration' (zero-field cooling then switching on a field). The mean activation energy and the width of its distribution for fiux expulsion are 14 and 28 meV and for flux penetration are 34 and 67 meV. The smaller activation barrier for flux expulsion relative to flux penetration can be explained quantitatively in terms of flux pinning at the surface image potential and is also consistent with a model of randomly coupled superconducting grains. Hard superconductors show time-dependent changes in their magnetization (diamagnetic susceptibility) under a magnetic field gradient. These changes, typically logarithmic in time, can be modeled as thermally activated Auxon motions over a distribution of pinning barriers where the apparent activation energy increases as equilibrium is reached. The driving force is the Lorentz force associated with the field gradient. Miiller, Takashige, and Bednorz stirred renewed interest in Aux creep when their report of large time-dependent effects in Laz(Sr)Cu03 reached the wide community now attracted to the field of high-T, superconductivity. They interpreted the flux creep as evidence of a superconducting-glass (SCG) state with a random local-order parameter associated with the phase difference between the superconducting wave functions in adjacent coherent superconducting regions. Moreover, they identified the ergodic limit on the SCG H-T phase diagram as a de Almeida- Thouless glass transition beyond which the system is Auidlike, i.e., the time scale for relaxation is very short on a laboratory scale. Critical to the understanding of the time-dependent 'eAects, whether as conventional Aux creep or as manifestations of an SCG state, is an appreciation of the preparation of the initial magnetization states. The initial magnetization can be set by zero-field cooling (ZFC) then suddenly switching the field on, MzFC(T, tp), or by field cooling (FC) then suddenly removing the field, M„(T,tp) (Both of these magnetizations are also functions of the field involved but that dependence will not be explicitly expressed because its effects are not discussed here. ) In the former case the time dependence describes Aux penetration, in the latter case, Aux expulsion. It is of interest to compare these two processes. It is known that in the absence of an applied field, the coherent superconducting region in Y 1:2:3is the macroscopic specimen, whether it is a single crystal or a Josephson-coupled polycrystalline sample. However, in the presence of a relatively weak field, not only are the grains of a polycrystal decoupled (Hq (100 Oe), s but the coherent superconducting regions may become much smaller than the grain size. Our Aux-creep experiments were done at fields of 1 kOe and greater and, therefore, probe the intragranular pinning processes, not those of the weak links between the grains. We have studied flux flow in a variety of polycrystalline high-T, superconductors and observed a significant difference between flux expulsion and penetration in several of these. Here we report the results for YBa2Cus07 — s (Y 1:2:3). Flux expulsion is found to be a much easier process, penetration more difficult, in terms of the activation energies derived. In Y 1:2:3 flux expulsion is characterized by a mean activation energy of 14 meV, Aux penetration by 34 meV. It is not possible to interpret this difference in terms of a simple flux-creep model in which the same pinning barriers are encountered by Auxons entering or leaving the material. We show that it is possible to explain the data quantitatively by extending the simple flux-creep model to include surface-pinning efrects. '

Magnetization curves have been measured as a function of temperature for a polycrystalline sample of ${\mathrm{Ba}}_{0.6}$${\mathrm{K}}_{0.4}$${\mathrm{BiO}}_{3}$. These measurements reveal significant reversibility at modest fields for all temperatures studied. The irreversibility line (${H}^{\mathrm{*}}$,${T}^{\mathrm{*}}$) is determined for this material and shows that ${H}^{\mathrm{*}}$ is less than 0.8 T at all temperatures. The logarithmic magnetic relaxation rate A=dM/d ln(t) has been studied for this material, as a function of both field and temperature. Analysis using the flux-creep model reveals the average pinning energy ${U}_{0}$ to be 0.08\ifmmode\pm\else\textpm\fi{}0.02 eV. This extremely small pinning energy is used to explain the small critical current, its strong dependence on field, and the significantly reversible behavior in this material.

The superconducting $\mathrm{Y}{\mathrm{Ba}}_{2}{\mathrm{Cu}}_{3}{\mathrm{O}}_{7\ensuremath{-}x}$ phase has been produced through a novel rapid solidification processing route yielding a high-quality granular superconducting phase. A detailed study of the magnetic properties indicates important features relevant to current carrying capacity. Low-field magnetization data reveal that weakly coupled superconducting grains decouple at fields of approximately 20 Oe so that the magnetization measurements at higher fields are reflective of intragranular processes exclusively. The high-field data reveal significant irreversibility as well as nonergodic susceptibility behavior consistent with the recently proposed superconducting glass state.

We show that the anomalously small impurity exchange splittings obtained from previously reported multiple-scattering Xa cluster calculations for the dilute alloys CuM (M represents a 3d transition metal) were due to incomplete convergence to self-consistency. Proper self-consistent calculations produce exchange splittings that are in excellent agreement with other calculations, which we illustrate with CuMn. We compare in detail the results of our calculations with those obtained with other approaches and discuss briefly the role of many-body effects in the CuM alloy series. The calculation of electronic structures of dilute transition-metal impurities in a nonmagnetic host is a problem in determining the extent to which the impurity 3d electrons maintain an atomic character in the metallic environment. In general terms, the local alloy electronic structure is the result of a competition between the tendency toward localization of the 3d charge about the impurity, which is driven by the intra-atomic electronic correlations, and charge delocalization, caused by mixing between the impurity 3d orbitals and the conduction electrons of the host. For example, there are well-defined moments associated with several of the transition metals in a copper environment which for chromium and manganese have been observed to prevail well below liquid-helium temperatures. ' In contrast, the magnetism of the transition metals is almost completely absent in an aluminum environment, with only chromium and manganese showing signs of quasimagnetic behavior in the form of spin fluctuations. The varied electronic properties of the transition metals in the respective environments underscore the role of the host in determining the local electronic structure of the alloy. The manifold differences between the electronic structures of copper and aluminum are reflected through the bonding among neighboring atoms which, in turn, influences the available bonding channels between the transition-metal impurities and the respective hosts. The formal approaches to this problem begin from one of two extreme points of view. The Friedel-Anderson virtual-bound-state model ' considers the 3d orbitals as resonances in the host conduction band such that even in the case of a magnetic impurity only the gross features of 3d atomic character remain. At the other extreme the Schrieffer-Hirst configuration-based model asserts that the impurity-host interaction leads to a coupling among different 3d occupancies within each of which there is a hierarchy of term, spin-orbit, and crystal-field splittings. The magnetic limit of the Schrieffer-Hirst theory then corresponds to the predominance of one 3d configuration over all others, and the model predicts that the aforementioned intra-atomic splittings within this dominant configuration are experimentally observable. The virtual bound-state approach thus places the burden of localization upon the strength of the impurity potential (e.g. , the Friedel sum rule ), while the configuration-based approach presumes the existence of localized many-electron atomic states whose correlations must be broken down by the conduction electrons. In a previous paper, a model was developed for the local electronic structure of dilute CuM (M represents a 3d transition metal) alloys based upon the multiple scattering A'a (MS-A'a) method. ' Electronic structure calculations were carried out for nineteen-atom complexes that included the substitutional impurity and the twelve nearest and six next-nearest copper neighbors in the face-centered cubic lattice. The calculations revealed (a) trends in local magnetization and spin occupancy of crystal-field levels that are in agreement with nuclear-magnetic-resonance (NMR) measurements, " (b) significant impurity-host d-d hybridization, which was subsequently observed in photoemission studies, ' but (c) the reduction of the impurity intra-atomic exchange splitting by an order of magnitude from the atomic value. Here we show that the anomalous result (c) was due to incomplete convergence to selfconsistency, and that fully self-consistent calculations yield excellent agreement for the impurity exchange splitting with other cluster calculations, ' as well as with the self-consistent matching Green's-function calculations of the Julich group. '