Local Magnetic Field Distribution Research Articles

Characterization of superconducting rings using an in-field hall probe magnetic mapping system

A Hall probe magnetic imaging system that works in magnetic fields in the range -1 T to 1 T has been implemented, and it has been used to characterize the superconducting behavior of single domain melt textured YBa/sub 2/Cu/sub 3/O/sub 7/ rings. We show that in addition to the analysis of the evolution of the local magnetic field distribution when the external magnetic field is cycled, the hysteretic behavior of the magnetic moment can also be investigated after integration of the local magnetic field. The critical current density has been determined through the critical state model and it has been compared to that calculated by inversion of the Biot-Savart law. A remarkable agreement is achieved with both methods.

An improved method for quantitative magneto-optical analysis of superconductors

We report on the analysis method to extract quantitative local electrodynamics in superconductors by means of the magneto-optical technique. First of all, we discuss the calibration procedure to convert the local light intensity values into magnetic induction field distribution and start focusing on the role played by the generally disregarded magnetic induction components parallel to the indicator film plane (in-plane field effect). To account for the reliability of the whole technique, the method used to reconstruct the electrical current density distribution is reported, together with a numerical test example. The methodology is applied to measure local magnetic field and current distributions on a typical YBa2Cu3O7−x good quality film. We show how the in-plane field influences the MO measurements, after which we present an algorithm to account for the in-plane field components. The meaningful impact of the correction on the experimental results is shown. Afterwards, we discuss some aspects about the electrodynamics of the superconducting sample.

Atomic and magnetic structures, disorder effects, and unconventional superexchange interactions inA2MnGaO5+δ(A=Sr,Ca)oxides of layered brownmillerite-type structure

Crystal and magnetic structures of complex manganese oxides Sr2GaMnO5+delta (deltasimilar or equal to0.01,0.52) and Ca2GaMnO5+delta (deltasimilar or equal to0.05) were studied by neutron powder diffraction (ND) and muSR technique in the temperature range 2-300 K. The crystal structures contain single MnO2 layers separated by three nonmagnetic cation-oxygen layers. The principal difference between the deltasimilar or equal to0 and deltasimilar or equal to0.5 compounds is the Mn valence: Mn3+ or Mn4+, and the structure of the (GaO1+delta) buffer layer, which is formed by tetrahedra or partially filled octahedra, respectively. The magnetic moments of the manganese ions are coupled antiferromagnetically in the MnO2 plane, but antiferromagnetically (G type) or ferromagnetically (C type) between the planes for the reduced and oxidized compositions, respectively. The transition from the G- to C-type magnetic structure by oxygen doping is explained by strong diagonal 180degrees superexchange antiferromagnetic interaction between Mn4+-ions in the adjacent layers through additional oxygen atoms in the GaO buffer layer. The magnetic moments in Sr-based samples are appreciably reduced in comparison with the spin-only values of the corresponding Mn ion. By using complementary information on local magnetic field distribution from muSR we show that the reduced magnetic moments seen by ND are caused by the presence of locally flipped Mn spins and a short-ranged (40 Angstrom) antiferromagnetic phase. The magnetic disorder can be caused by the disorder observed in the oxygen positions of the GaO1+delta layer, because the coupling between the MnO2 layers is mediated by the geometry of the superexchange path through these oxygen atoms.

Deconvolution of 2D transport currents from the local magnetic field distribution above flat superconducting samples

Measurements of the magnetic field distribution (magnetic images) above flat superconducting samples provide information on the pattern of current flow within the material. While several deconvolution procedures to calculate the current flow have been proposed, we show here that they do not always give accurate results in situations where the current flow extends to regions that are outside of the magnetic image window, as with transport currents in a conductor. We describe two new procedures which overcome the problem. The most accurate (and most direct) approach is to incorporate magnetic images at different heights above the sample. The second method relates to the more conventional case using only one field image at a finite height above a thin sample (but of finite thickness). The results compare favourably to existing approaches.

A STUDY OF 2D ISING FERROMAGNETS WITH DIPOLE INTERACTIONS

A two-dimensional Ising model with competing short range ferromagnetic and long range dipolar interactions is used to study the transition properties and phase diagram in ultrathin magnetic films. Monte Carlo simulations in systems with exchange and dipolar interactions reveal a ground state of striped phases with varying width. By raising the temperature, the domain walls are smeared out by fluctuations, leading to a random domain mesoscopic phase with no long-range order, and finally the domains are melted to the high-temperature disordered phase. Local magnetic field distributions and specific heat calculations reproduced transition points consistent with the previous phase diagram of the model. The resemblance to the phase diagram in ultrathin magnetic films, such as in Fe/Cu(100), is discussed.

Mesoscopic ferromagnet-superconductor junctions and the proximity effect

We have measured the electrical transport of submicron ferromagnets (Ni) in contact with a mesoscopic superconductor (Al) for a range of interface resistances. In the geometry measured, the interface and the ferromagnet are measured separately. The ferromagnet itself shows no appreciable superconducting proximity effect, but the ferromagnet/superconductor interface exhibits strong temperature, field, and current bias dependences. These effects are dependent on the local magnetic field distribution near the interface arising from the ferromagnet. We find that the temperature dependences can be qualitatively described by a modified version of the Blonder-Tinkham-Klapwijk theory for normal-superconductor transport.

A method for decreasing transport ac losses in multifilamentary andmultistrip superconductors

A new method is proposed for decreasing transport ac losses inmultifilamentary superconductors by the decoupling of the filaments using amagnetic material in the form of thin layers surrounding the individualfilaments. For a superconductor with an elliptical cross section, the magneticmaterial surrounding the filaments affects the local magnetic fielddistribution that both reduces the critical current of the filaments andinduces the transport ac losses in the magnetic material. Even by taking intoaccount any detrimental influences of the presence of the magnetic materialaround the filaments, the analysis of the experimental data supported bycomputer modelling confirmed that for a Bi2223 tape with 100 filamentsindividually covered by magnetic material, such as iron powder, the transportac losses should be 65 times lower than for the same multifilamentaryconductor without the magnetic coating on the filaments. With an increasingnumber of filaments, the ac loss decrease would be even larger.

Two-dimensional NMR study of surface water dynamics in hydrated silica spheres

Partially hydrated LUDOX silica spheres (LUDOX colloidal silica HS-40, E.I. DuPont de Nemours Co.) of 15 nm diameter represent a system where the wetting layer of water is quite well defined in the sense that the study of the structure and dynamics of this layered water is not obscured by the much stronger signal of bulk water. When placed in a homogeneous magnetic field, the magnetic susceptibility differences of the silica and water create a distribution of local magnetic fields at the solid-liquid boundary. These fields encode the NMR frequency of the resonant nuclei and label water molecules in the surface layer. The distribution of local fields was determined by the deuteron two-dimensional (2D) separation of interactions NMR technique. The characteristic autocorrelation time ${\ensuremath{\tau}}_{c}$ for the motion of water molecules in the surface layer was studied by measuring the deuteron quadrupolar spin-lattice and spin-spin relaxation times, and taking their ratio. The ${\ensuremath{\tau}}_{c}$ characterizing the fluctuations of the electric-field gradient tensor at the deuteron site was determined at room temperature for a set of hydrations corresponding to an estimated thickness of surface water from 2 to 22 water monolayers. The magnitude of ${\ensuremath{\tau}}_{c},$ which is on the order of ${10}^{\ensuremath{-}8}\mathrm{s},$ demonstrates that the quadrupolar relaxation mechanism is the intramolecular rotational diffusion. This dynamic process slows down only insignificantly with a decreasing thickness of the wetting layer. A search for slower motions was made by the 2D exchange NMR of deuterons in a 6% hydrated LUDOX. No slow molecular motions were detected within the observation window of the experiment, demonstrating that the motional frequencies are faster than $2\ensuremath{\pi}\ifmmode\times\else\texttimes\fi{}{10}^{5}{\mathrm{s}}^{\mathrm{\ensuremath{-}}1}.$

Nuclear magnetic resonance study of the internal magnetic field distribution in water base ionic and surfacted ferrofluids

This work reports a nuclear magnetic resonance (NMR) study of both hydrogen and deuterium on two different kinds of ferrofluids. One of them is a colloidal suspension of surfacted magnetic grains dispersed in a mixture of light and heavy water. The second one is an ionic ferrofluid composed by positively charged magnetic grains of Co–Fe2O4 dispersed in the same solution of water. A model of the local magnetic field distribution in the interstitial (bulk) water is worked out in order to account for the broadening of the NMR spectra observed in both samples.

Two-dimensional nuclear magnetic resonance study of a hydrated porous medium: An application to white cement

Structure and dynamics of a hydrated white cement were investigated by the T1-weighted lineshape, two-dimensional (2D) exchange, and 2D separation of inhomogeneous and homogeneous lineshapes nuclear magnetic resonance (NMR) techniques. T1 weighting of the proton spectrum eliminates the strong bulk water signal from the pores so that the weaker spectrum of the solid cement matrix becomes observable. The proton spectrum of the solid cement fraction exhibits a characteristic Pake doublet shape and shows a close similarity to the powder spectra of pure calcium hydroxide and the crystalline water of gypsum. The 2D exchange NMR spectrum of white cement demonstrates the existence of slow chemical exchange processes of protons in the solid matrix on a sub-kHz frequency scale. This exchange is orders of magnitude slower than the exchange between the surface and bulk water. The 2D NMR separation of inhomogeneous and homogeneous lineshapes technique demonstrates that the absorption lines of a set white cement are inhomogeneously broadened due to the existence of a distribution of static local magnetic fields. The major contribution to these fields comes from the dipolar fields of the paramagnetic electrons whereas the magnetic susceptibility differences of the liquid and solid fractions add a small part at the solid-liquid boundary where the direction of the external magnetic field is changed.

Local Magnetic Field Distribution in a Polycrystalline Sample Exposed to a Strong Magnetic Field

Abstract A model for a polycrystalline material is proposed to describe the local magnetic field distribution that gives rise to susceptibility broadening in nuclear magnetic resonance. The following contributions to the broadening have been accounted for: (i) broadening due to various orientations of the crystallites containing the resonating spin; (ii) broadening due to different orientations of neighboring crystallites; and (iii) broadening due to variations in the distribution of neighboring crystallites. The effect of each contribution on the NMR spectra taken with and without magic angle spinning has been studied for axially symmetric susceptibility tensors. Numerical simulations of the overall magnetic field distribution have been carried out for different packing densities and anisotropies Δχ of the susceptibility tensor. The overall field distribution appeared to be symmetric with a first moment that is not affected by the presence of neighboring crystallites. The MAS line-narrowing factor was found to decrease for higher packing density and increasing Δχ. The proposed method also allows the calculation of local gradients associated with the local magnetic fields.1H NMR experiments on adsorbed molecules in NaX zeolite have been used to test the proposed model and to examine the limitations that magnetic susceptibility forces upon spectral resolution.

NMR of3He in Porous Glasses

Abstract A study of 3 He in porous glasses is performed in which the observed chemical shifts and lineshapes are attributed to the local magnetic field distribution in the pores and to the distribution of pore shapes. The observed relaxation times T 1 and T 2 are explained by the residual oxygen concentration in the pores and by the diffusion of 3 He in the inhomogeneous local field, respectively. The study indicates that in principle sample tubes containing 3 He in the pores as a built-in lock substance could be produced but the production is not straightforward. 3 He proves to be an ideal nucleus for studying the shape of void structures in solids.

Structures of single vortex and vortex lattice in a d-wave superconductor.

The structures of a single vortex and vortex lattice in a superconductor with ${d}_{{x}^{2}\ensuremath{-}{y}^{2}}$ symmetry are studied self-consistently employing a recently developed Ginzburg-Landau theory. Near a single vortex, we found that an $s$-wave component of the order parameter is always induced, and it causes the local magnetic-field distribution and the $d$-wave order parameter to have a fourfold anisotropy. It is shown that there is a strong correlation between the structure of a single vortex and the shape of the vortex lattice. Our numerical calculation indicates that the structure of the vortex lattice is always oblique except for temperatures very close to ${T}_{c}$ where it becomes triangular. The possible connection of the result with experiment is also discussed.

Muon-spin-rotation and relaxation studies in (TMTSF)2-X compounds.

Muon-spin-rotation and relaxation (\ensuremath{\mu}SR) studies of the organic compounds (TMTSF${)}_{2}$-X (X=${\mathrm{PF}}_{6}$, ${\mathrm{NO}}_{3}$, and ${\mathrm{ClO}}_{4}$) at ambient pressure are reported. We observe spin-density-wave (SDW) states in all three compounds under zero external magnetic field. The onset of the SDW is extremely sharp, which may indicate a first-order transition. The sublattice magnetization (or SDW amplitude) in the ${\mathrm{PF}}_{6}$ compound exhibits significant reduction with increasing temperature at low temperatures, which demonstrates the existence of collective low-energy excitations, in addition to the single-particle excitations across the SDW gap. The large spin-wave stiffness we observe in this system is incompatible with a Heisenberg model for a localized spin system; this demonstrates the importance of using an itinerant-electron picture to describe the magnetic behavior of this system. The broad distribution of local magnetic fields deduced from the \ensuremath{\mu}SR time spectra is consistent with that expected from an incommensurate SDW. The magnitude of the internal field at T\ensuremath{\rightarrow}0 is approximately the same for the three systems, suggesting a common SDW amplitude in these systems. Transverse-field \ensuremath{\mu}SR measurements in the relaxed-state ${\mathrm{ClO}}_{4}$ system show no visible enhancement of the relaxation rate in the superconducting state down to 0.1 K, providing a lower limit for the superconducting penetration depth ${\ensuremath{\lambda}}_{\mathrm{ab}\ensuremath{'}}$\ensuremath{\geqslant}12 000 \AA{}.

Susceptibility effects in porous media in the presence of flow.

Magnetic resonance (MR) imaging of microvascular flow is an important topic in biomedicine because it permits access to the functional state of a biologic system. The internal heterogeneous magnetic field due to susceptibility differences within tissues is one of the factors that can affect signal intensity. A glass bead phantom simulating a porous medium was used to experimentally study the effect of the internal magnetic field on MR flow measurements. A physical model was developed to simulate the paths of the moving spins and the local magnetic field distribution in the medium to estimate the signal intensity with spin-phase analysis. The susceptibility variation inside the glass bead phantom was estimated by comparing the simulation results with the experimental data. Experiments were also performed with a tissue-simulating phantom, and the results indicated that the effect of the internal field inhomogeneity on in vivo microvascular flow measurements was negligible.

Local magnetic field distribution in polycrystalline YBa2Cu3O7 and its influence on bulk critical currents

The transport critical current density Jc has been measured in high quality polycrystalline YBa2Cu3O7 samples made from extruded preforms. The hysteretic response of Jc to applied magnetic fields was studied as a function of temperature (77–87 K) and sample morphology for various field sweeps in the 0–5 KOe range. A variety of samples show a basic Jc ∼ H−n behavior where n depends on temperature, but is independent of the other variables. A simple model of the local magnetic field distribution is presented and compared to the data, and the theoretical implications of the power law behavior are considered.

Influence of a transport current on the local magnetic field distribution in sintered YBa2Cu3Ox

The influence of a transport current on the magnetic flux-line distribution in sintered YBCO was studied by means of μSR. Pronounced differences between zero-field-cooled (ZFC) and field-cooled (FC) signals and irreversible behavior were observed. In the ZFC case even a small transport current (10 A/cm2) tends to orderirreversibly the inhomogeneous flux-line distribution considerably, suggesting a broad distribution of pinning barriers. However, for a FC sample no noticeable change in the flux distribution in the presence of a transport current (up to 40 A/cm2) was detected, indicating that the FC state represents a stable flux-line configuration.

μSR in oxygen deficient YBa2Cu3O x (6.5⩽x⩽7.0)

A systematic μSR study of the local magnetic field distribution in a series of oxygen deficient YBa2Cu3O x samples with 6.5⩽x⩽7.0 is reported. Special attention was given to perform the experiments under the same conditions, so that the oxygen content of the measured samples was the only parameter varied. The behavior of the depolarization rate σ as a function of the oxygen contentx was found to have strong similarities with the behavior of the critical temperatureT c as a function ofx. In particular, two step-like increases of σ were observed abovex=6.7 and 6.9. The temperature dependence of the normalized depolarization rate σ(T)/σ(0) is well described by the two-fluid model forx⩾6.781(1) and clearly deviates from this behavior forx⩽6.704(1). Our results are compared to those obtained by other groups.

Magnetic and electronic properties of YBa 2Cu 3O x (6.5 < x < 7.0) studied by muon spin rotation

A systematic study of the local magnetic field distribution p( B) in the perovskite-type superconductor YBa 2Cu 3O x is presented. Special attention is given to the second moment 〈 ΔB 2 〉 of p( B) which is closely related to the superconducting carrier density n s and the effective mass m ∗ of the superconducting carriers. The influence of temperature, anisotropy and hole doping are investigated in detail. It is found that the temperature dependence of the magnetic penetration depth in YBa 2Cu 3O x is in agreement with conventional s-wave pairing and suggests strong coupling. The anisotropic behavior of p( B) is well described by a theory for uniaxial type II superconductors, with an anisotropy ratio γ ∼- 5(1). Moreover, the zero-temperature values λ ab (0) = 130(10) nm and λ c (0) ∼- 500–800 nm (penetration depth for screening currents flowing in the ab-planes and along the c-axis, respectively) were extracted from our data. μSR experiments performed on oxygen deficient samples indicate that the structural and electronic properties of YBa 2Cu 3O x are strongly correlated via the oxygen content x.

Muon-spin rotation experiments in high-Tc superconductors and related materials

Recent muon-spin rotation µSR experiments in high-T c superconductors and related antiferromagnetic materials are reviewed. The possibilities and the limitations of the µSR method for investigating these materials are briefly discussed. In a high-T c superconductor, µSR is an ideal tool with which to study the local magnetic field distribution at the muon site, allowing a determination of the London penetration depth. It is further shown that µSR experiments may contribute to the microscopic understanding of the superconducting glass state in the high-T c oxides. In the related antiferromagnetic materials µSR is a sensitive method for detecting frozen local magnetic moments.