Low-field Behavior Research Articles

Effects of surface-roughness related process parameters on the magnetoresistive characteristics of spin valves

The low-field magnetoresistance behavior was investigated for sputter-deposited NiFe 80 /spl Aring//Cu 28 /spl Aring//NiFe 40 /spl Aring//FeMn 150 /spl Aring/ spin valve multilayers under various process conditions (buffer layers, argon sputtering pressure, and Cu-sputtering power). Topological dependence of MR characteristics was observed by atomic force microscope (AFM) surface analyses. We discuss the effects of the process parameters on the bias field and the field sensitivity in terms of topological coupling associated with the interface roughness. In addition, we could control the MR characteristics utilizing a combination of the process parameters which significantly affect the surface and interface roughness of multilayers.

Universal Behaviour of Magnetoconductance due to Weak Localization in Two-Dimensional Systems - Example of GaInAs Quantum Wells

Weak localization corrections to conductivity of a two-dimensional electroii gas are studied by measurements of the magnetic field dependence of the conductivity in GalnAs quantum wells. We observe that, when presented as a function of the normalized magnetic field (x = B/Btr , where B is the magnetic field, B« = h/4eτD, D is the dichusion constant and τ is momentum relaxation time), dicherent samples show very similar high field behaviour. A theoretical description is developed that allows one to describe in a consistent way high and low field behaviour. The theory predicts universal (B -1 /2 ) behaviour of the conductivity correction for all 2D systems in the high field limit (r > 1). Low field behaviour depends strongly on spin and phase relaxation mechanisms. Comparison of the theory with experiment confirms the universal behaviour in the high fleld limit and allows one to estimate the spin and phase relaxatioii times for dicherent GalnAs quantum wells.

Electronic instability during vortex motion in cuprate superconductors Regime of low and high magnetic fields

The critical velocity ν ϕ ∗ associated with the electronic instability predicted by Larkin and Ovchinnikov displays a cross-over effect from magnetic-field independent behavior at high fields to the proportionality ν ϕ ∗ ∼B − 1 2 at low fields. This low-field behavior results from the fact that ν ϕ ∗ multiplied with the inelastic quasiparticle scattering time τ in must reach at least the intervortex distance, thereby ensuring spatial homogeneity of the nonequilibrium quasiparticle distribution. At high magnetic fields the flux-flow instability becomes unobservable due to the onset of fluctuation effects contributing to the resistivity.

Microwave losses of YBCO thin films in high magnetic fields

A parallel plate resonator technique was employed to investigate changes in the microwave surface impedance between 20 K and 60 K, when d.c. magnetic fields up to 4 T were applied parallel to the YBCO film surface. By using a simple model to describe the observed behaviour, we can extract values for the vortex viscosity /spl eta/ and pinning constant /spl kappa//sub p/, as a function of temperature in three sets of films. As a result of low energy deuterium ion implantation in one of the films, the low field behaviour was degraded but at high fields the pinning properties improved quite drastically. A much smaller effect was observed in field dependence of the dc screening current as measured using vibrating sample magnetometry. >

Low-magnetic-field critical behavior in strongly type-II superconductors.

A description is proposed for the low-field critical behavior of type-II superconductors. The starting point is the Ginzburg-Landau functional in presence of an external magnetic field H. A set of fictitious vortex variables and a singular gauge transformation are used to rewrite this finite H Ginzburg-Landau functional in terms of a complex scalar field of zero average vorticity. The continuum limit of the transformed problem takes the form of an H=0 Ginzburg-Landau functional for a charged field coupled to a fictitious ``gauge'' potential, which arises from long-wavelength fluctuations in the background liquid of field-induced vortices. A possibility of a phase transition involving zero-vorticity degrees of freedom and formation of a uniform condensate is suggested. A similarity with the superconducting (Higgs) electrodynamics and the nematic-smectic-A transition in liquid crystals is noted. The experimental situation is discussed.

Majority carrier injection from ITO anodes into organic light-emitting diodes based upon polymer blends

The current-field characteristics of organic light-emitting diodes (LEDs), formed by sandwiching blends of hole-transporting molecules and various polymer binders between indium-tin oxide (ITO) and Al contacts, have been measured. It is concluded that the rate-limiting step is hole injection at the ITO anodes. At high fields, temperature-independent tunneling injection prevails, the efficiency of which scales with the mobility of the charge carriers in the transport layer. Increasing the polarity of the matrix enhances injection because of barrier lowering. The low field behaviour ( E < 10 6 V cm −1) is controlled by a superposition of thermionic injection and fluctuation induced tunneling, the latter being more pronounced with polar systems.

Investigation of the magnetic field and stress dependence of 180° domain wall motion in pipeline steel using magnetic Barkhausen noise

Magnetic Barkhausen noise (MBN) measurements were performed on 2% Mn pipeline steel samples as a function of magnetic field ( H) and as a function of orientation of an applied sweep field for various tensile and compressive stress values. For the magnetic field dependent measurements, the H field and stress were applied along the same axis, which was aligned within 10° of the sample's magnetic easy axis. Tensile stresses between 210 MPa and 0 and compressive stresses between 0 and -200 MPa were applied. The MBN signal was observed to initially increase with H followed by a decrease and eventual suppression of the MBN signal at higher H values. The application of tensile stress increased the total MBN signal and increased the field at which suppression occurred, while the application of compressive stress decreased the total signal with a corresponding decrease in the suppression field value. The stress and low-field behaviour of the MBN signal was correlated with the irreversible differential permeability ( μ IDP ) measured in a similar 2% Mn pipeline steel sample. The behaviour of the MBN signal as a function of H and stress was attributed to the effect of these parameters on the relative population and/or size of 180° domain walls, that is, the total 180° domain wall area, within the sample.

Low-field behavior of magnetic and transport properties of laser irradiated Nd-Ba-Cu-O ceramic superconductors

The low-field behavior of magnetic and transport properties of high-power laser-irradiated (Q-switched ruby laser, 694 nm, 30 ns) Nd-Ba-Cu-O (NBCO) ceramic superconductors prepared by the coprecipitation technique have been investigated. Laser-irradiated NBCO samples contribute significantly to the flux pinning for low applied magnetic fields. A well-defined field valueH′ c was observed for different laser-irradiated samples which is characteristic of a cross-over between regions dominated by the inter- and intragranular effects. The value ofH′ c for a laser-irradiated sample is increased with increasing number of laser pulses. Thermal modeling of laser heating reveals that on irradiation the surface melts up to a depth of 1μm and laser-induced evaporation occurs at energy density 2.5 J/cm2. SEM studies showed grain growth due to laser-induced surface sintering which improves the interconnectivity among the superconducting grain structure and thus attributed to physical densification and consequent reduction in the total number of weak links. The increase of flux trapping and transport critical current density after laser irradiation is determined by the rigidity of the flux line lattice which may be due to the presence of additional pinning centers caused by the creation of laser-induced mobile defects and their clusters.

Field and oxygen dependence of the magnetic irreversibility line in YBa2Cu3O7- delta.

We report on dc magnetic susceptibility measurements in polycrystalline YBa[sub 2]Cu[sub 3]O[sub 7[minus][delta]] as a function of the oxygen content in the limit of small [delta]. We focus mainly on the detailed behavior of the zero-field-cooled--field-cooled irreversibility line at fields below 7 kOe. We find that this line shows a crossover from a [ital H][sup 2] dependence at higher fields to the more commonly observed [ital H][sup 2/3] behavior at lower fields. This feature is analogous to the anisotropy induced crossover from the high-field Gabay-Toulouse line to the low-field de Almeida-Thouless behavior, characteristic of the spin-glass transition. Our results are discussed in the light of a dirty-limit vortex-glass model where the grain charging energy plays a major role. Data concerning the behavior of the resistive transition as a function of the distribution of the oxygen vacancies are also presented.

Transition from intact to short decoupled vortices in the vortex liquid of YBa2Cu3O7- delta.

Superconducting transitions were studied in single-crystal ${\mathrm{YBa}}_{2}$${\mathrm{Cu}}_{3}$${\mathrm{O}}_{7\mathrm{\ensuremath{-}}\mathrm{\ensuremath{\delta}}}$ with magnetic fields in the range 0.02--8 T applied along the c axis and six contacts mounted on two ab-plane surfaces to measure voltages on opposite sides of the crystal. In the beginning of the transition curves, a change was observed at about 0.3 T from a high-field region, where vortices are preserved over the length of the crystal, to a low-field behavior where vortices move independently in different layers. In the high-field region a peak was observed in ${\mathit{V}}_{\mathrm{bot}}$, which can be qualitatively understood in terms of breaking of long vortices by increased thermal fluctuations. At about 0.3 T we also observed a crossover in the magnetic-field dependence of an apparent activation energy from a 1/B behavior at high fields to -lnB at low fields. From the data in the low-field region new quantitative support is obtained for a Kosterlitz-Thouless-type transition where vortex-antivortex pairs dissociate and move independently in different layers of the crystal. The correlation length of these vortices is estimated to be 650\ifmmode\pm\else\textpm\fi{}150 \AA{}.

Magnetocrystalline anisotropy in Y(Co0.85Al0.15)2 with the C15 cubic Laves phase structure

The temperature dependence of the first magnetocrystalline anisotropy constant K1 has been deduced from the analysis of isothermal magnetization curves I(H) for polycrystalline Y(Co0.85Al0.15)2 samples with the C15 cubic Laves phase structure. The magnitude of K1 is equal to 7.6*104 erg cm-3 at T=7 K and falls rapidly with increasing temperature. K1 has a negative sign over the temperature range investigated from 7 to 20 K, i.e. below TC=25 K. The results of the anisotropy study are used to explain the low-field magnetic behaviour of the Y(Co0.85Al0.15)2 alloy.

Low-field behavior of critical current in Sm1?x ?Ba2?Cu3+x ?O7?? ceramic superconductor

The transport critical currentI c carried by ceramic Sm1−x Ba2−Cu3+x −O7−δ (SBCO) superconducting samples (prepared by the coprecipitation technique using organic carbonate in the presence of stable polymer) in low magnetic fields (0–100 Oe) have been investigated at different temperatures close to the critical transition temperatureT c . A significant reduction of the transport current-carrying capacity has been observed for magnetic fields smaller than the lower critical fieldH cl . A semilog plot of the field dependence of the transport critical current densityJ ct reveals a well-defined field valueH′ c characteristic of a crossover between regions dominated by the inter- and intragranular effects. Microwave measurements show evidence of a reduction in the total number of weak links between the network of the superconducting grain structure for Cu-rich SBCO samples. The enhancement ofJ tc for Cu-rich SBCO samples is due to the increase in hole concentration caused by partial replacement of Sm by Cu which caused physical densification between the network of superconducting grain structure in SBCO.

Field-induced superconductor-to-insulator transitions in Josephson-junction arrays.

We have studied phase transitions in fabricated 2D arrays of Josephson junctions where the charging and Josephson coupling energies are about equal. We find a crossover from superconducting behavior in low fields to insulating behavior for fields above a critical value of 0.1--0.2 flux quanta per unit cell. The quantitative aspects agree well with predictions from a recent theory for quantum vortex motion by M. P. A. Fisher and with measurements on films. Similar transitions are found around values f=n/m, where f is the flux per cell and n,m=1,2,3. The field dependence is periodic in f with period 1.

Magnetic properties of chemical remanent magnetization in synthetic and natural goethite: Prospects for a natural remanent magnetization/thermoremenant magnetization ratio paleomagnetic stability test?

Magnetic properties of chemical remanent magnetization in synthetic and natural goethite: Prospects for a natural remanent magnetization/thermoremenant magnetization ratio paleomagnetic stability test?

Low-field scaling behaviors of global flux pinning in Tl-Ba-Ca-Cu-O thin films.

The scaling behaviors of global pinning force density (F p =J c ×B) in dc sputtered Tl-Ba-Ca-Cu-O superconducting thin films as functions of both the reduced magnetic field and temperature were investigated by direct transport measurements for fields up to 4 kOe. It was found that, depending on the film granularities (originating from different deposition and annealing conditions and characterized by the temperature and magnetic-field dependencies of critical currents), vastly different scaling behaviors were manifested

Growth of thin thermal silicon dioxide films with low defect density

Abstract The use of thin gate oxides in scaled MOS-VLSI devices demands good control over oxide thickness, oxide defects, hot carrier instability and breakdown behaviour. In the quest to grow high quality SiO 2 films with low defect density, an investigation on the effect of growth and annealing ambients on oxide properties is reported. Thin SiO 2 films (10–60 nm) grown at 900–1000°C in O 2 /O 2 + N 2 were characterized by C-V and I-V techniques using Al/SiO 2 /p-Si and poly-Si/SiO 2 /p-Si structures. Quality oxides with a very low density of physical defects due to contamination/pinholes (responsible for dielectric breakdown at fields lower than the intrinsic breakdown fields), H 2 O-related and intrinsic defects (acting as electron and hole traps) have been demonstrated. The effect of post-oxidation annealing in N 2 and O 2 on defect-related low-field and intrinsic breakdown behaviour was studied. The occurrence of low-field pre-breakdowns can be reduced by a short O 2 anneal, in agreement with our earlier results. Oxide device worthiness was tested by fabricating N-MOSFETs.

Magnetization process and hysteresis in pure ion

The magnetization properties of polycrystalline pure Fe specimens with different grain sizes are presented. The normal magnetization curve, differential permeability along minor loops and hysteresis loss were measured in the induction range 10 −5−1 T. Nonlinear low-field behavior is found and brieftly discussed in relation to modifications of the standard Rayleigh law framework.

Unified theory of magnetoresistance in Fe and magnetic layer structures

A unified explanation is given of the long-standing question of the origin of the low-field magnetoresistance behavior in pure Fe and the more recently observed “large” magnetoresistance effects seen in magnetic layered structures. It is shown that these effects are due to the scattering of the predominant d-conduction electrons at the magnetic boundary between two antiparallel magnetic regions.

Low-field diamagnetic characteristics of sintered high Tc superconductors

We consider a three-dimensional (3D) equivalent network of Josephson junctions (JJs) and inductances for a sintered high T c cylindrical superconductor in an external magnetic field applied in the axial direction after zero-field cooling (ZFC). Given the particular symmetry of the problem, it is shown that a 2D equivalent network made of concentric superconducting loops containing junctions is sufficient to describe the low-field diamagnetic behavior of this class of superconductors. We thus develop a numerical analysis to study the stationary magnetic states of the system, taking into account the field dependence of the maximum Josephson current I Jthat can flow in each junction. From this analysis a critical-state picture follows. We finally describe flux penetration in the presence of thermally activated processes.

The “hinge” behaviour of the current-voltage characteristics of polycrystalline high temperature superconductors

Polycrystalline high temperature superconductors often exhibit a linear relationship between In ( V) and In ( I). We observe that measurement made at a variety of temperatures and magnetic fields commonly produce a family of linear behaviours which pass very close to a “hinge point” of extrapolated from the linear region to higher currents. We have investigated this behaviour by fitting data taken from two phases of BiSCCO to an empirical formula which contains temperature and magnetic field dependent parameters describing the intercept, slope and curvature of the In( V) versus In( I) behaviour. We show that the concept of the “hinge” is still valid even the curvature is non-zero. Equivalent pairs of magnetic field and temperature give the same slope and intercept while spanning two clearly distinguishable high and low magnetic field regions of behaviour. This evidence indicates the need for a theory in which low field and high field behaviour are due to the same physical phenomena.