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An extended critical state model fits data obtained from saturation magnetization curves of a sintered Bi2Sr2CaCu2O superconductor yielding the lower critical and first vortex entry fields, Hc1 and H1, respectively. Hc1 varies linearly and inversely with temperature at 50 Kelvin, and is less than H1. The surface barriers disappear above 50 Kelvin, and irregularities in the magnetization curves render the critical state model invalid.

A study was conducted to analyze the magnetoresistance in glass-coated soft magnetic Fe-Ni-Cu microwires. The microwires were prepared using Taylor's technique. Experimental results indicated that negative magnetoresistance reached 8.15% at 77 K and 6.35% at 300 K in a magnetic field of 9.0 T. Findings also showed that magnetoresistance decreases upon annealing the sample at 500 degrees C.

The transverse relaxation time T2 of protons in water suspensions of iron-oxide particles increases with the waiting time tw after the sample is inserted in the gap of the spectrometer magnet. Such a T2 increase becomes significant if the particles are aggregated into large clusters, for which field-induced formation of cluster-chains will occur and T2 should increase with increasing the length of chains. T2 increases with tw even for small particles, for which no chain formation may be induced, and for large clusters when tw is too small to form long enough chains. The T2 increase is accompanied by a significant echo-time dependence. All this is experimentally and theoretically studied. [ABSTRACT FROM AUTHOR]

Hard magnetic materials are usually measured and used in an open magnetic circuit, for which demagnetizing effects are present. The applied field Ha has been conventionally corrected to the volume averaged internal field H by a formula H=Ha-NmM, where M is volume averaged magnetization and Nm is magnetometric demagnetizing factor calculated for the case of uniform magnetization as a function of body shape. Since Nm>0, such a correction always increases the slope of the descending M(Ha) curve without influencing the value of the coercive field with respect to M. However, we show by numerical calculations that if the hard magnetic material is granular and modeled as a rectangular prism, the magnetostatic interactions among the grains will result in a formula H=Ha-NghmM+ΔHc, where the demagnetizing factor for granular hard magnets Nghm can be either positive or negative and the usually positive coercivity increment ΔHc is correlated to collective magnetic reversal of longitudinal grain chains. [ABSTRACT FROM AUTHOR]

We show both experimental evidences and Monte Carlo modeling of the effects of interparticle dipolar interactions on the hysteresis losses. Results indicate that an increase in the intensity of dipolar interactions produce a decrease in the magnetic susceptibility and hysteresis losses, thus diminishing the hyperthermia output. These findings may have important clinical implications for cancer treatment. [ABSTRACT FROM AUTHOR]

Superparamagnetic iron oxide particles in commercial products Resovist and SH U555C, used as magnetic resonance imaging contrast agents, are polydispersed and covered with a thick organic coating so that the average particle sizes were reported between 3 and 62 nm with different definitions. It is unclear which size determines the transverse relaxation rate 1/T2 of water suspensions of such particles. Comparing the measured 1/T2 and average sizes of different definitions determined from magnetization curve, transmission electron microscopy image, x-ray diffraction, and dynamic light scattering, it is found that the 1/T2 behavior is basically determined by the diameter of bare single-crystal magnetic particles having the particle-volume-weighted average volume (about 14 or 9 nm for Resovist or SH U555C) and is slightly influenced by their thick organic coating. This is explained by the low partial density of the coating substance and the overwhelming water occupation adjacent to magnetic particles. [ABSTRACT FROM AUTHOR]

The size distribution of superparamagnetic iron oxide nanoparticles in two powder samples is determined from the measured magnetization curve fitted by a core-shell model and a uniform model, both based on the Langevin function with a log-normal particle volume distribution. Different average sizes are evaluated from the fitting parameters and compared to those determined by other techniques. Conceptual discussions are presented on different models and approaches, from which the core-shell model fitting is recommended for the magnetic size determination of superparamagnetic nanoparticles. [ABSTRACT FROM AUTHOR]

The frequency dependent low-field circular ac susceptibility χ=χ′-jχ″ of a nearly nonmagnetostrictive Co68.2Fe4.3Si12.5B15 amorphous wire annealed at 470 °C is determined by impedance Z measurements under stepwise decreased longitudinal dc field Hdc from 216 to -216 A/m after premagnetization at 16 kA/m. It is found that compared with the results of the classical eddy-current model, the measured low-frequency loss factor is larger by an anomaly factor η between 1.5 and 3, depending on the value of Hdc. However, the χ(f) curve at each value of Hdc may be well fitted by an eddy-current model of bamboo-domain wall displacements if the material conductivity is replaced by a greater effective one and a proper value of effective domain size is chosen. The physical mechanism of such a fitting is discussed based on previous studies on anomalous dc circular susceptibility. This anomalous χ(f,Hdc) leads to an anomalous magnetoimpedance Z(Hdc) with a two-peak feature when frequency is high. [ABSTRACT FROM AUTHOR]

We present calculations of the alternating current (ac) susceptibility in rectangular thin-film type-II superconductors, based on the critical-state model and on the minimization of magnetic energy. First, we simulate a homogeneous superconductor and give an analytical approximate expression for both real and imaginary parts of the ac susceptibility as a function of the amplitude of the ac applied field. Second, ac susceptibility is calculated in a superconductor composed by two parts connected by a region of lower critical-current density. We find two peaks can appear in the imaginary ac susceptibility although, because of the strong demagnetizing fields, these peaks overlap in some cases. Detailed explanations of the imaginary part of the ac susceptibility are given. [ABSTRACT FROM AUTHOR]

A simple and universal formulation for the transport V(I) curve is given for a superconducting cylinder of radius a with a power law E(J), E=(Ec/Jc)|J/Jc|n-1J. This formulation is a function of Ec, Jc, n, a, and the current sweeping rate R. Concepts of the full penetration and the relevant voltage as well as several influencing factors in using this formulation are discussed. [ABSTRACT FROM AUTHOR]

The field amplitude and frequency dependent complex alternating current susceptibility χ(Hm, f) of YBa2Cu3O7-δ films metalorganically deposited on different substrates with different processes has been measured at 77 K and studied in relation with their microstructures and some modeling results. It is shown that χ(Hm) for films with well aligned grains and a high Jc is of Bean type with a characteristic f dependence for a power-law E(J), so that thermally activated collective flux creep is the dominant dissipation mechanism. The Jc of these films may be well determined by χ(Hm, f) measurements. For films with misaligned grains and intermediate values of Jc, χ(Hm, f) is anomalous and able to be roughly simulated by a linear-exponential E(J). This phenomenon should be related to the presence of weak links and Josephson vortices, but to look for its physical mechanism is still challenging. [ABSTRACT FROM AUTHOR]

The circular magnetization process of FeSiBCuNb amorphous and nanocrystalline wires has been followed by the impedance measurements as a function of the amplitude and the frequency of driving current. The domain structure and consequently, the magnetization process have been tailored by suitable thermal processing. The different circular coercivity mechanisms are deduced by analyzing the magnetic losses. We find that the coercivity is due to domain-wall pinning for wire with axial domain, while the coercivity is from domain nucleation for one with transverse domain structure. In addition, the influence of axially applied field H is different for wires with different domain structure. The application of tensile stress to a wire with positive magnetostriction can be taken as an axial induced anisotropy, which makes the circular permeability decrease and the coercivity increase. Finally, the influence of driving current frequency on the circular magnetization process has been also studied for the wire with transverse domain structure. The results indicate that the frequency effect is from the eddy-current damping. [ABSTRACT FROM AUTHOR]

Two pieces of Fe[SUB73.5] Cu[SUB1] Nb[SUB3] Si[SUB13.5] B[SUB9] amorphous wires were submitted to thermal treatment without (S1) or under (S2) applied tensile stress, so inducing nanocrystalline structure characterized by two different types of magnetic anisotropy. Sample S1 exhibits longitudinal domain structure with small anisotropy constant, while a transverse domain structure with large transverse anisotropy is induced by stress annealing for sample S2. The specific magnetoimpedance response of each nanocrystalline wire has been studied as a function of the applied torsion, &Xgr;. Both samples show the giant magnetoimpedance (GMI) effect, with GMI ratios of 190% and 145% at &Xgr; = 0 for wires S1 and S2, respectively. Applied torsion modifies the form as well as the amplitude of magnetoimpedance, which is discussed in terms of the competition between the magnetoelastic anisotropy induced by the processing and the helical anisotropy induced by the torsion. The torsion impedance has been modeled for the case of sample S1 under large enough helical anisotropy. The observed giant torsion-impedance effect of 320% and 235% for samples S1 and S2, respectively, is proposed to be employed in magnetoelastic torsion sensors. The use of the GMI effect is further emphasized to be a powerful tool to explore the magnetic structure and the magnetic anisotropy. [ABSTRACT FROM AUTHOR]

After a positive high field premagnetizing, the circular susceptibility xφ of annealed nearly nonmagnetostrictive amorphous wires determined by measuring ac inductance is anomalously large and asymmetric with respect to the sign of the dc bias field H[SUBz]. This phenomenon is explained by comparing with another circular susceptibility χ[SUBφ,v[SUB2]] obtained by measuring seond harmonic longitudinal magnetization in terms of a model of connected (for low H[SUBz]) or coupled (for elevated H[SUBz]) donut structures induced by alinged magnetic inclusions. [ABSTRACT FROM AUTHOR]

The magnetoimpedance (MI) effect has been investigateded in Fe[sub 63.5]Cr[sub 10]Si[sub 13.5]B[sub 9]Cu[sub 1]Nb[sub 3] alloy wire exhibiting an ultrasoft magnetic character after suitable thermal treatment at 525 °C. It has been measured as a function of the sample length and of the distance to the ends of the wire (MI profile). The observed dependence on both parameters is correlated with the local particularities of the magnetization process: complex domain structures emerging near the ends to decrease the magnetostatic energy modify the local magnetic susceptibility and consequently the MI response. The present study points out the importance of geometrical dimensions of samples and measuring conditions when reporting numerical values in MI studies. This wire, showing large magnetoimpedance together with optimized soft magnetic behavior and improved corrosion properties seems to be indicated as a sensing element in GMI based sensors. [ABSTRACT FROM AUTHOR]

The demagnetizing problem should be studied numerically in nonellipsoids with nonzero susceptibility χ, except for a few limiting cases where analytical treatment turns out to be practical. In this work, for an infinitely long bar with rectangular cross section 2a×2b and χ=∞ and -1 in a uniform transverse applied field H[sub a] along dimension a or b, analytical expressions for the surface pole or current distributions and the fluxmetric and magnetometric demagnetizing factors N[sub f,m] are derived using a technique of conformal transformation. From the new as well as the existing formulas, N[sub f,m](χ=∞,0,-1) are evaluated, plotted, and tabulated as functions of a/b. © 2002 American Institute of Physics. [ABSTRACT FROM AUTHOR]

The effectiveness of ac susceptibility measurements in determining the conductivity of conducting disks was evaluated. It was shown that ac susceptibility measuerements were very effective in the case of low frequency nonmagnetic materials. In the case of weakly magnetic materials, the correlation of demagnetizing factors with ac susceptibility measurements allowed a reasonable degree of accuracy to be achieved.

Presents a study that measured the real and imaginary parts of alternating current susceptibility of a sintered superconductor before and after powdering. Implication of the discovery of high-critical-temperature superconductors; Details of the study; Discussion of the models used in the study.

Presents a study that developed formulas for the initial magnetization curves and hysteresis loops based on the exponential-law model and valid for a general specimen shape which includes the slab and cylinder. General space-dependent supercurrent densities for different cases and magnetization stages; Expressions derived from the exponential-law model for different parameters; Discussion on the shielding effect of supercurrent.

Deals with a study which calculated the initial magnetization curves and complete hysteresis loops for hard type-II superconductors. General expressions for magnetization; Derivation of M(H) from J(x); Computed M(H) curves; Determination of J[subc](H) from the width of the hysteresis loop.

Presents information on a study which analyzed the effects of sequential pressure cycling on the critical currents in a YBa[sub2]Cu[sub3]O[sub7-8] superconductor. Experimental procedure; Direct current measurements carried out using a vibrating sample magnetometer; Application of the magnetic field along the longest axis of the sample, to minimize the demagnetization effects.

Presents a study which investigated the alternating current and direct current magnetic susceptibilities of YBa[sub2]Cu[sub3]O[sub7-x] (YBCO). Details of the experimental techniques used; Information on the normal state resistivity of YBCO.

The transport eddy-current loss of a nearly nonmagnetostrictive CoFeSiB amorphous wire under a large longitudinal bias field can be several times greater than the value commonly expected by the classical model. Related to the overlarge circular susceptibility under the same condition, this is attributed to the formation of disklike soft-magnetic microstructures around hard magnetic inclusions randomly embedded in a soft magnetic matrix. [ABSTRACT FROM AUTHOR]

Simple analytic formulas are proposed, that provide a close approximation to the fluxmetric and magnetometric demagnetizing factors of long cylinders with infinite susceptibility as functions of aspect ratio. © 2001 American Institute of Physics. [ABSTRACT FROM AUTHOR]

Deals with the origin of induced anisotropy and magnetostriction in metallic glasses. Temperature dependence of induced anisotropy for cobalt-rich alloys; Predictions of single- and two-ion models; Analysis of annealing effect and anomalous temperature dependence.

Presents the abstract of the study 'Are the High T[subc] Superconducting Materials or Grain Boundary Percolating Network Superconductors?'

Presents the abstract of the study 'Some Novel Melt-Spun Superconducting Materials.'

Presents an abstract of the article 'Magnetic and transport properties of ion-beam-mixed amorphous Fe-Zr alloys,' by K.V. Rao, N. Karpe, D.-X Chen, R. Goldfarb, J. Bøttiger, K. Pampus and B. Torp.