高温超导体的磁化与磁滞损耗

利用三种临界态模型(Bean模型、Kim模型和指数模型),采用比较简单的方法,以Bi2223高温超导体为例,给出了平板状超导体的初始磁化曲线和磁滞回线的解析表达式.对不同温度和磁场下的磁化强度进行了编程计算,对计算结果进行讨论.利用推导的公式,讨论了温度和外加磁场对高温超导体的磁滞损耗的影响. 关键词： 磁滞损耗 磁通钉扎 高温超导体 临界态模型     

Magnetoresistance of porous polycrystalline HTSC: Effect of the transport current on magnetic flux compression in intergranular medium

The hysteretic dependences of the magnetoresistance of porous (38% of the theoretical density) granular high-temperature superconductor (HTSC) Bi_1.8Pb_0.3Sr_1.9Ca₂Cu₃O _x have been analyzed in the model of the effective intergranular field. This effective field has been defined by the superposition of the external field and the field induced by magnetic moments of superconducting grains. The magnetic flux compression in an intergranular medium, characterized by the effective field, controls the hysteretic behavior of the magnetoresistance. It has been found that the magnetoresistance hysteresis width for the studied porous HTSC depends on the transport current, in contrast to the superconductor of the same composition with high physical density (more than 90% of the theoretical value). For a porous superconductor, a significant current concentration occurs in the region of the grain boundaries, which is caused by features of its microstructure. A current-induced increase in the effective boundary length results in a decrease in the flux compression, a decrease in the effective field in the intergranular medium, and a magnetoresistance hysteresis narrowing with increasing current.     

Application of magnetic Compton scattering for spin‐specific magnetic hysteresis measurement

An application of magnetic Compton scattering as a new tool to measure a spin‐specific magnetic hysteresis (SSMH) loop is introduced and its validity demonstrated. The applied magnetic field dependence of the integrated intensity of magnetic Compton scattering spectra, which reflect only the spin‐dependent magnetic properties of magnetically active electrons, was interpreted as the spin‐specific hysteresis. The spin magnetization of amorphous Tb₃₃Co₆₇ film was observed and its SSMH loop exhibited qualitative agreement with the ordinal magnetic hysteresis loop measured using a conventional vibrating sample magnetometer.     

Influence of eddy currents on magnetic hysteresis loops in soft magnetic materials

In this paper an attempt has been made to extend the Jiles and Atherton (J–A) quasi-static hysteresis model to describe magnetisation of a material with an alternating magnetic field. In low – industrial – and medium frequency of magnetic field it is possible to ignore the magnetic relaxation and resonance. The field penetration is assumed to be uniform through the material. The influence of eddy currents on the hysteresis loop could be considered and calculated using the method of successive reactions of eddy currents, where a reaction is an additional magnetic field, called reaction H_d, induced in the material by the eddy currents according to rot J=γ∂B/∂t where γ is the electrical conductivity. The reaction field H_d was added to the basic field H₀∝Iz_1, where I is the current intensity in the magnetising coil of z₁ number of windings. By solving the J–A equation for the magnetic field H_w=H₀+H_d it has achieved an extension of the hysteresis loop at an increased frequency of the current, caused by increased losses of the eddy currents. At the frequency f→0 Hz , the hysteresis loop approaches the shape of the quasi-static one.     

Flux penetration in high-Tc superconductors: Implications for magnetic suspension and shielding

Two new effects associated with flux penetration and pinning in superconducting YBa₂Cu₃O_x are demonstrated. One of these is focussing of magnetic field lines by the superconductor and the other is magnetic suspension. The existence of stable equilibrium in suspension is shown to be related directly to hysteresis observed in the force-separation relation for a magnet and the superconductor. We also observe that the levitation height of a magnet increases with the magnet volume, contrary to results in the literature.     

Influence of experimental methods on crossing in magnetic force–gap hysteresis curve of HTS maglev system

The crossing in magnetic levitation force–gap hysteresis curve of melt high-temperature superconductor (HTS) vs. NdFeB permanent magnet (PM) was experimentally studied. One HTS bulk and PM was used in the experiments. Four experimental methods were employed combining of high/low speed of movement of PM with/without heat insulation materials (HIM) enclosed respectively. Experimental results show that crossing of the levitation force–gap curve is related to experimental methods. A crossing occurs in the magnetic force–gap curve while the PM moves approaching to and departing from the sample with high or low speed of movement without HIM enclosed. When the PM is enclosed with HIM during the measurement procedures, there is no crossing in the force–gap curve no matter high speed or low speed of movement of the PM. It was found experimentally that, with the increase of the moving speed of the PM, the maximum magnitude of levitation force of the HTS increases also. The results are interpreted based on Maxwell theories and flux flow-creep models of HTS.     

Modeling of two-phase magnetic materials based on Jiles-Atherton theory of hysteresis

The Jiles-Atherton (JA) theory of hysteresis has been extended in the present paper to model hysteresis in two-phase magnetic materials. Two-phase materials are those that exhibit two magnetic phases in one hysteresis cycle: one at lower fields and the other at higher fields. In magnetic hysteresis, the transition from one phase to the other i.e. low field phase to high field phase depends mainly on the exchange field. Hence, the material-dependent microstructural parameters of JA theory: spontaneous magnetization, M_S, pinning factor, k, domain density, a, domain coupling, α, and reversibility factor, c, are represented as functions of the exchange field. Several cases based on this model have been discussed and compared with the measured data from existing literature. The shapes of the calculated and measured hysteresis loops are in excellent agreement.     

Magnetoelectric coupling in antiferroelectric and magnetic laminate composites

We researched the properties of magnetoelectric composites between antiferroelectric Pb_0.94La_0.04(Zr_0.55Sn_0.3Ti_0.15)O₃ and magnetic Terfenol-D. The magnetic field enhances the electric field-induced strain and polarization of the composite. The magnetic moment induced by the electric field increases at the electromechanical resonance frequency because the antiferroelectric ceramics exhibit ferroelectric behaviors under a high electric field. The induced magnetic moment increases with decreasing thickness ratio and shows a hysteresis loop with the bias magnetic field. Due to the antiferroelectric characteristics, it also shows a hysteresis loop with the bias electric field, which could be used in the magnetic switch controlled by electricity.     

Drift of domain walls in a harmonic magnetic field

It is shown that a two-step form of the dynamic magnetization curve (and the hysteresis loop) established for a multiaxial ferrite-garnet wafer with a low quality factor (Q < 1) and considerable anisotropy in the plane (K _p /K _u = 14) in the frequency range of 25–1000 Hz is explained by the reconstruction of the dynamic domain structure, particularly by the established features of the drift of domain boundaries in the harmonic magnetic field.     

Magnetization reversal in granular thin films

The paper discusses the physics of magnetization reversal in granular magnetic films. It gives an overview of the key physical properties that determine the collective and macroscopically observable magnetization reversal behavior. In particular, the multitude of observable hysteresis loops is reduced to three key physical quantities, namely the single grain switching field distribution D(h_s), the inter-granular exchange coupling constant J_ex, and the magnetostatic interaction constant J_ms. By varying the relative influence of these quantities, many different shapes of hysteresis loops can occur, which is documented by experimental examples. The regime of partially and strongly correlated reversal is discussed in detail, and minor loop measurements are presented that show scaling behavior for strongly correlated magnetization reversal in the vicinity of hysteresis loop criticality.     

Unusual magnetic hysteresis behavior of oxide spinel MnCo2O4

Magnetic hysteresis behavior of the oxide spinel MnCo₂O₄ has been studied at different temperatures below its T_c≈184 K. Normal hysteresis behavior is observed down to 130 K whereas below this temperature the initial magnetization curve, at higher magnetic fields, lies outside the main loop. No related anomaly is observed in the temperature variation of magnetization or coercivity. However, the anisotropy field overcomes the coercivity below 130 K. The unusual magnetic hysteresis behavior of MnCo₂O₄, at low temperatures, may be associated with irreversible domain wall movements due to the rearrangement of the valence electrons.     

The use of Lorentz microscopy for the determination of magnetic reversal mechanism of exchange-biased Co30Fe70/NiMn bilayer

Lorentz transmission electron microscopy (LTEM) combined with in-situ magnetizing experiments is a powerful tool for the investigation of the magnetization of the reversal process at the micron scale. We have implemented this tool on a conventional transmission electron microscope (TEM) to study the exchange anisotropy of a polycrystalline Co₃₅Fe₆₅/NiMn bilayer. Semi-quantitative maps of the magnetic induction were obtained at different field values by the differential phase contrast (DPC) technique adapted for a TEM (SIDPC). The hysteresis loop of the bilayer has been calculated from the relative intensity of magnetic maps. The curve shows the appearance of an exchange-bias field reveals with two distinct reversal modes of the magnetization: the first path corresponds to a reversal by wall propagation when the applied field is parallel to the anisotropy direction whereas the second is a reversal by coherent rotation of magnetic moments when the field is applied antiparallel to unidirectional anisotropy direction.     

Nonmonotonic behavior of magnetoresistance, R(H) hysteresis, and low-temperature heat capacity of the BaPb0.75Bi0.25O3 superconductor in a magnetic field: Possible manifestations of phase separation

The transport properties (R(T) and R(H) dependences at various values of the transport current in magnetic fields up to 65 kOe) and low-temperature heat capacity in magnetic fields up to 90 kOe of the BaPb_0.75Bi_0.25O₃ superconductor (T _C ≈ 11.3 K) are investigated with the goal of clarifying the mechanisms determining the nonmonotonic behavior and hysteresis of its magnetoresistance R(H). The type of R(H) hysteretic dependences for BaPb_0.75Bi_0.25O₃ is analogous to that observed in granular high-T _c superconductors (HTSCs); however, unlike classical HTSC systems, the field width of the magnetoresistance hysteresis loop for polycrystalline BaPb_0.75Bi_0.25O₃ depends on the transport current. This means that although the mechanisms responsible for the magnetoresistance hysteresis (the influence of the magnetic flux trapped in superconducting regions on the effective field in Josephson interlayers) are identical in these objects, the transport current in BaPb_0.75Bi_0.25O₃ may considerably affect the diamagnetic response of the superconductor. A considerable effect of transport current on the field in which the R(H) dependences have a peak and exhibit hysterestic properties is observed. Such a behavior can be adequately interpreted using the model of the spatially inhomogeneous superconductor-insulator state proposed by Gorbatsevich et al. [JETP Lett. 52, 95 (1990)]. The nonmonotonic dependence of quantity C/T (C is the heat capacity) on the magnetic field discovered in the present study also agrees with the conclusions based on this model.     

Tb0.3Dy0.7Fe2合金磁畴偏转的滞回特性研究

研究了不同载荷作用下Tb_0.3Dy_0.7Fe₂合金在压磁和磁弹性效应中磁畴偏转的滞回特性. 基于Stoner-Wolhfarth模型的能量极小原理, 采用绘制自由能-磁畴偏转角度关系曲线的求解方法, 研究了压磁和磁弹性效应中载荷作用下的磁畴角度偏转和磁化过程, 计算分析了不同载荷作用下磁畴偏转的滞回特性. 研究表明, 压磁和磁弹性效应中磁畴偏转均存在明显的滞回、跃迁效应, 其中磁化强度的滞回效应来源于磁畴偏转的角度跃迁; 压磁效应中预加磁场的施加将增大磁化强度的滞回, 同时使滞回曲线向大压应力方向偏移; 磁弹性效应中磁畴偏转的滞回存在两个临界磁场强度, 不同磁场强度下合金具有不同的磁畴偏转路径和磁化滞回曲线, 临界磁场强度的大小取决于预压应力的施加. 理论分析对类磁致伸缩材料磁畴偏转模型的完善和材料器件的设计应用非常有意义.     

Influence of Al and Ni on the magnetic properties of Finemet alloys

Crystallization behavior and soft magnetic properties of the FeSiBCuNbM (M=Al or Ni) Finemet alloys are investigated by X-ray diffraction, differential scanning calorimetry, hysteresis loop tracer, and vibrating sample magnetometry. The nanocrystalline alloys are prepared by annealing melt-spun amorphous ribbons at different temperatures. Results indicate that the partial substitution of Ni or Al for Nb results in the increase of saturation magnetic induction density (B_s) of the alloys. The alloys with Al or Ni show favorable combination of soft magnetic properties. The partial substitution of Ni for Nb enhances the B_s value, while Al decreases coercivity. The mechanism underlining the magnetic behavior is discussed.     

Study of the magnetic interaction in nanocrystalline Pr-Fe-Co-Nb-B permanent magnets

The magnetic properties of an isotropic, epoxy resin bonded magnets made from Pr-Fe-Co-Nb-B powder were investigated. The magnetization reversal process and magnetic parameters were examined by measurements of the initial magnetization curve, major and minor hysteresis loops and sets of recoil curves. From the initial magnetization curve and the field dependencies of the reversible and irreversible magnetization components derived from the recoil loops it was found that the magnetization reversal process is the combination of the nucleation of reversed domains and pinning of domain walls at the grain boundaries and the reversible rotation of magnetization vector in single domain grains. The interactions between grains were studied by means of δM plots. The nonlinear behavior of δM curve approve that the short range intergrain exchange coupling interactions are dominant in a field up to the sample coercivity.The interaction domains and fine magnetic structure were revealed as the evidence of exchange coupling between soft α-Fe and hard magnetic Nd₂Fe₁₄B grains.     

Magnetoresistance hysteresis in granular HTSCs as a manifestation of the magnetic flux trapped by superconducting grains in YBCO + CuO composites

Hysterestic behavior of the magnetoresistance of granular HTSCs and its interaction with the magnetic hysteresis are studied by measuring magnetoresistance R(H) and critical current I _c(H) of composites formed by HTSC Y_0.75Lu_0.25Ba₂Cu₃O₇ and CuO. A network of Josephson junctions is formed in such composites, in which the nonsuperconducting component plays the role of barriers between HTSC grains. Hysteretic dependences R(H) of magnetoresistance are studied in a wide range of transport current density j and are analyzed in the framework of the two-level model of a granular superconductor, in which dissipation takes place in the Josephson medium and the magnetic flux can be pinned both in grains and in the Josephson medium. The interrelation between the hysteresis of critical current I _c(H) and the evolution of the hysterestic dependence R(H) of the magnetoresistance upon transport current variation is demonstrated experimentally. The effect of the magnetic past history on the hysteretic behavior of R(H) and the emergence of a segment with a negative magnetoresistance are analyzed. It is shown for the first time that the R(H) dependences are characterized by a parameter that is independent of the transport current, viz., the width of the R(H) hysteresis loop.     

A study on the magnetic properties of two-phase particulate magnetic composites

The magnetic properties of two-phase particulate magnetic composites with a hard ferromagnetic component are studied theoretically and experimentally. The magnetic properties considered here are phase-distribution sensitive properties, including remanence, coercivity and the shape of hysteresis loop. These properties depend mainly on the properties of its constituents, volume fractions, phase distribution, packing fraction and orientation distribution for anisotropic particles. With fixed packing fraction and orientation distribution, the magnetic properties of the two phase mixture can be calculated in terms of its component properties, volume fractions and phase distribution. Here, the component properties include not only remanence B_r and coercivity H_c but also a variable m which is the rate of change of magnetic induction B with respect to field intensity H. For two-phase systems satisfying B - H relation of the type B = B_r + mH where m is a constant, the equations for calculating the magnetic properties B_r, H_c, etc., in terms of m are derived. The method for calculating m is also developed for the cases of parallel and series distributions. Bounds for m-values were also established. A modified Landauer's type equation is developed to calculate m-values in terms of the component properties of the mixture. Experiments were conducted to verify the theoretical calculations. Good agreements between the theoretical calculations and experimental results were obtained.     

Structure and magnetic properties of cobalt-intercalated layered InSe crystals

We study the structure and magnetic properties of Co _x InSe layered crystals electrochemically intercalated by cobalt in a constant magnetic field. It is found that impurity clusters consisting of cobalt nano-particles with the fcc structure are formed in the intercalates under investigation on the Van der Waals planes in the space between the layers. Intercalates Co_0.1InSe obtained by implantation in a magnetic field exhibit a change in their magnetic properties (dependence of the magnetic moment in the magnetic field strength has the form of a hysteresis loop, which is typical of ferromagnetic materials).     

Dependence of the coercive force on the magnetic field sweep rate in (PrDy)(FeCo)B alloys

The dependence between the coercive force and the average hysteresis loop record time was revealed in sintered (Pr_{1 – x} Dy _x )₁₃(Fe_{1 – y} Co _y )₇₉B₈ magnets. The coercive force was established to grow by 22% with an increase in the average hysteresis loop sweep rate within a range of 1.1 × 10²–3 × 10⁵ Oe/min, obeying a logarithmic dependence on the loop passage velocity with saturation at low rates. Some domain structure transformations produced by a magnetic field in the process of magnetization were established by magnetic force microscopy.