Abstract Magnetic Relaxation Research Articles

High-frequency magnetic field on crystal field diluted S = 1 Ising system: magnetic relaxation near continuous phase transition points

Magnetization relaxation and the steady state response of the S = 1 Ising model with random crystal field to a time varying magnetic field with a frequency ω is modelled and studied here by a method that combines the statistical equilibrium theory with the theory of irreversible thermodynamics. The method offers information on the relaxation time (τ) of the system as well as the temperature (θ) and ω dependencies of the complex (AC or dynamical) susceptibility (i.e., χ(ω) = χ′(ω) − iχ″(ω)). The so-called low- and high-frequency regions are separated by τ because τ−1 → 0 as θ approaches the critical temperatures (θc). One can choose to keep the frequency ω fixed and observe the low-frequency behaviors followed by the high-frequency behaviors when θ → θc. It is shown that χ(ω) exhibits different behaviors in low- and high-frequency regimes that are separated by the quantity ωτ: χ′(ω) converges to static susceptibility and χ″(ω) → 0 for ωτ ≪ 1. However, in the high-frequency region where ωτ ≫ 1, χ′(ω) vanishes and χ″(ω) displays a peak at the critical temperature (θc). Besides the above, the logarithm of the susceptibility components versus log(ω) is also plotted. From these plots, one plateau (a step-like) region and a shifted peak with rising temperature is observed for the real and imaginary parts, respectively.

Magnetic Forces Relaxation Step Change Characteristics of YBCO Bulk with Mechanical Impact Load over a NdFeB Guideway

Magnetic force relaxation of YBCO bulk above the NdFeB permanent magnet guideway (PMG) with impact load has been investigated. An experimental setup has been built using a single YBCO bulk and a symmetrical center NdFeB PMG. There are two experimental methods: the case of magnetic levitation force relaxation measuring perturbed with impact load in zero-field-cooling (ZFC); and the case of magnetic levitation and guidance force relaxation measuring synchronously perturbed with impact load in field-cooling (FC). The results show that there is magnetic levitation and guidance forces relaxation step change at the time of the impact load. Two times impact loads are applied for each method. The first step change range is much larger than the second one. The Bean critical model and Anderson–Kim theory are used to analyze it.

Energy equation for the analysis of magnetization relaxation to equilibrium

Magnetization relaxation starting from a generic non-equilibrium state is analytically described. An equation for the energy decay is obtained. On this basis, an approximate expression for the magnetization motion during the ringing process is obtained in terms of Jacobi elliptic functions with time-dependent parameters.

Anomalous magnetic relaxation in rapidly quenched Nd–Fe–B ribbons

Abstract Magnetic relaxation (magnetic viscosity) experiments were conducted in nanocrystalline single and two-phase NdFeB melt spun ribbons. On hysteresis loops, at applied demagnetizing fields at some degree lager than H c , an anomalous non-monotonic variation M ( t ) has been found for all investigated samples either with exchange coupled or decoupled grains. On recoil curves, the same non-monotonic variation of the magnetization has been found in the case of suppressed fields lower than H c .

Magnetic relaxation in partly penetrated critical states of type-II superconductors

Magnetic relaxation in a thin flat type-II superconductor of arbitrary shape placed in an external magnetic field perpendicular to its plane is analyzed in the general case when flux penetration is not necessarily complete. We derive an expression for the magnetic relaxation rate that generalizes the well-known formula of Beasley et al. obtained for superconductors in the fully penetrated critical state. Our result allows one to extract the effective height of the activation barrier from experimental data even when the critical state has a complicated structure, in particular when the applied magnetic field is less than the field of full flux penetration into the sample.

About magnetic relaxation of partially penetrated screening current in superconductors with various models of flux creep

Magnetic relaxation of the partially penetrated screening current during flux creep has been studied. A theoretical analysis was based on the macroscopic description of the flux creep using the power and exponential equations for current-voltage characteristics of superconductors. The analytical flux-creep solutions were written and compared with numerical simulations of corresponding problems. Equations describing the relaxation of the electromagnetic field, magnetic moment, moving penetration boundary are derived. It is shown that peculiarities of relaxation phenomena are determined by dynamics of the electric field on the surface of the superconductor. The performed analysis allows to formulate nontrivial conjugation conditions that take place on moving boundary of screening current. In accordance with these conditions the electromagnetic field induced inside a superconductor by external perturbations smoothly approaches its undisturbed values. The essential role of the low electric field area of current-voltage characteristics in the flux relaxation and primarily in high-temperature superconductors is shown.

Magnetic relaxation in a superconducting plate with rotating flux lines

Magnetic relaxation in a superconducting plate placed in an external magnetic field parallel to its plane is analyzed in the following situation: The external field is switched on and then rotated through some angle in the plane of the plate without changing its magnitude, and then its magnitude and direction are kept fixed. Since the rotation causes flux-line cutting, the so-called general critical state is established in the sample. The relaxation of just this state is investigated. We find expressions for the electric fields and relaxation rates of the magnetic field components and currents in the plate. It is shown that study of the magnetic flux decay allows one to extract the effective height of the activation barrier against flux-line cutting from experimental data.

Investigation of glass behaviour of vortex arrays in superconducting submicron YBCO powders by magnetic relaxation measurements

Magnetic relaxation measurements are made in the remanent state of pure and Zn-doped submicron YBCO powders. A characteristic feature of these samples is dipolar interaction between single vortices trapped in different particles. Similar to other systems with dipolar interactions, such as powders of fine ferromagnetic particles, our samples show glassy behaviour: different metastable states with various pinning properties can be prepared by varying the strength of the magnetizing field, i.e. history of the sample. It is found also that the relation between the pinning energy and the magnetic moment can be described by an expression similar to the vortex glass interpolation formula.

Study of Flux-Creep in Bulk Melt-Textured YBa2Cu3Ox in the Regime of Individual Pinning

Magnetic relaxation, with magnetic fields applied parallel to the c axis, was measured in a high-quality melt-textured YBa2Cu3Ox bar. Zero-field-cooled magnetization-versus-time data were obtained within a regime of field and temperatures dominated by single vortex pinning. The activation pinning energy U = UM was calculated from experimental data and compared to U = U0ln(Jc/J), where U, is a constant, Jc is the critical current density, and J the current density. This expression for U obtains within the individual pinning regime. A logarithmic dependence of UM on J was found, and the data indicated that a scaling function of UM, g(T/Tc) = (1−T/Tc)1/2, was appropriate for describing the single-vortex pinning regime.

Conductivity and NMR Study of LaGa1−xMnxO3 Single Crystals

AbstractElectrical conductivity and magnetic relaxation has been studied in single crystals of Mn doped LaGaO3 with doping concentrations of 0%, 0.5%, 2%, 10% and 50%. The resistivity of the crystals strongly depends on temperature and can be described with the hopping model. Both activation energy and resistivity coefficient grow significantly with decrease in Mn ion concentrations. Dramatic changes are observed in 71Ga NMR spin-lattice relaxation times.

Crystal Structure and Curie Temperature of Sm2Fe17—yGayCx

Magnetic relaxation and flux pinning behaviors have been investigated on the c-axis oriented [YBa2Cu3O7 (24 Angstrom)/PrBa2Cu3O7 (24 Angstrom)](12) multilayers in the perpendicular magnetic fields (H parallel to c-axis) up to 2 T. The critical current density, J(c), value is of the order of 10(6) A/cm(2) in the investigated temperature range and at magnetic field H approximate to 0. The proximate logarithmic dependence of magnetization on time is observed in this multilayers. The magnetic relaxation rate Q = dM/dln t decreases monotonically with increasing temperature. The normalized magnetic relaxation rate S shows a maximum value in S-T curves at low magnetic fields. The behaviors of Q and S are found to be strongly dependent on the magnetic fields. A scaling form of vortex activation energy, U-eff(J, T) = U(i)G(T)F(J(i)/J) was used to analyze the flux pinning behavior at the given magnetic field. Here U-i and J(i) are scaling constants of activation energy and current density, respectively, and G(T) is empirically chosen as G(T) = (1-(T/T-r)(2))(3/2) With T-r being the irreversibility temperature. The current density dependence of U-eff is in agreement with the prediction of collective-pinning theory. The possible pinning mechanisms are proposed. (C) 1997 Elsevier Science B.V.

Quantum creep of Josephson intergrain vortices in Bi2Sr2Ca2Cu3O10

Magnetic relaxation caused by the creep of Josephson intergrain vortices is studied in Bi2Sr2Ca2Cu3O10 high-Tc superconductor in the temperature interval 1.8–30 K for applied magnetic fields between 15 and 25 Oe. It is found that the magnetic moment M logarithmically decays with time t. At high enough temperatures the normalized logarithmic decay rate S=−(1/MJ0)(dMJ/dlnt) linearly increases with temperature, while at T<T0(H), a temperature-independent relaxation rate S≈1.7·10−3 is found, manifesting the occurrence of quantum creep of the Josephson vortices. The “crossover” temperature T0 from the quantum tunneling to the classical thermally activated regime of the vortex motion rapidly decreases from T0=10 K to T0<2 K when the field is increased from H=15 Oe to H=25 Oe. No thermal enhancement of the tunneling rate was found in contrast to the quantum creep behavior of Abrikosov intragrain vortices measured in the same sample (Physica C 222 (1994) 149).

Magnetic relaxation and pinning potential in superconducting Rb3C60 powder

Magnetic relaxation in various magnetic fields and at different temperatures was carried out in Rb3C60 powder withT c=30.5 K, and its pinning potential as functions of temperature and field is determined from these relaxation measurements. ForH=5 kOe, the apparent pinning potential,U o, is 12 meV at 5 K, and increases to about 47 meV with temperature up to 12.5 K. Then it keeps at 45 meV betweenT=12.5 K and 20 K, and at last decreases with temperature. For 3 kOe, a maximumU o of 35 meV was observed atT=20 K. AtT=15 K, theU o rapidly decreases from ≈60meV to 24 meV as field increases from 1 kOe to 2 kOe, then it shows field-independent and always remains at 24 meV in our measuring field range.

Study of the magnetic relaxation on superconducting YbBa2Cu3O7−δ films

Magnetic relaxation was studied on YbBa2Cu3O7−δ epitaxial film for fields up to 5.0 Tesla applied normal to the film plane. The pinning energies were determined in the frame of collective pinning theories. A comparison with YBa2Cu3O7−δ films shows similar weak pinning mechanisms. The results are consistent with a single pinning regime description.

Magnetic relaxation in very thin films of Dy deposited onto crystalline Cu(111)

Magnetic relaxation measurements of very thin Dy films of thickness 20 A and 40 A have been performed in the temperature range 1.8 K–20 K. Below the blocking temperature, the magnetic viscosity decreases when decreasing the temperature and becomes non-thermal below a few Kelvins.

Magnetic relaxation in Pd3Fe following low-temperature electron irradiation

Magnetic relaxation measurements are made in the temperature range 4 to 460 K on both ordered and disordered Pd3Fe following irradiation with 3 MeV electrons. The temperature of irradiation is 40 K and the dose ranges from 1022 to 1023 e− m−2. Well-defined relaxation processes are observed at 34 and 74 K in the ordered samples, which are attributed to the reorientation of intrinsic interstitial-type defects. On anneal to higher temperatures, a relaxation evolves at 230 K, again in the ordered samples; this is interpreted as a divacancy reorientation.

Magnetic Relaxation in YBaCuO Systems Prepared by Modified Melt-Textured Growth Method

Magnetic relaxation in YBa 2 Cu 3 O 7 high-T c superconductor system samples prepared by a modified melt-textured growth method was investigated. The relaxation rate S, given by M?1 dM/d log(t), is independent of the magnetic field and temperature below a given temperature T 0 , which decreases with increasing magnetic field. The pinning potential, evaluated from temperature-independent S values, increases with temperature, but the behavior is not intrinsic. However, at temperatures above T 0 , S increases with temperature, giving a potential dependent on log(J c ).

Magnetic relaxation and intrinsic pinning in a single crystal of Bi2Sr2CaCu2Ox.

Magnetic-relaxation experiments were performed on Bi[sub 2]Sr[sub 2]CaCu[sub 2]O[sub [ital x]] single crystals with the direction of the field parallel to the [ital ab] plane. Based on the relaxation data, we have obtained relationships between the activation energy [ital U] and the current density [ital j] by an approach we developed previously. We found that the activation energy has a logarithmic dependence on [ital j] in a wide regime of driving force. It has been reported that CuO[sub 2] planes in high-[ital T][sub [ital c]] superconductors can act as strong intrinsic pinning centers and that the relation [ital U][similar to][ital U][sub 0]ln([ital j][sub [ital c]]/[ital j]) may describe such a pinning mechanism. Our experimental results have shown good agreement with such a physical model of intrinsic flux pinning.

Effective flux-creep activation energy and anisotropy in a Bi2Sr2CaCu2Ox single crystal

Magnetic relaxation studies have been performed on a Bi2Sr2CaCu2Op single crystal for field orientations parallel and perpendicular to the c-axis. The current density dependence of the effective flux-creep activation energy is derived from the flux-creep data through an expansion of the effective activation energy U about a magnetization current Jo. Although the current densities are much lower in the H∥ab orientation, the overall U(J) behaviors are similarly logarithmic for both field orientations. We discuss the nature of the effective activation energy and the so-called “apparent activation energy”, Uo, contrasting them to the actual activation energy, Up, and our data is used to illustrate problems pith the definition of Uo. A value ≈ 764 is found for the ratio of the critical vortex current densities in the tpo field orientations, {Jc}ab{Jc}c.

Non-logarithmic relaxation in flux-grown YBa2Cu3O7−δ single crystal

Abstract Magnetic relaxations have been measured for up to 20 h for a flux-grown YBa 2 Cu 3 O 7- δ single crystal, with the applied field (up to 7 T) parallel to the c -axis. A ln( t ) relaxation of the magnetisation, found from the Anderson-Kim flux creep model, only describes our data for low temperatures and low fields. However, we have found that a power-law relaxation ( M )( t )∝ t -α ), which points to a logarithmic current-dependence of the effective activation energy [ U ( J )= U 0 ln ( J 0 / J )], fits the data well for a wide range of temperatures and applied fields. Values for U 0 ( B, T ) have been obtained from the power-law relation, and compared with the values derived from the logarithmic relaxation rates. Discrepancies between them are discussed. Possible connections between our experimental data and Feigel'man's collective flux creep theory are also discussed.