Permeability Relaxation Research Articles

Jeans Instability of the Self-Gravitating Viscoelastic Ferromagnetic Cylinder with Axial Nonuniform Rotation and Magnetic Field

The effects of nonuniform rotation and magnetic field on the instability of a self gravitating infinitely extending axisymmetric cylinder of viscoelastic ferromagnetic medium have been studied using the Generalised Hydrodynamic (GH) model. The non-uniform magnetic field and rotation are acting along the axial direction of the cylinder and the propagation of the wave is considered along the radial direction, while the ferrofluid magnetization is taken collinear with the magnetic field. A general dispersion relation representing magnetization, magnetic permeability and viscoelastic relaxation time parameters is obtained using the normal mode analysis method in the linearized perturbation equation system. Jeans criteria which represent the onset of instability of self gravitating medium are obtained under the limits; when the medium behaves like a viscous liquid (strongly coupled limit) and a Newtonian liquid (weakly coupled limit). The effects of various parameters on the Jeans instability criteria and on the growth rate of self gravitating viscoelastic ferromagnetic medium have been discussed. It is found that the magnetic polarizability due to ferromagnetization of medium marginalizes the effect of non-uniform magnetic field on the Jeans instability, whereas the viscoelasticity of the medium has the usual stabilizing effect on the instability of the system. Further, it is found that the cylindrical geometry is more stable than the Cartesian one. The variation of growth rate against the wave number and radial distance has been depicted graphically.

A New Method to Identify Reservoirs in Tight Sandstones Based on the New Model of Transverse Relaxation Time and Relative Permeability

Relative permeability and transverse relaxation time are both important physical parameters of rock physics. In this paper, a new transformation model between the transverse relaxation time and the wetting phase’s relative permeability is established. The data shows that the cores in the northwest of China have continuous fractal dimension characteristics, and great differences existed in the different pore size scales. Therefore, a piece-wise method is used to calculate the fractal dimension in our transformation model. The transformation results are found to be quite consistent with the relative permeability curve of the laboratory measurements. Based on this new model, we put forward a new method to identify reservoir in tight sandstone reservoir. We focus on the Well M in the northwestern China. Nuclear magnetic resonance (NMR) logging is used to obtain the point-by-point relative permeability curve. In addition, we identify the gas and water layers based on new T2-Kr model and the results showed our new method is feasible. In the case of the price of crude oil being low, this method can save time and reduce the cost.

Broadband magnetic losses of nanocrystalline ribbons and powder cores

Finemet type alloys have been investigated from DC to 1GHz at different induction levels upon different treatments: as amorphous precursors, as ribbons nanocrystallized with and without an applied saturating field, as consolidated powders. The lowest energy losses at all frequencies and maximum Snoek's product are exhibited by the transversally field-annealed ribbons. This is understood in terms of rotation-dominated magnetization process in the low-anisotropy material. Intergrain eddy currents are responsible for the fast increase of the losses with frequency and for early permeability relaxation of the powder cores. Evidence for resonant phenomena at high frequencies and for the ensuing inadequate role of the static magnetic constitutive equation of the material in solving the magnetization dynamics via the Maxwell's diffusion equation of the electromagnetic field is provided. It is demonstrated that, by taking the Landau–Lifshitz–Gilbert equation as a constitutive relation, the excellent frequency response of the transverse anisotropy ribbons can be described by analytical method.

Study of the Internal Morphology of Cation-Exchange Membranes by Means of Electroosmotic Permeability Relaxations

The effect of an ac sinusoidal perturbation of known amplitude and frequency superimposed to the usual dc applied electric voltage difference on the electroosmotic flow through three cation-exchange membranes with different morphology has been studied. A dispersion of the electroosmotic permeability on the frequency of the applied ac signal has been found for the three membranes investigated, observing that the electroosmotic permeability reaches maximum values for some characteristic values of the frequency. These characteristic frequency values, which are related to relaxation processes in heterogeneous media, depend on the membrane system and permit to obtain information about the different structures of the membrane system. Thus, the study of the electroosmotic permeability relaxation can be used as a method to study the internal morphology of a cation-exchange membrane in a given electrolyte medium.

An approach to the magnetic relaxation processes in lithium ferrites

The relaxation of the initial magnetic permeability has been measured in polycrystalline Li x Fe 3− x O 4 samples, with x ranging from 0 to 0.5, by means of the magnetic disaccommodation (DA) technique. We have found that there is no abrupt transition for a given composition, but there is a progressive modification of the characteristic relaxation processes of magnetite. These results have been interpreted on the basis of the increasing amount of Li ions in the spinel lattice and hence, the resulting modifications on their proximities.

Influence of Na doping on the magnetic relaxation processes of magnetite

The relaxation of the initial magnetic permeability was measured in polycrystalline Na-doped magnetite samples, with nominal composition Na x Fe 3− x O 4 ( x ranging from 0 to 0.05), by means of the magnetic disaccommodation (DA) technique. We found that the increasing amount of Na ions modifies the DA spectra and a very different behaviour depending on the sintering atmosphere. These results were discussed in terms of the presence of Na ions in the magnetite lattice, giving rise to certain modifications in their neighbourhood.

Effect of tetravalent substitutions on the magnetic disaccommodation in magnetite

AbstractThe relaxation of the initial magnetic permeability has been measured in polycrystalline tetravalent‐doped magnetite with nominal composition MexFe3‐xO4 (Me: Ti, Sn, Si) with 0 < x < 0.4. The samples have been sintered at 1400 ºC in a reducing CO2 atmosphere. The time decay of the initial permeability after sample demagnetization has been represented by means of isochronal curves in the temperature range between 80 K and 500 K.The isochronal disaccommodation spectra for Ti and Sn‐doped magnetite show the splitting of the well known process III of vacancy‐doped magnetite (at 300 K) into three peaks at 240 K (IV peak), 270 K (IV') and 300 K (III). This behaviour is ascribed to the presence of tetravalent cations in octahedral sites within the spinel lattice. Two peaks at 400 K (II) and 440 K (I) also emerge, owing to the existence of pairs Me4+‐Fe2+ cations as well as long range diffusion of ferrous cations. The differences found among these substitutions are explained in terms of the different cationic radii of tin and titanium. On the other hand only the III and IV processes are detected in Si‐doped magnetite samples, due to the preferential occupation by Si cations of tetrahedral sites according to their smaller size. (© 2006 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Effect of sintering conditions on the magnetic disaccommodation in barium M-type hexaferrites

The relaxation of the initial magnetic permeability has been measured in polycrystalline hexaferrites with nominal composition BaO·6Fe 2O 3 (i.e. M-type). The samples have been sintered at different temperatures in CO 2 atmosphere and with different manufacturing conditions. In temperature range between 80 and 500 K, the magnetic disaccommodation shows presence of different relaxation processes, depending on both the sintering temperature and sintering time. The analogies and differences between the results obtained are discussed in terms of similar phase formation and different crystallite size.

Study of the magnetic disaccommodation in La doped YIG

The relaxation of the initial magnetic permeability of La doped yttrium iron garnet (YIG) samples with nominal composition Y3-xLaxFe5O12 (0< x <0.6) is analysed in this work. The results show a very different behaviour depending on the sintering atmosphere used for the fabrication of the samples. Concretely, for the samples sintered in air, it just have been detected the usual relaxation peak found in YIG at 130 K. However, a new relaxation peak appears around room temperature for the samples sintered in CO2 atmos- phere when the La content is at least of 0.3. These results have been interpreted in terms of the formation of a secondary perovskite phase when the La solubility limit is reached. It is well known that Yttrium Iron Garnet (YIG) is a key material for the development of microwave devices due to its electrical and magnetic properties and the possibility of introducing a wide range of dopants for tailoring its properties. It is very usual the substitution of iron by metallic cations in tetrahe- dral or octahedral sites of the lattice (1-3). However, the substitution of part of yttrium ions in dodecahe- dral sites with a rare earth element has been also studied so far (4, 5), showing that the total substitution of Y by lighter 4f rare earth elements La, Ce, Pr and Nd is not possible, and that, above the solubility limit, a secondary perovskite phase forms together with the magnetic garnet. In this sense, it is essential to determine the solubility limit of each extrinsic cation in order to assure that single YIG phase is ob- tained. Recently, our group has shown (6) that the magnetic disaccommodation technique is a powerful tool to analyse the garnet-perovskite transition in rare earth element doped YIG. This technique consists in the time variation of the mobility of domain walls after a magnetic shock, and is shown by a temporal evolution of the magnetic permeability after a demagnetization stage. This kind of phenomena, which is strongly temperature-dependent, is undesirable from the technical point of view, as it represents an im- portant source of magnetic losses in these materials, but it is very useful for basic research because such studies yield information about lattice symmetry and dynamics. Its origin has been attributed to either the rearrangement or the diffusion of anisotropic point defects, such as lattice vacancies and interstitials, within the Bloch walls. In this work we have studied the effect of La doping on the magnetic disaccom- modation of YIG samples fabricated under different sintering atmospheres.

Relationship between the giant magnetoimpedance effect and the relaxation of magnetic permeability

Temperature dependence of the magnetoimpedance and its relaxation has been measured in amorphous ferromagnetic wires. The presence of several peaks in the frequency dependence of the magneoimpedance is ascribed to the magnetoelastic resonance of standing waves in the circular direction. A core-shell model that includes a term accounting for the magnetic looses interprets the analysis of the phase angle of the complex impedance. It is also observed a time evolution of the impedance that reveals the existence of magnetic after effect with a maximum at temperatures close to 350 K. (© 2004 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Magnetic aftereffects in hexagonal ferrites with BaO·9Fe/sub 2/O/sub 3/ stoichiometric initial composition

We have measured the relaxation of the initial magnetic permeability after sample demagnetization, in the temperature range 80 K < T < 420 K, in polycrystalline barium hexaferrites with a Ba:Fe ratio in the initial composition of 1:18 corresponding to stoichiometric W phase. The materials were sintered under different temperature and oxygen pressure conditions. The results represented by isochronal curves show the existence of up to four relaxation processes. Additional thermogravimetric and X-ray diffraction studies reveal the successive formation of M, X, and W hexagonal phases with the increase of the sintering temperature. In this framework, we ascribe the relaxation processes to the progressive increase of ferrous cations in the different sites within the hexagonal lattice.

Domain wall permeability limit for the giant magnetoimpedance effect

The magnetoimpedance (MI) effect is based on the change of inductance and resistance under the effect of an external magnetic field. In bulk homogeneous wires or ribbons these two components of the impedance are related to the penetration depth of the electromagnetic field in the material. From simple considerations it is shown that the maximum MI ratio is then proportional to the square root of the relative permeability of the material. That limit is reached for frequencies at which the penetration depth is of the order of the transverse dimensions of the sample. At low operating frequencies, typical of thick geometries, the permeability is limited by the microeddy currents associated with domain wall displacements. The permeability relaxation equivalent to these local eddy currents can be calculated and used in the classical expressions for MI. The real part of the permeability is highly reduced at the relaxation frequency and gives rise to a decrease of the inductance, while the imaginary part contributes to the resistance, with a maximum at the same frequency.

Magnetic disaccommodation in Sn substituted magnetite

The relaxation of the initial magnetic permeability has been measured in polycrystalline Sn-doped magnetite with nominal composition Sn x Fe 3− x O 4 with x ranging from x=0 to 0.6. In the temperature range between 80 and 500 K, the time decay of the initial permeability after sample demagnetization has been represented by means of isochronal disaccommodation curves, which show the presence of different relaxation processes at 250 K (IV′ peak), 275 K (IV), 300 K (III), 400 K (II) and 440 K (I). This behavior is explained on the basis of the disaccommodation of vacancy-doped magnetite and another similar tetravalent substitution, as the previously analyzed Ti-doped magnetite.

Giant magneto-impedance in soft magnetic “Wires”

The giant magneto-impedance, GMI, effect consists in the huge change of the impedance, both real and imaginary components, up to around 600% relative change in the presence of static magnetic fields (a few Oe) or applied stresses. Sensitivity of up to 500%/Oe has been observed in the very low (less than 1 Oe) field region. This effect was detected a few years ago in extremely soft amorphous magnetic wire-shaped materials. In the low frequency region, up to around 50–100 kHz, changes of impedance may be ascribed to the magnetoinductive effect due to the circular magnetization process. At higher frequencies (0.1–10 MHz) although lower than for on-magnetic conductors, the skin effect becomes dominant because of the large permeability. At the microwave regime, ferromagnetic resonance induces absorption phenomena. Variations of permeability originated by the static applied field result in changes in the skin depth and finally in the impedance. The main characteristics of GMI as single-peak or two-peak behaviors are described considering the magnetic anisotropies, the magnetization processes and the domain structure according to existing models. The influence on the GMI of the permeability relaxation with frequency is also discussed. Finally, attention is paid to the recent studies in the microwave regime, on novel materials as heterogeneous microwires and thin film layered structures. The main technological applications for magnetic field sensors devices based on GMI are outlined. In summary, GMI opens a new branch of research by combining classical electrodynamics and micromagnetism of soft magnets.

Temperature Dependence of Complex Permeability and Domain Wall Relaxation in Polycrystalline Perovskite La0.7Sr0.3MnO3

The magnetic spectra of the polycrystalline perovskite La 0.7 Sr 0.3 MnO 3 have been measured at various temperatures and in the frequency range 10 Hz-100 MHz using an impedance analyzer. The complex permeability and the relaxation of magnetic domain walls have been investigated. A pronounced dispersion of the relaxation in the spectra has been observed. The relaxation frequency reduces from 1.47 MHz to 772 kHz with decreasing temperature from 296 to 77 K. Meanwhile, the static permeability becomes smaller from 42 to 26. The relaxation is ascribed to the displacements of domain walls, and the damping mainly originates from the eddy currents associated with wall motions.

Temperature Dependence of Complex Permeability and Domain Wall Relaxation in Polycrystalline Perovskite La0.7Sr0.3MnO3

Temperature Dependence of Complex Permeability and Domain Wall Relaxation in Polycrystalline Perovskite La0.7Sr0.3MnO3

Magnetic permeability relaxation in amorphous Fe 62.5Co 6Ni 7.5Zr 6Cu 1Nb 2B 15

The high-frequency initial magnetic permeability and its relaxation have been studied in Fe 62.5Co 6Ni 7.5Zr 6Cu 1Nb 2B 15 after heat treatment (both in furnace or via DC Joule heating). The magnetic permeability steadily increases in correspondence of both structural relaxation and nanocrystallisation of the alloy. The intensity of the permeability relaxation (measured between fixed times after demagnetisation) is reduced by structural relaxation but exhibits a sharp maximum immediately before the onset of nanocrystallisation. The nanocrystalline alloy is characterised by a nonzero, if reduced, permeability relaxation.

Giant magnetoimpedance in soft magnetic amorphous and nanocrystalline materials

Recent results on giant magnetoimpedance (GMI) are reviewed. After a brief outline on the existent theories which explain the phenomenon, a general overview of experimental data on amorphous wires and ribbons is given. Furthermore, the GMI effect is observed in soft magnetic crystalline and nanocrystalline samples, and few investigations done in different systems are presented. To conclude, a recently observed effect in amorphous materials, the so-called magnetoimpedance aftereffect, is reported and explained as a consequence of the relaxation of the circular magnetic permeability.

Time evolution study of the domain structure in amorphous Fe64Co21B15 and Co77B23 using dynamic 3D-neutron depolarization

The most recent results on the relaxation of magnetic permeability in amorphous ferromagnets have been obtained using the ac technique. The magnetic relaxation is attributed to the existence of structural defects, which are typical for the amorphous metallic state. The short-time magnetic relaxation in amorphous Fe64Co21B15 and Co77B23 alloys was measured by the 3D-neutron depolarization technique after impulsive sample saturation. The behaviour of the magnetic decay shows that this effect has a predominant diffusive character, ascribed to the activated process of directional ordering of defects.

Reversible and Irreversible Relaxation of Magnetic Permeability in Photoexcited Yttrium Iron Garnet

Photoinduced variation of magnetic complex permeability µ=µ'-jµ'' at low frequency, e.g., 0.14 kHz is studied as a function of temperature in yttrium iron garnet (YIG). We have observed a new phenomenon in YIG occurring solely after illuminated at low temperature; photoinduced disaccommodation. We discussed an origin of the observation, taking account of the charge state of oxygen vacancy and an interaction between lattice relaxation and domain wall.