Showing papers on "Saturation (magnetic) published in 1994"

Shear stresses in magnetorheological fluids: Role of magnetic saturation

Abstract: The interparticle forces and resulting shear stresses in a magnetorheological fluid are calculated. The field due to a linear chain of particles in a fixed average magnetic induction Bave is determined from a finite element analysis in which the nonlinearity and saturation of the particle magnetization are incorporated. The shear stresses are then computed from the field using Maxwell’s stress tensor. The stresses obtained for all but the lowest magnetic inductions are controlled by the saturation of the magnetization in the contact regions of each particle. Identifying the maximum shear stress as a function of shear strain with the yield stress gives values in agreement with results reported for typical fluids. For high magnetic inductions the yield stress plateaus due to the complete saturation of the particle magnetization; the stress scales as the square of the saturation magnetization in this regime.

Magnetic field sensors using GMR multilayer

Abstract: Wheatstone bridge magnetic field sensors using giant magnetoresistive ratio (GMR) multilayers were designed, fabricated, and evaluated. The GMR ranged from 10% to 20% with saturation fields of 60 Oe to 300 Oe. The multilater resistances decreased linearly with magnetic field and showed little hysteresis. In one sensor configuration, a permanent magnet bias was placed between two pairs of magnetoresistors, each pair representing opposite legs of the bridge. This sensor gave a bipolar bridge output whose output range was approximately GMR times the bridge source voltage. The second sensor configuration used shielding on one resistor pair, and it gave a bridge output dependent on the magnetic field magnitude, but not polarity, and the output range was approximately one half GMR tines the bridge source voltage. Field amplifications of 3 to 6 were accomplished by creating a gap in a low reluctance magnetic path, thus providing the full range of outputs with 1/3 to 1/6 of the intrinsic saturation fields of the GMR multilayers. >

Creep induced magnetic anisotropy in nanocrystalline Fe-Cu-Nb-Si-B alloys

Abstract: Amorphous Fe-Cu-Nb-Si-B ribbons have been nanocrystallized under tensile stress applied along the ribbon axis. This results in a strong magnetic, creep-induced anisotropy with an easy ribbon axis for low and, a hard ribbon axis for high Si-concentrations, respectively. The analysis of the compositional dependence shows that the anisotropy energy correlates linearly with the saturation magnetostriction of the precipitated BCC-FeSi grains. It is therefore proposed that the induced anisotropy mainly originates from the magneto-elastic anisotropy of the BCC grains due tensile back stresses exerted by the anelastically deformed amorphous matrix. >

The influence of particle size variations on the magnetic properties of sediments from the north-eastern Irish Sea

Abstract: This paper considers the magnetic properties of a range of recent and contemporary sediments from the north eastern part of the Irish Sea. Principal component ordinations of the results show a close link between magnetic property variations and particle size. The magnetic properties of a subset of samples, particles sized by a combination of sieving and pipette analysis, confirm that variations in ferrimagnetic (‘magnetite’) grain size parallel those in particle size, despite the fact that the magnetic grains in the fine grades have diameters 1–2 orders of magnitude smaller than those of the particle size fraction in which they occur. This is best explained by postulating that the fine magnetic grains occur in the clay fraction but are present in declining concentrations in the coarser grades up to 4o as an artefact of the pipette method. Most samples have a biomodal distribution of magnetic minerals, with a coarse mode associated with heavy minerals in the sands or coarse silts, and a fine mode in the clays. Magnetic susceptibility (x) and saturation isothermal remanent magnetization (SIRM) largely pick out the coarse mode where present; anhysteretic remanent magnetization (ARM) largely picks out the fine mode. The results open up the possibility of normalizing samples from these environments for particle size and, more specifically, clay content, by means of ARM or ARM/x values. The most likely source for the uniform and almost exclusive stable single domain magnetite, which dominates the magnetic properties of the clays, is thought to be bacterial magnetosomes. The measurements as a whole do not appear to hold much promise for discriminating sediment source types.

Ni45 Fe55 metal-in-gap thin film magnetic head

Abstract: The present invention provides a novel high magnetic moment material for the pole pieces as well as a metal-in-gap configuration for the pole tips of either an inductive magnetic head only or the inductive portion of a MR head. The novel material is Ni 45 Fe 55 . In the MIG configuration each pole piece of the inductive head or the inductive head portion of a MR head has a combination of layers, each combination of layers including a first layer of high magnetic moment material Ni 45 Fe 55 adjacent to a transducing gap and a second layer of low magnetic moment material such as Permalloy (Ni 81 Fe 19 ) further away from the gap. Since both layers are made of NiFe all the desirable properties of this type of material can be employed as well as simplifying its construction with similar plating baths. The saturation of the first layers is 50 to 60 percent higher than the saturation of the second layers. The present invention avoids effects of magnetostriction in spite of the high magnetic moment of the first layers. By appropriately selecting the thickness ratio of the second layer with respect to the first layer the magnetostriction of the laminated structure can be reduced substantially to zero. When this thickness ratio is in the order of five to nine the magnetostriction is reduced to, or slightly below, zero. If the inductive head is employed for write functions only then the second pole tip or both pole tips can be constructed of the high moment Ni 45 Fe 55 material without any thickness ratio or MIG configuration constraints.

Effect of duct bias on transport of vacuum arc plasmas through curved magnetic filters

Abstract: The plasma output of a 90° magnetic macroparticle filter of vacuum arc plasma was monitored by a Langmuir probe as a function of bias of the duct wall and guiding magnetic field. Maximum plasma transport through the filter was found at a positive bias of about 20 V. A relatively small magnetic field of 10–30 mT is sufficient for effective guiding of the plasma, and further increase of the guiding field improves the filter efficiency only gradually. The potential of a floating duct changes from negative to positive when the guiding field is increased. This can be explained by the balance of electron and ion flux transverse to the magnetic field. Saturation in the plasma output at high guiding field (≳120 mT) is observed for carbon but not for heavy elements. The transport of plasma through bent ducts is made possible by the magnetic pressure of the guiding field, and by sheath and space‐charge electric fields.

Plastic card provided with magnetic stripe

Abstract: A plastic card provided with a magnetic stripe, which is very difficult to forge or alter and, even tough subjected to forgery or alteration, enables the forgery or alteration to be easily detected in appearance. The plastic card includes a card substrate and a hologram-magnetic stripe including, provided on said card substrate in the following order, an adhesive layer, a first magnetic recording layer, a second magnetic recording layer, a transparent continuous thin film layer and a hologram forming layer. The first magnetic recording layer and the second magnetic recording layer are such that one of the magnetic recording layers has a coercive force at least twice as high as the other magnetic layer and a Curie point at least 100° C. below the other magnetic layer and, when the magnetic recording layers are heated at a temperature in the range of from the lower Curie point to 30° C. below the lower Curie point, they become substantially identical to each other in saturation writing current value.

Synthesis and characterization of Fe16N2 in bulk form

Abstract: Using molecular beam epitaxy and ion implantation, Japanese workers have formed Fe16N2 in thin Fe films. They report a large magnetic induction for this nitride, ≥2.8 T. In the present study, Fe16N2 has been prepared in bulk form by treating Fe powder with NH3/H2 gas mixtures at temperatures in the range 660–670 °C. The γ phase alloy, which forms under these conditions, was quenched to room temperature to form the α’‐Fe‐N phase and then heat treated at 120–150 °C to form the α‘‐Fe‐N phase. The α’ phase has been prepared in 80% purity, the other phase being nonmagnetic γ‐Fe‐N. The α‘ phase has been prepared in 50% purity, the impurity phases being α‐Fe and γ‐Fe‐N. Magnetic measurements give saturation magnetizations at room temperature of 250±10 emu/g (2.6μB/Fe) for the α’ phase, 285±10 emu/g (2.9μB/Fe) for the α‘ phase, and essentially zero for the γ phase. Mossbauer measurements confirm that nitrogen austenite is nonmagnetic. The anisotropy of Fe16N2 is small but detectable.

Giant magnetoresistance peaks in CoNiCu/Cu multilayers grown by electrodeposition

Abstract: Giant magnetoresistance (GMR) of CoNiCu/Cu multilayers grown by electrodeposition was measured as a function of the copper layer thickness and effects of the order of 14% were obtained. The copper layer thickness ranged from 0.7 to 3.5 nm. Two peaks in the magnetoresistance were observed. One was centered at a copper thickness of ∼1.0 nm and the second was centered at ∼2.3 nm. Comparison of the field dependence of the magnetoresistance with the field dependence of the magnetization, as determined by vibrating‐sample magnetometer, suggests that the saturation field for GMR and the magnetization are similar for the larger copper thicknesses, but are strikingly different near 1.0 nm copper thickness. This observation suggests that the GMR is affected by different factors depending on the thickness of the copper layer.

Magnetic anomalies in single crystal Fe3O4 thin films

Abstract: Fe3O4 thin films were reactively sputtered onto 〈100〉 and 〈110〉 MgO substrates. X‐ray diffraction data indicate single crystal Fe3O4 films under a tensile stress. Magnetization data show good agreement with bulk Fe3O4 for Ms, but also show the presence of a large anisotropy component manifest in the lack of saturation in fields up to 70 kOe. Conversion electron Mossbauer spectroscopy data taken at zero field also show good agreement with bulk Fe3O4 for the values of the hyperfine fields and isomer shifts of the two Fe3O4 sites, indicate good stoichiometry, but indicate the presence of a large anisotropy component randomizing the moments. In‐plane torque measurements on 〈100〉 oriented Fe3O4 are consistent with bulk Fe3O4 magnetocrystalline anisotropy. However, in‐plane torque measurements on 〈110〉 oriented Fe3O4 show the presence of a uniaxial anisotropy superimposed on the crystalline terms. This anisotropy, modeled as being due to the stress induced by epitaxy, changes the easy axis from the 〈111〉 direct...

Magnetization transfer imaging with pulsed off‐resonance saturation: Variation in contrast with saturation duty cycle

Abstract: Magnetization transfer contrast (MTC) can be generated in magnetic resonance images of water-containing macromolecular structures. Results obtained with the pulsed off-resonance saturation technique for achieving MTC are presented, allowing comparison of this technique with others. The magnetization transfer effect is demonstrated in newborn piglet brain and bovine muscle tissue, as well as in human subjects. MTC increases nonlinearly with the duty cycle of the off-resonance irradiation, approaching a maximum value corresponding to continuous irradiation. However, approximately half of the available contrast can be obtained at a given saturation offset frequency, with 16% irradiation duty cycle. Examples of MTC in various human tissues demonstrate the specificity of this technique at power levels within U.S. Food and Drug Administration guidelines. Plots of MTC versus the irradiation offset frequency demonstrate the effects of magnetization transfer and are compared with the curve generated by a control phantom that does not show magnetization transfer. Magnetization transfer imaging with pulsed off-resonance saturation is effective and easily implemented and may be useful in the investigation of saturation transfer phenomena in tissue.

Soft X-ray magnetic circular dichroism and magnetic films

Abstract: Soft X-ray magnetic circular dichroism is the difference in the total absorption cross-section of positive and negative helicity circularly-polarized soft X-rays at the magnetically interesting L 2 and L 3 absorption edges of 3d transition metals or the M 4 and M 5 absorption edges of the rare earths. The absorption edges can be measured using a variety of techniques including transmission, partial electron yield, total electron yield, partial fluorescence yield, sample current, and reflection. The different measurement schemes represent different probing depths, allowing depth dependent studies, and are complementary to each other. A description of the relative merits of the various measurement schemes is presented. In addition, complications such as beam instability, saturation effect, and incident beam Bragg scattering into the detectors are discussed.

Radio frequency reactive sputtered iron nitrides using ammonia gas: Structure and magnetic properties

Abstract: Iron nitrides of γ’‐Fe4N, e‐Fe2‐3N, and ζ‐Fe2N have been fabricated using rf sputtering with reactive NH3 gas, which offers significant advantages over the conventional N2 gas. All but ζ‐Fe2N nitride films have relative high saturation moments with a large in‐plane anisotropy. The Mossbauer results are consistent with structural analyses and those from bulk samples.

Magnetic and transport properties of the R3Ni system (R = Y, Gd, Tb, Dy, Ho, Er)

Abstract: The electrical resistivity and magnetic susceptibility have been measured for Y3Ni, Gd3Ni, Tb3Ni, Dy3Ni, Ho3Ni and Er3Ni single crystals in a wide temperature range. A T3-dependence and a strong negative curvature of the electrical resistivity were observed for lower temperatures and a saturation effect for higher ones. Y3Ni exhibits also a very pronounced temperature dependence of the magnetic susceptibility. After subtracting the temperature independent term, Y3Ni obeys a Curie-Weiss law with μeff=1μB per formula unit for the temperature range 10–50 K and with μeff=0.68μB for the range 50–420 K. It may indicate the existence of a magnetic moment for the Y-Ni system in spite of such a low Ni concentration. A complex magnetic behaviour and enhancement of the effective magnetic moment in relation to the theoretical value, calculated assuming only R3+-ion contribution, for Gd3Ni, Tb3Ni, Dy3Ni, Ho3Ni and Er3Ni, was observed.

Structural and magnetic characterization of the new ternary phase Tb3(Fe1−xTix)29

Abstract: We report the existence and characterization of the stable ternary phase Tb3(Fe1-xTix)29. The structural characterization by X-ray powder diffraction is evidence for a monoclinic structure (P21/c space group) with refined lattice parameters a=10.583(1) AA, b=8.5116(7) AA, c=9.6736(9) AA and beta =97.018(5) degrees . A large magnetovolume effect has been observed in the volume thermal expansion at the order temperature Tc=455 K. The anisotropy has been measured by using the singular point detection technique. Two distinct anisotropy fields of relatively high intensity have been detected. The HA values measured at 6.4 T and 1.9 T at 293 K and both increase markedly with decreasing temperature. Below 200 K, the approach to saturation corresponding to the highest HA develops into a first-order magnetization process.

Ultrafine maghemite particles. I. Studies of induced magnetic texture

Abstract: A sample with an orientational magnetic texture was prepared by freezing a ferrofluid containing maghemite particles in a magnetic field. The degree of alignment of the easy directions obtained by field-cooling was examined by Mossbauer spectroscopy for varying strengths of the freezing field. We compare the results with the predictions of a simple model assuming that the intrinsic magnetic anisotropy in the particles is uniaxial and that the particles are non-interacting. Good agreement between the experimental results and the theoretical model is found. This allows a determination of the magnetic anisotropy energy constant of the particles, which is an important parameter in the model. The anisotropy energy constant is also determined from the reduction of the magnetic hyperfine field relative to the saturation value caused by collective magnetic excitations, as well as from a decay of remanence measurement. Good agreement between the estimates of the anisotropy energy constant by the three methods is found.

Novel bulk iron garnets for magneto-optic magnetic field sensing

Abstract: We report measurements of the magneto-optic response function and frequency response for three bulk iron garnet crystals grown by a flux technique. The samples were the product of an intensive effort to develop iron garnet compositions with properties specifically optimized for magnetic field sensing. Sensitivity enhancement was achieved through both bismuth substitution (for increasing the saturation Faraday rotation) and gallium substitution (for reducing the saturation magnetization). One sample exhibited a value of magneto-optic sensitivity of 25/spl deg//mT for 1.3 /spl mu/m light. Frequency response measurements indicate that bismuth substitution actually improves performance (compared to unsubstituted yttrium iron garnet) in contrast with gallium, which causes substantial degradation. >

Magnetic properties of ferromagnetic GdN

Abstract: A polycrystalline GdN sample was prepared by the direct reaction of gadolinium metal and nitrogen gas at 1600°C and at a pressure of 1300 atm. We measured magnetization and the magnetic field dependence of the susceptibility and specific heat of GdN. Susceptibility and magnetization show ferromagnetic behavior for GdN with the saturation value reached at about 20 kOe. Specific heat measurements revealed a ferromagnetic transition in GdN at T c = 58 K with an additional broad peak around 20 K. Experimental results were explained with an exchange interaction mechanism.

Anomalous perpendicular magnetoanisotropy in Mn4N films on Si(100)

Abstract: Ferrimagnetic Mn4N films were deposited on Si (100) substrate by dc reactive magnetron sputtering from sintered Mn target. Highly (002) textured Mn4N ordered phase is formed in situ at studied substrate temperatures of 150–250 °C without further annealing. Anomalous perpendicular magnetoanisotropy exists in these face‐centered cubic films with larger coercivity measured perpendicular to the film (2000–3000 Oe) than that parallel (1100–1300 Oe), as is the remanence. Coercivity in either direction decreases, while the saturation flux density (from 240 to 610 G) increases with increasing substrate temperature. The anomalous perpendicular magnetoanisotropy is attributed to (1) the stress‐induced anisotropy due to in‐plane tensile stress coupled with a reverse magnetostriction, and (2) the shape anisotropy due to columnar grain structure.

Determination of proton magnetization transfer rate constants in heterogeneous biological systems

Abstract: Two procedures are currently in use for the determination of proton magnetization transfer rate constants between macro-molecular tissue components and water. The first method assumes that there are only two spin baths (macromolecular plus solvent) and that during off-resonance irradiation complete saturation of the “immobile” proton spin bath occurs (S. H. Koenig, R. D. Brown, 111, R. Ugolini, Magn. Reson. Med. 29, 311 (1993)). This approach neglects the possibility of incomplete saturation and polydispersity, and yields an apparent magnetization transfer rate constant, Kapp. The second approach utilizes a formalism which can account for polydispersity and incomplete saturation of the immobile spin bath (K. Kuwata, D. Brooks, H. Yang, T. Schleich, J. Magn. Reson., in press). In this work magnetization transfer rate constants derived by the use of both methods for two systems, ocular lens tissue and cross-linked bovine serum albumin (BSA) were compared. For both samples Kapp was dependent on B2, off-resonance irradiation frequency and power when the first method was used. The second method provided values of the magnetization transfer rate constant that were similar to the values obtained by the first method, as the limit of complete saturation was approached.

Interfacial quality and giant magnetoresistance in MBE-grown Co/Cu(111) superlattices

Abstract: We report Co-59 NMR measurements on three MBE-grown Co/Cu(111) multilayer films that show a progression of saturation magnetoresistance from DELTAR/R = 4% to DELTAR/R = 40%. Our results demonstrate unambiguously that the increase in magnetoresistance is correlated with an improvement in the quality of the interfaces in these MBE-grown materials.

Effect of uniaxial stress on the reversible and irreversible permeabilities of 2% Mn pipeline steel

Abstract: The results of a study of the effects of constant uniaxial stress on the irreversible and reversible differential permeabilities of minor and saturation major hystersis loops are presented, extending an earlier study which studied those permeability components on the initial magnetization curve. Tension was found to increase both the reversible and irreversible components of the saturation major loop differential permeability in the low magnetization region, and to decrease them in the high magnetization region. The opposite effect was found for compression. This effect was explained as the result of changes in the domain structure of the sample when stressed and a resulting change in the ratio of 90/spl deg/ of 180/spl deg/ domain walls. The differences between the upper and lower branches of the saturation major hysteresis loop reversible relative differential permeability were found to increase in tension and decrease in compression, with more complicated behavior occurring in the irreversible component. Minor loop behavior was also found to vary depending on the magnetization of the sample. In the low magnetization region tension was found to produce higher values and larger variations in both components of the relative differential permeability than the unstressed case, while compression produced lower values and smaller variations. The opposite behavior was found to be true in the high magnetization region, while an intermediate behavior with little or no change in both the relative differential permeability components was found to exist between the two extreme cases. The minor loops were measured from set values of the applied field, and tension was found to increase the corresponding values of magnetization, with compression causing a decrease in the magnetization values. The behavior of the minor loops was found to be related to the major loop behavior in the same region. >

Magnetic properties of maraging steel in relation to deformation and structural phase transformations

Abstract: Magnetic properties in annealed and cold rolled conditions have been investigated for maraging steel grade 18%Ni-2400. The austenite content, coercive field, saturation magnetisation and remanence were determined after ageing for 1 h in the temperature range from 400 to 800°C. The results show that the degree of deformation imparted to martensite influences both the amount of reverted austenite and the magnetic properties obtained following ageing. Transmission electron microscopy was carried out in order to investigate the structural changes taking place during reversion of austenite.

The effect of the distribution of vacancies on the magnetic properties of γ – Fe2O3 particles

Abstract: Samples of γ-Fe2O3 have been prepared from uniform α-Fe2O3 particles obtained by homogeneous hydrolysis in solution and directly by spray pyrolysis methods. The crystalline structure of these samples was analyzed by using x-ray and electron diffraction. It was concluded that, depending on the preparation method and parameters, different degrees of order in the distribution of vacancies were present. The study of the magnetic properties of these samples included the measurement of both intrinsic (saturation magnetization) and extrinsic properties (coercive force and magnetization behavior in the approach to saturation region). It was found that all the above-mentioned quantities depended on the degree of order in the distribution of vacancies.

The development of nanocrystalline Fe73.5Cu1Nb3Si13.5B9: magnetism and structural disorder

Abstract: In order to study the effect of the initial degree of topological disorder on the formation of the nanocrystalline state, two amorphous ribbons of composition Fe 73.5 Cu 1 Nb 3 Si 13.5 B 9 produced in different laboratories were investigated. Measurements of hysteresis loops, disaccommodation and magnetostriction were performed on as-cast and nanocrystalline samples. An analytically solvable model describing the stress dependence of the coecivity is discussed. Two types of analysis of the approach to saturation are used to estimate the distribution function of the magnetization vectors. From the pinning field, the dimension of long-range order stresses in the nanocrystalline state is deduced.