Showing papers on "Saturation (magnetic) published in 1988"

Magnetic properties of some synthetic sub-micron magnetites

Abstract: The grain size dependence of various mineral (rock) magnetic parameters has been determined, using a series of essentially pure, fine-grained (single domain, SD) and ultrafine-grained (superparamagnetic, SP) magnetites. The parameters measured include low-field susceptibility, frequency-dependent susceptibility, saturation remanence (SIRM), anhysteretic susceptibility (XARM), and coercivity of remanence ((B0)CR). The magnetites were produced in experiments designed to simulate possible pedogenic and biogenic pathways of magnetite formation. Their mean grain sizes range from 0.012 um to 0.06 um, and hence span the SP/SD boundary. Isothermal magnetic measurements were performed on two separate subsets of differing packing densities. The response of the magnetic parameters is modified by interaction effects, but they display continuous variation across the entire grain size range, confirming their value for rapid magnetic granulometry. Within the fine and ultrafine end of the magnetite grain size spectrum, susceptibility, frequency dependent susceptibility and XARM are notably responsive to grain size change. In terms of magnetic response (and also possibly of grain size, shape and absence of cation substitution), these synhtetic magnetites represent close analogues of those found in some soils and sediments.

Magnetic properties of amorphous ferromagnets (invited)

Abstract: Some magnetic properties of amorphous ferromagnets are well described within the random‐anisotropy real‐space model. This model assumes that the neighboring spins are ferromagnetically coupled with each other, and that there is a local magnetic anisotropy whose axes are correlated over a small length Ra due to short‐range structural order. The system is characterized by a small parameter λ∼R2aK/A which depends on temperature and on the concentration of magnetic atoms via the local anisotropy K and exchange constant A. In zero magnetic field the local magnetization smoothly rotates over the solid with a characteristic length Rf =Ra/λ2. The zero‐field susceptibility is very sensitive to the exchange, the anisotropy, and the amorphous structure: χ∝A3K−4R−6a. The magnetization law in approaching saturation (M→M0) is universal (M0−M)∝1/(H)1/2 for H<2A/M0R2a. These and other predictions of the model seem to be in a good agreement with many recent experimental results.

Saturation magnetisation, coercivity and lattice parameter changes in the system Fe3O4-γFe2O3, and their relationship to structure

Abstract: This study has characterised the oxidation products of a fine-grained single domain magnetite which was made synthetically by a colloidal method. Changes in the intrinsic magnetic properties (saturation magnetisation, saturation remanence, and coercive force) during progressive oxidation are correlated with lattice parameter changes and an oxidation mechanism. It is proposed that magnetite oxidises to hematite via at least two metastable maghemites. The first of these, formed on low temperature oxidation by the formation of a magnetite/maghemite solid solution, is a face centered maghemite with lattice parameter a= 8.3419±0.0006 A. A second maghemite, produced on oxidation at higher temperatures, has a primitive cubic structure and a lattice parameter a = 8.3505±0.0005 A. Maghemite cation distributions are derived to explain the reduced saturation magnetisations of between 56 and 74 Am2 kg-1 observed, and a maghemite structure containing an increase in tetrahedral Fe3+ ions and up to 3 octahedral vacancies per 32 oxygen unit cell is proposed.

Planar inductors on magnetic substrates

Abstract: Using the method of current images, a simple formula is derived that describes the enhancement effect of a thick magnetic substrate of permeability mu on the inductance of planar inductors. It is shown that a maximum of 100% enhancement of the inductance over the corresponding air-core value can be obtained. This occurs in the limit mu >>1, in which case the inductance becomes independent of mu . Previous and new experimental data as well as a finite-element result are found to be in very good agreement with the theoretical result. >

Properties of ion‐beam‐sputtered Ni/Fe artificial lattice film

Abstract: This paper reports the results of experimental and analytical investigations into the properties of ion‐beam‐sputtered Ni/Fe artificial lattice films. Through structure analysis, it is known that the Fe layer has poor crystallinity and the Ni layer has a face‐centered‐cubic structure with strong preferred orientation. Additionally, the Ni/Fe lattice film is found to possess good soft‐magnetic properties with lower coercivity less than 1 Oe and uniaxial anisotropy as the lattice period is decreased. The saturation magnetization and magnetostriction are also confirmed to be controlled by changing the lattice period and the thickness of Ni and Fe layers. As a result of the simplest model analysis, the very small magnetostriction obtained in this experiment is thought to be realized by the balance in magnetostriction among Ni, Fe, and Ni‐Fe interdiffusion layers. Furthermore, a good domain structure is observed in the Ni/Fe lattice film with very small magnetostriction.

Enhancement of the superconducting critical current from saturation in Nb‐Ti wire. I.

Abstract: The flux pinning characteristic of processed Nb‐Ti monofilamentary wires was investigated at 4.2 K and higher temperatures by an ac measuring method. Saturation of the global pinning force, as in Nb3Sn, was observed at high fields for specimens heat treated after initial heavy cold work. A transition from the saturation to a strongly pinned nonsaturation occurred after the specimen was finally cold worked. This transition directly demonstrates that the saturation is not caused by a shearing deformation of the fluxoid lattice assumed in Kramer’s model [J. Appl. Phys. 44, 1360 (1973)], because this proposed mechanism primarily depends on intrinsic superconducting properties which are not changed by the final cold working. From the measurements of the elastic and plastic behavior of the fluxoid lattice by using the ac method, it was also found that it was not the shearing deformation of the fluxoid lattice but rather the depinning that occurred in the saturation: The saturation is expected to be caused by a ...

Coercivity in ThMn12 ‐type magnets

Abstract: Fe‐rich and RE‐containing phases with the ThMn12 structure show interesting intrinsic magnetic properties. The single‐phase composition Fe80Mo12Sm8 has a saturation magnetization of 10 kG, a Curie temperature of about 200 °C, and an anisotropy field of about 90 kOe, with the saturation magnetization and the Curie temperature being further improved by a Co substitution. In this contribution we report on the first experiments to create coercivity in these samples. As preparation techniques we applied mechanical alloying and rapid quenching which both result in microcrystalline material. The highest coercivity (3.8 kOe) so far was obtained for a multiphase sample prepared by mechanical alloying and a successive heat treatment at 800 °C. The dependence of coercivity on composition for these samples is discussed. Results for rapidly quenched material are also presented.

Magnetic and structural properties of Fe(100)/Ag(100) single‐crystal multilayer films with ultrathin Fe layers

Abstract: Multilayer single‐crystal films with 57Fe(100)/Ag(100) periods have been grown with Fe layers only 1.7 monolayers thick, using molecular‐beam epitaxy techniques. Reflection electron diffraction and Auger spectroscopy were used to establish the crystalline and chemical perfection of these samples. SQUID magnetometer measurements were made over 0–40 kOe and 10–300 K ranges. At 10 K it is easier to saturate the magnetization normal to the film than in its plane. Furthermore, there is a significant perpendicular remanence Mr. This indicates that the easy axis of magnetization is normal to the film. The strong temperature dependence of Mr suggests that thermally assisted domain‐wall motion exists in these samples. Mossbauer studies of the same sample (reported elsewhere) are consistent with the above behavior.

Cylindrical domains in small ferromagnetic spheres with cubic anisotropy

Abstract: Magnetization configurations are computed for small ferromagnetic spheres with cubic anisotropy. A cylindrically symmetrical configuration is found, which has a lower energy than a single domain, for a sphere radius larger than about 27.7 nm, for cubic cobalt. In magnetite, the changeover from a single domain to the cylindrical configuration is at a radius of about 27.0 nm. This problem has been studied particularly for cubic iron oxides, used in magnetic recording. >

Soft magnetic properties of Fe-C/Ni-Fe multilayered films

Abstract: The paper examines soft magnetic properties and film structures of Fe‐C/Ni‐Fe multilayers formed by the dual ion‐beam sputtering method. The following results are obtained. (1) The addition of C to Fe improves the soft magnetic properties of Fe, making its magnetostriction constant zero while keeping its high saturation magnetic flux density. (2) The insertion of Ni‐Fe intermediate layers between Fe‐C layers further improves the soft magnetic properties to a relative permeability of 3000, with a saturation magnetic flux density of 2.0 T. (3) The improvements in the soft magnetic properties are due to reductions in the magnetic anisotropy dispersions and the mean anisotropy field, which in turn are due to reduction of crystallite diameter.

High saturation ternary cobalt-iron basalt alloys

Abstract: A study was made of the manner in which ternary additions to cobalt iron alloys impart ductility to the brittle binary alloys. Results show that tantalum or niobium are more effective than the conventional vanadium addition, and additions as low as 0.2 wt.% impart sufficient ductility to permit cold rolling. The magnetic characteristics of alloys made with Ta and Nb are reported. An increase of 4% in the saturation flux compared with the 49CoFe2V Permendur alloy is obtained. Improved coercive force and permeability, compared with conventional cobalt iron alloys, were also found. >

Magnetic properties of Ni/Fe//SiO2 multilayer film

Abstract: This paper discusses the magnetic properties of ion‐beam‐sputtered Fe/SiO2 multilayer films. Decreasing Fe layer thickness, while maintaining a constant SiO2 layer thickness, results in good soft‐magnetic properties and nearly zero magnetostriction. These results are thought to be due to smaller crystal grains and a change in crystal orientation in the Fe layers. In Fe/SiO2 multilayer films consisting of 7‐nm Fe and 2.5‐nm SiO2 layers, magnetization of 1.55‐T, hard‐axis coercivity of 0.3 Oe, and relative permeability of 4000 are achieved under magnetostriction of less than ±10−6. After 250 °C annealing, relative permeability increases up to 5000. Furthermore, a magnetic domain structure with easy‐axis direction and extended 180° walls suitable for thin‐film heads can be obtained.

The nature of magnetic single domains in volcanic glasses of the KBS Tuff

Abstract: Magnetic and optical variations in volcanic glasses of the East African Rift have been investigated by means of experimental and analytical study. The results add to the formal understanding of magnetism in volcanic glasses and give us new perspectives on the nature, origin, and significance of fine-grained magnetic minerals and on the magnetic petrology of glassy rocks. The samples that we have studied in detail, glass shards from the KBS Tuff of northern Kenya, contain a superparamagnetic precipitate of microcrystals, ∼2 to ∼10 nm in size. The precipitate is likely to have formed at elevated temperature, during or subsequent to eruption, about 1.9 m.y. ago. We have studied these microcrystals by means of transmission electron microscopy (TEM), ac and dc magnetic methods, electron spin resonance (ESR), Mossbauer spectroscopy, and optical spectroscopy. Our efforts have concentrated on determining the magnetic mineralogy and understanding the mechanism(s) responsible for the optical and magnetic behavior of these glasses. The particle size determination for the larger of these microcrystals has been achieved using TEM imaging and Mossbauer spectroscopy. The mineralogy is nonstoichiometric magnetite, based on Mossbauer, X ray diffraction, and electron diffraction results. The ratio of saturation remanent magnetization to saturation magnetization for the larger microcrystals is very nearly that predicted by single-domain theory for a uniaxial anisotropy, the origin of which may lie with stress or surface effects. We interpret magnetic and ESR results to infer the possible existence of microcrystals or amorphous clusters of Fe atoms, ∼1 nm or smaller, in KBS glasses that appear homogeneous in high-resolution TEM images. Results from optical absorption experiments show that the color variations in KBS shards arise due to optical absorption by the precipitate, a result consistent with classical electromagnetic theory.

Thermomagnetic switching in amorphous rare‐earth transition‐metal alloys with high compensation temperature

Abstract: The thermomagnetic switching process in amorphous Tb‐FeCo alloys has been investigated theoretically for typical compositions with a Curie temperature of 500 K and compensation temperatures ranging from 360 to 420 K. The temperature and radial dependence of the relevant magnetic parameters such as the saturation magnetization, the exchange constant, the uniaxial anisotropy, and the domain wall energy have been calculated in terms of the respective sublattice magnetizations inferred from the mean field theory and data obtained experimentally. The condition for domain stability has been derived and the radial dependence of the forces controlling the domain wall position is calculated yielding domain diameters for writing and erasure as a function of the applied field, the wall energy, and the laser‐induced temperature. Heating above the Curie temperature is shown to offer an alternative possibility to control the domain diameter and to achieve high domain regularity at low applied magnetic fields.

The effect of Ga substitution on the magnetic properties of Nd2Fe14B, Pr2Fe14B, and PrCo5

Abstract: R2Fe14−xGaxB (R=Pr and Nd) and PrCo5−xGax alloys were synthesized and examined by powder x‐ray diffraction at room temperature and by magnetometry in the temperature range 4.2–1100 K. Lattice parameters, Curie temperature, spin‐reorientation temperatures, anisotropy fields, and saturation magnetizations were determined. Phase analysis revealed that single‐phase materials exist only for x≤1 in the case of 2:14:1 alloys and for x≤2 for 1:5 alloys. An increase in lattice parameters is observed upon Ga substitution in all cases. About 20 K increase in the Curie temperature is observed for 2:14:1 alloys, whereas there is a dramatic decrease in 1:5 alloys. Saturation magnetization values decrease upon Ga substitution. The anisotropy field is slightly enhanced by Ga addition in the Nd2Fe14−xGaxB system. Spin‐reorientation phenomena in Nd2Fe14−xGaxB and PrCo5−xGax are strongly affected by Ga addition. The observed behavior is discussed in terms of preferential substitution of Ga atoms into the Nd2Fe14B‐type cryst...

Stress-induced changes in the magnetic properties of some nickel-copper and nickel-cobalt alloys

Abstract: This paper presents results on the magnetic properties of NiCu and NiCo alloys with compositions in the range 70%–100% Ni. Measurements include coercivity initial susceptibility, maximum differential susceptibility, magnetostriction, and Barkhausen effect emissions. Results revealed a surprising increase in the magnetostriction as a function of magnetic induction B as the copper content increased. The saturation magnetostriction λs was nevertheless found to decrease with copper content. The addition of cobalt reduced the magnitude of the magnetostriction resulting in a change of sign from negative to positive at between 20% and 30% cobalt. Coercivity was found to be little affected by elastic stress, but strongly affected by plastic deformation such as occurs, for example, in cold working. A similar result was true for initial susceptibility. Maximum differential susceptibility was, however, found to be strongly dependent on elastic stress. Results were interpreted in terms of stress induced uniaxial anis...

MAGNETIC PHASE TRANSITION OF HELICAL CsCuCl3 IN HIGH MAGNETIC FIELD

Abstract: Magnetization measurements of CsCuC13 is performed using pulsed high magnetic fields at various temper- atures below TN. Magnetization shows a small jump at 12.5 T and saturation at 31 T at 1.1 K when Ho // c, while a small plateu at around 12 T when Holc. The results are quite different from those reported so far in other triangular lattice substances and seem to be due to other type of incommensurate phase transition. Hexagonal ABX3 type compounds show compli- cated but interesting magnetic behaviors due to trian- gular lattice structure, where A = Rb, Cs, X = C1, Br and B = Mn, Fe, Co, Ni, Cu. They have been inves- tigated for more than two (l) but still remain some attractive problems. ype of crystal struc- ture, magnetic moments couple/each other strongly along the c-chain and then these compounds can be regarded as quasi-one dimensional magnets. The inter- chain exchange interaction is usually -weak compared to intrachain one. Because of the triangular network in the c-plane, the moments whose in-plane interaction is antiferromagnetic have frustration and then spin struc- ture becomes complicated. CsCuC13 is one of the materials of this category 121. Strong intrachain ferromagnetic exchange inter- action J1 .= 16.7 cm-' makes almost ferromagnetic but low pitch helical spin structure along the c-axis below TN = 10.6 K. This is considered to be due to the antisymmetric Dzyaloshinsky-Moriya interaction. In the c-plane, the-spin system shows a 120' structure caused by weak antiferromagnetic interchain interac- tion J2 = 2.7 cm-l. The magnetic moment lies in the plane due to D-M interaction whose vector is parallel to the c-axis, although dipolar anisotropy apparently makes spins prefer to direct to the c-axis.

Electromagnet and method of forming same

Abstract: A unitary superconducting electromagnetic structure (10) and method of forming same for use particularly at magnetic intensities greater than the magnetic saturation of iron and including an inner body (12) containing a coil pack (40) and an outer magnetic iron body (16) about the inner body (12) in concentric relation thereto. The outer body (16) has an end assembly (18) adjacent each end of the inner body (12) and the outer iron body (16) increases the magnetic field intensity at intensities substantially above the saturation of the magnetic iron in the outer body (16). The design of the electromagnetic structure (10) is based on a computer program which utilizes accurate information concerning the mechanical, thermal, electrical and magnetic properties of the materials proposed for the electromagnet including particularly the thermal and magnetic properties of the iron for predicting precisely the magnetic permeability of the iron at all values of the magnetic field.

Magnetostriction and Perpendicular Magnetic Anisotropy of Amorphous GdFeCo Thin Films

Abstract: The saturation magnetostriction constant λs, and the perpendicular magnetic anisotropy Ku of amorphous Gdx(Fe100-yCoy)100-x(9.5

Hexagonal ferrite materials for phase shifter applications at millimeter wave frequencies

Abstract: The effect of anisotropy field and saturation induction on phase shift at millimeter-wave frequencies has been examined for hexagonal ferrite materials. The analysis was done for both uniaxial and planar anisotropy at three particular frequencies of technological interest, 30, 60, and 90 GHz. The work was concerned primarily with materials; a simple thin slab device configuration was used for the analysis. Limitations on materials parameters due to propagation cutoff, phase error degradation over a specified bandwidth, and low field loss were also considered. The results indicate that any improvement in phase shift over conventional cubic ferrites will require a high saturation magnetization, comparable to that of Li-Zn ferrite, and moderate (6-10-kOe) to large (20-30-kOe) anisotropy fields, depending on the frequency. >

Hollow substantially hemispherical permanent magnet high-field flux source

Abstract: A hollow hemispherical flux source produces a uniform high-field in its central cavity. The hemisphere is comprised of a selected magnetic material whose magnetization is azimuthally symmetrical. The magnetic orientation (α) in the hemispherical permanent magnet shell is given by the equation: α=2θ where θ is the polar angle. A planar plate of high saturation, high permeability material closes the open end of the hemisphere. An axial bore hole through the hemisphere and/or through the planar plate provides the requisite accessability.

Magnetic and structural characterization of copper/cobalt multilayers

Abstract: Copper/cobalt multilayers were deposited by magnetically enhanced dc triode sputtering onto single‐crystal sapphire substrates. According to the binary‐phase diagram, no stable alloys of copper and cobalt exist. Because the multilayers are not exposed to high temperatures, we expect no alloy formation at the interfaces. X‐ray diffraction techniques demonstrate crystalline layering existing down to monolayer thicknesses of copper and cobalt. The saturation magnetization behavior of the multilayers was modeled using the perpendicular and in‐plane hysteresis curves obtained using a vibrating sample magnetometer. A torque magnetometer was used to determine the net magnetic anisotropy. Here we discuss the correlation between the magnetic behavior and the structural properties.

Saturation of Kelvin–Helmholtz fluctuations in a sheared magnetic field

Abstract: Kelvin–Helmholtz unstable flows are numerically investigated in the context of a sheared E×B flow profile and a sheared magnetic field in the collisional, electrostatic limit. In the extreme form of this limit, density fluctuations are small and the system is described by the nonlinear E×B vorticity dynamics. In order to focus on the role of magnetic shear localization, the computations are confined to two dimensions. For weak magnetic shear the fluctuations become turbulent and saturate by nonlinear cascade to small (dissipative) scales. In a strong magnetic shear regime near the linear stability boundary, nonlinear spatial broadening allows direct access to resistive shear dissipation, leading to saturation at small amplitude with nearly all the fluctuation energy in the longest‐wavelength mode. This is in accordance with previous investigation using a statistical closure analysis [Phys. Fluids 29, 231 (1986)]. The amount of broadening is proportional to the linear growth rate. The fluctuation amplitude...

A 151Eu Mossbauer study of the magnetic hyperfine interactions in the metallic compound Eu2Co12P7 containing trivalent europium

Abstract: Magnetic susceptibility measurements of Eu2Co12P7 show a ferromagnetic phase transition at the Curie temperature of TC=151+or-0.5 K, which is attributed to the magnetic order of the cobalt sublattice. 151Eu Mossbauer spectra reveal an unusual magnetic hyperfine structure splitting of the Van Vleck ion Eu3+. The temperature dependence of the magnetic flux density at the 151Eu nuclei was determined and compared with theoretical values. The exchange field acting on the europium ions may be assumed to follow the Brillouin function, where the best agreement is achieved for J=1. An exchange constant of mu BBexch(0)/k=7.5+or-0.1 K was deduced from the saturation value of the magnetic flux density Bsat=19.0+or-0.1 T. The possible contributions of Fermi contact terms of the magnetic flux density are discussed.

High Field Magnetization of GdB6

Abstract: High field magnetization of GdB 6 is investigated up to 550 kOe. The saturation magnetization of 7±0.5 µ B is obtained around 490 kOe at 1.3 K and the system can be regarded as a typical cubic antiferromagnet with the nearest neighbor exchange energy J 0 / k of -0.78±0.02 K. Beside the standard antiferromagnetic magnetization, two small anomalous step-magnetizations are found near the saturation field H c . They are explained by the local spin flip of the disordered Gd located at the B 6 cluster site. Two kinds of the disordered states, Type-I and Type-II, are proposed corresponding to two steps. The disordered Gd spin couples with the surrounding host spins with the antiferromagnetic exchange energy J '/ k of -1.2 K and -1.3 K for Type-I and Type-II, respectively.

Zero magnetostriction iron films

Abstract: Iron films with low coercive force and near-zero magnetostriction have been prepared by ion beam sputtering. With an increase in Ar ion acceleration voltage, the dominant crystalline orientation of the films changed from the [100] to the [110] plane. The saturation magnetostriction constant, lambda /sub s/, changed from positive to negative values, corresponding to the orientation change. The lambda /sub s/ of annealed films, whose lattice constants agreed well with those of the bulk, were close to the calculated values. The coercive force had a minimum of about 2 Oe near zero lambda /sub s/. The results indicate that the soft magnetic properties of iron films can be improved by reducing lambda /sub s/, which in turn is controlled by the acceleration voltage of the ion-beam-sputtering system. >

Saturation of green absorption in titanium-doped sapphire laser crystals

Abstract: Saturation of the pump-band absorption in Ti:sapphire has been observed by using 10-nsec, π-polarized pulses of 532 nm with fluences up to 4 J/cm2. A Frantz–Nodvik method was used for fitting the data and determining the saturation fluence (7.6 J/cm2) and the absorption cross section (4.9 × 10−20 cm2) at 532 nm. The peak absorption cross section was calculated to be 6.4 × 10−20 cm2 at 490 nm. This value is in reasonably good agreement with other determination methods.