Showing papers on "Coercivity published in 1971"

5PbO·3GeO2 CRYSTAL; A NEW FERROELECTRIC

Abstract: Dielectric constants and refractive indices of a 5PbO · 3GeO2 crystal, which was grown from the melt by by the Czochralski method, were measured as a function of temperature. The crystal was identified as a new type of ferroelectric with spontaneous polarization Ps = 0.046 C/m2, coercive field Ec ∼ 2.3 × 106 V/m at room temperature, and Tc = 177°C.

Calculation of intrinsic coercivity of magnetic domain walls in perfect crystals

Abstract: The critical field strength at which a classical domain wall moves through a perfect ferromagnetic crystal of simplified structure is calculated as a function of the ratio between anisotropy energy K and ferromagnetic coupling energy C , taking into account the discrete nature of the spins. When K approaches C intrinsic wall pinning becomes appreciable with critical fields approaching the anisotropy field when K \gg C . For K/C \geq 2/3 , the wall has a thickness of one atomic distance and thus resembles a ferroelectric wall.

Device implications of the theory of cylindrical magnetic domains

Abstract: This paper applies the theory of cylindrical magnetic domains1 to cylindrical domain devices. The stability conditions are examined as bounds to the region of possible device operation and it is found that the plate thickness, h = 4l, and the domain diameter, d = 8l, where l is the ratio of the wall energy per unit area to 4π times the saturation magnetization squared, are preferred values. When the effects of wall coercivity and mobility are examined, it is found that the preferred plate thickness and domain diameter are even more strongly preferred, that the wall motion coercivity should be less than one percent of 4π times the saturation magnetization, and that a domain coercivity and mobility may be defined. Consideration of the Neel temperature and the desired absolute domain size in addition to the static stability conditions shows that domain materials having some antiferromagnetic character and induced uniaxial anisotropy are preferred. Where appropriate, domain methods for measuring material parameters are described.

Domain Walls in Ferromagnetic Dy and Tb

Abstract: The spin structure and energy of ferromagnetic domain walls in Dy and Tb have been calculated, using an iterative method which takes into account exchange interactions, magnetocrystalline anisotropy, and magnetoelastic anisotropy. The domain walls are found to be very thin (less than 20 atom layers) and to have relatively high energy (∼10 erg/cm2), as compared to the ferromagnetic transition metals. An intrinsic coercive force for wall motion is predicted for these very thin walls; it is analogous to the Peierls‐Nabarro force on a dislocation, and is estimated to be of the order of 1 kOe for Dy and 30 Oe for Tb at absolute zero.

MnAlGe Films for Magneto‐Optic Applications

Abstract: MnAlGe is a uniaxial ferromagnetic compound with a saturation 4πMs of 3600 G, and Curie temperature of 245°C. We have studied polycrystalline thin films of this compound formed on various substrates by getter sputtering. The optimum substrate temperature to obtain films with uniform magnetic properties was 500°C. Films prepared at this temperature have a substantial component of magnetization normal to the plane of the substrate, and a coercive force of approximately 2000 Oe. Information was written into films approximately 700‐A thick by Curie point writing or with a fine permanent magnet wire, and read with good optical contrast by means of the polar Faraday effect. The relatively low Curie temperature is advantageous in reducing the power required for Curie point writing for magneto‐optic memory or holographic applications.

Theoretical and experimental aspects of coercivity versus particle size for barium ferrite

Abstract: The Stoner-Wohlfarth coherent rotation model has been extended to particle sizes approaching the superparamagnetic critical size S c for the case of barium ferrite. It is shown that at particle volumes up to 100 S_{c} , the coercivity factor will be lower than the Stoner-Wohlfarth factor of 0.48. Experimentally, micro-powders of barium ferrite with variable particle size have been formed using the glass-ceramic process. The particle size was determined with X-ray and electron microscope techniques. Although coercivities in excess of 5000 Oe were measured, the comparison between experiment and theory is not good. This has been attributed to the fact that the particles are poorly crystallized even though formation proceeded by a precipitation and crystal growth process.

Noncubic magnetic anisotropies in bulk and thin-film garnets

Abstract: Magnetic bubbles have been reported in bulk grown, chemical vapor deposition (CVD), and liquid phase epitaxy (LPE) garnets, implying the existence of noncubic anisotropies in all of these materials. Anisotropy magnitudes, directions, nonuniformities, and annealing behaviors and effects of strain relief on the anisotropies of several potentially useful garnet materials are presented and compared. These observations indicate that the noncubic anisotropies in the bulk materials and LPE films are similar, and are consistent with the pair-ordered model. Annealing reduces the growth induced anisotropy of LPE films and may be useful for increasing wall mobility, decreasing coercivity, and altering bubble sizes to match circuits.

Critical Fields Determining Magnetic Coercivity in Microparticles of SmCo5 and LaCo5

Abstract: Magnetic hysteresis curves measured by a specially designed magnetometer on a single particle taken from a ground SmCo5 powder show that the coercivity is determined by pinning of domain walls. Heat treatment at 100°C in air reduces the pinning force. Magnetizing the particle in a strong field increases the pinning force. A wall‐pinning model is proposed to explain this effect. The coercivity of a particle taken from the powder after electrolytic etching seems to be determined by nucleation of domain walls. Heat treatment at 100°C in air does not significantly influence the field at which nucleation occurs. The coercivity of an LaCo5 particle is determined by pinning. The pinning force is reduced by hydrogen absorption.

Review of magnetic properties of Fe-Ni alloys

Abstract: During the past two decades, improved understanding of the fundamental magnetic behavior of Fe-Ni alloys has made them one of the most versatile class of soft magnetic materials. No longer is the concern limited to high permeability and low coercfve force at room temperature. Alloys have now been custom-crafted to meet high permeability requirements at cryogenic temperatures, exhibit a skewed hysteresis loop for pulse transformer use, or a square hysteresis loop combined with stress insensitivity and controlled coercive force for memory applications. These examples are discussed in terms of the relationship of magnetic properties to structure and composition in Fe-Ni alloys.

Magnetic hardness of the intermetallic compound SmCo5 as a function of particle size

Abstract: The dependence of the coercivity, H c, remanence coercivity, H R, and remanence, M R, on particle size has been investigated at various temperatures from 77°K to 297°K. The results show that H c and H R increase as the particle size and temperature decrease. There is strong evidence for the existence of a critical particle size ∼45 μm below which the coercivities H c and H R increase very rapidly. This is also supported by the observation that there is an abrupt increase in the remanence, M R, at a particle size ∼45 μm.

Interpretation of hysteresis loops of cobalt-rare-earth single particles

Abstract: The magnetic behavior of single particles of Co 5 Sm is being studied in an effort to understand the origin of the coercive force in cobalt rare earths. It has been found that the complex hysteresis loops often observed, containing several magnetization discontinuities, can be analyzed as the linear sum of single-discontinuity hysteresis loops, implying that regions of a particle can act magnetically independently.

Magnetic and Structural Properties of Epitaxial and Polycrystalline GdIG Films Prepared by rf Sputtering

Abstract: Magnetic, magneto‐optical, and structural properties of rf‐sputtered gadolinium iron garnet films are presented The dependence of the Faraday rotation on film structure is explored and the behavior of coercivity and hysteresis loop characteristics with temperature discussed in some detail The coercive force was found to be independent of structural differences in the various films as monitored by x‐ray and electron diffraction techniques The high coercivities observed are attributed in part to a strain induced magnetoelastic interaction Hysteresis loops with squareness ratios of Mr/Ms=1 are obtained from (111)‐oriented films on (111) YAG substrates Domain patterns are highly temperature stable in the epitaxial films There is no evidence from magnetic and structural properties in these epitaxially grown films that cation diffusion from substrate to film is involved at crystallization temperatures below about 900°C The films are considered quite adequate for magneto‐optic memory applications

Cobalt‐Mischmetal‐Samarium Permanent Magnet Alloys: Process and Properties

Abstract: Several authors have reported successful fabrication of high‐energy product (15–20 MGOe) Co5R‐type cobalt‐samarium permanent magnets. In earlier papers we have described preparation of such magnets by a liquid‐phase sintering process sequence which includes powder preparation, magnetic alignment, pressing, sintering, and magnetizing. We have also described preparation of ternary compositions, primarily Co–Sm–Pr, by this process. In an effort to utilize more abundant rare‐earth metals, the cobalt‐mischmetal‐samarium system has been studied. Initial results indicate that a substantial fraction of the samarium may be replaced with mischmetal and still obtain high magnetic properties (BHmax 20 MGOe). Complete documentation of magnetic properties, i.e., saturation, remanence, coercive force, intrinsic coercive force, (BH)max, and temperature coefficients are presented along with a discussion of the fabrication techniques utilized for preparation of the samples.

Magneto‐Optical Properties of Sputtered Mn5Ge3 Films

Abstract: The Faraday rotation and optical absorption of ferromagnetic Mn5Ge3 thin films have been measured in the visible and near infrared. Apart from a gradual rise in optical density in the near uv, the absorption is featureless to 2.5 μ. The Faraday rotation behaves similarly. At 500 mμ the room‐temperature absorption coefficient α and the 5°K saturation rotation F are 3.5×105 cm−1 and 2.1×105 deg/cm, respectively. The rotation decreases with rising temperature and vanishes at 40°C which agrees well with the bulk Curie temperature of 37°C. The in‐plane coercive field Hc decreases monotonically with temperature as Tc is approached. For a 900‐A‐thick film Hc is 260 Oe at 0°C. The dependence of magnetic and magneto‐optical properties on film structure is discussed.

Magnetooptic-measurement techniques for magnetic-bubble materials

Abstract: Magnetic domains in thin crystal platelets of rare-earth-substituted iron garnets or rare-earth orthoferrites are readily visible when the platelets are viewed between crossed polarizers in a microscope, due to the magnetic Faraday effect. Several interesting optical-measurement techniques using this effect for investigating mobility, coercivity, wall energy, anisotropy, and imperfections are reviewed and compared.

Magnetic and magnetooptical properties of sputtered Fe 5 Si 3 films

Abstract: Polycrystalline films of Fe 5 Si 3 have been prepared by RF sputtering from a sintered target composed of 62.5 at% Fe and 37.5 at% Si. The Faraday rotation and optical absorption were measured for these films in the visible and near infrared. At a wavelength of 6328A, room-temperature absorption (corrected for reflection) and specific Faraday rotation are 3.7 × 105cm-1and 1.3 × 105deg/cm, respectively. The rotation decreases with rising temperature and vanishes around 120°C, the Curie temperature of the films. From polar and in-plane hysteresis loop measurements the spontaneous magnetization is found to lie in the film plane. Square hysteresis loops are observed when the external field is applied in the plane of the film, and the in-plane coercive force decreases monotonically with temperature at an average rate of -2 Oe/°C as T c is approached.

Method of making a thin film having a high coercive field

Abstract: An isotropic thin film having a high coercive field for use as a magnetic memory, and comprising a non-ferromagnetic substrate, at least one chromium layer having a thickness smaller than 10,000 A overlying said substrate and at least one cobalt layer having a thickness smaller than 1,000 A overlying the chromium layer. In a process for the fabrication of the thin film, the chromium layer or layers and the cobalt layer or layers are deposited on the non-ferromagnetic substrate by evaporation under a vacuum.

Coercivity of heat-treated Pr-Co power compacts

Abstract: The variation of coercivity as a function of the temperature of annealing or sintering is reported for compacts of a single-phase PrCo 5 powder and also for a mixture of PrCo 5 with an additive richer in Pr. For single-phase PrCo 5 , the intrinsic coercive force decreases steadily with increasing heat-treating temperature and shows a minor peak near 1000°C. For magnets made with the additive, the curve of M H c versus T has two peaks, one at approximately 1050°C and another at 1120°C. This behavior can be correlated with thermal events in the Pr-Co phase diagram. It is interpreted in terms of a model which assumes that pinning sites for the domain walls exist and are concentrated in shells forming on the surface of PrCo 5 grains during the sintering. These shell are thought to consist of Pr-Co compounds having lower melting points than PrCo 5 and to be epitaxial layers on or between the PrCo 5 grains into which the walls can travel from the latter.

Dual layer magnetic recording tape

Abstract: MAGNETIC RECORDING TAPE HAVING A DUAL-LAYER MAGNETIZABLE COATING TO PROVIDE IMPROVED HIGH FREQUENCY RESPONSE WHEN USED IN CONVENTIONAL AUDIO RECORDING DEVICES, ESPECIALLY IN CASSETTE RECORDERS. A TYPICAL TAPE HAS AN OUTER MAGNETIZABLE LAYER 40 MICROINCHES IN THICKNESS AND 400-550 OERSTEDS IN COERCIVITY AND AN INNER LAYER 160 MICROINCHES IN THICKNESS AND 270-330 OERSTEDS IN COERCIVITY.

A mechanism for producing magnetic remanence in meteorites and lunar samples by cosmic-ray exposure.

Abstract: An irradiation of 3 x 1017 neutrons per square centimeter in a reactor core produced an increase in the coercive force of iron and kamacite of 16 to 21 percent. The alternating-current demagnetization spectrum of saturation isothermal remanence was shifted toward higher coercive forces. Similar neutron fluences produced by cosmic-ray exposure may be capable of converting soft isothermal remanence in meteorites and lunar samples to remanence with a higher coercive force.

The influence of the shape of domains on the ferroelectric hysteresis loop

Abstract: Simple models of the polarization reversal in ferroelectrics with domains in the shape of either circular cylinders or parallel strips, which grow only by sideways expansion, are used to calculate the hysteresis loops and particularly the coercive field as a function of the amplitude and frequency of a sinusoidal electric field. The small differences in results corresponding to various models indicate that the coercive field is only slightly influenced by the shape of the domain structure in any sample and may thus be considered as a characteristic of a given material.

Magnetic properties of commercial soft magnetic alloys at cryogenic temperatures.

Abstract: It is well known that the magnetic properties of soft ferromagnetic alloys are affected by temperature. In this respect a great deal of work is reported in the literature on the effects of elevated temperatures, but only a few references concerning low-temperature properties are available to design engineers. One of the most com­prehensive papers published on the influence of low temperatures is that of Gniewek and Ploge [1] which deals with Si-Fe and 48 % Ni-Fe strip, both oriented and non-oriented. The increasing use of electromagnetic devices at cryogenic temperatures has created a need for information about other soft magnetic alloys, e.g. the high Ni-Fe permalloys and Co-Fe alloys. This study has investigated the behavior of a number of commercial alloys at low temperatures in an effort to supply some of the desired information.

Cast Permanent Magnets of the Co5RE Type with Mixtures of Cerium and Samarium

Abstract: Previous studies of the effect of composition on the magnetic properties of arc‐cast alloys in the vicinity of Co3.5CuFe0.5Ce yielded maximum energy products from 4 to 10×106 G Oe, residual inductions from 4000 to 6400 G, and intrinsic coercive forces from 5000 to 12 000 Oe. The partial substitution of samarium for cerium in alloys in this vicinity results in an improvement in permanent magnet properties, with maximum energy products from 4.6 to 12.3×106 G Oe, residual inductions 4300 to 7150 G, and intrinsic coercive forces from 4000 to 13 400 Oe. Freezing points of these alloys were determined using differential thermal analysis. The dependence of magnetic saturation on temperature was determined for several of these alloys from room temperature to 1.5°K. The saturation increases gradually with decreasing temperature without anomaly. The temperature coefficient of saturation magnetization between 25° and −100°C was found to be 0.1%/°C for the alloy Co3.5CuFe0.5Ce, 0.06%/°C for the alloy Co3.5Cu1.35Fe0.4...

Method of preparing cobalt doped magnetic iron oxide particles

Abstract: HIGH COERCIVITY COBALT DOPED MAGNETIC IRON OXIDE PARTICLES ARE MADE BY DOPING AN IRON OXIDE WITH COBALT, CONVERTING THE MATERIAL TO A COBALT DOPED GAMMA FERRIC OXIDE CONTAINING A SMALL AMOUNT OF DIVALENT IRON AT A HIGH TEMPERATURE AND SLOW COOLING THE HOT MATERIAL IN AN INERT ATMOSPHERE.

High-performance magnets of rare-earth intermetallics due to an unusual magnetization process

Abstract: Due to the crystalline field, the rare-earth atoms in low symmetry may show a high magnetic anisotropy. In such materials, especially in Dy 3 Al 2 and TbNi 1-x Cu x ( 0 ), because of the high anisotropy, the energy of narrow walls can be smaller than that of Bloch walls. Such narrow walls are frozen as long as the applied field is smaller than a critical field approximately equal to the coercive field. The coexistence of a large remanent magnetization and a large coercive field associated with this new type of domain wall suggests that such materials may be highly feasible for permanent magnets.

Magnetic Aftereffect in Oxidized Ni–Fe Films

Abstract: Displaced hysteresis loops, loop contraction, and increased coercivity are measured in oxidized 76–24 Ni–Fe film at low temperatures at frequencies of 0.05, 20, and 100 Hz using the Kerr effect. Switching in constant applied field by slow wall motion is also observed. A calculation of the observed temperature and frequency dependence of coercivity and loop displacement, based upon the magnetic aftereffect of fine antiferromagnetic oxide particles which are exchange coupled to the ferromagnetic film and have a distribution of anisotropy and coupling energies, is described.

Growth and Some Magnetic Properties of GdIG Films

Abstract: The growth and magnetic properties of GdIG films grown by chemical vapor deposition on YIG and GdGaG substrates is described. Various aspects of the deposition technique including the design of a concentric tube reactor, hydrodynamics of the gas flow, substrates, and deposition parameters are discussed in detail. GdIG films have been grown at rates as high as 16 μ/h. They are epitaxial and are of the same orientation as the substrate. The films are strained because of mismatch between the film and substrate. A Faraday effect hysteresigraph has been used to measure the coercive force and the compensation temperature. The remanence has been measured as a function of temperature from −40° to 60°C and has been correlated with strains in the film. Diffusion has been observed between the film and substrate. This can be minimized by increased growth rates and by preheating substrates.

The magnetic and mechanical hardness of the intermetallic compounds SmCo5 and LaCo5

Abstract: The magnetisation, coercivity, HC, and remanence coercivity, HR, have been measured for the intermetallic compounds SmCo5 and LaCo5 The coercivities HC and HR for SmCo5 are very much greater than those for LaCo5 The differences in these parameters are much greater than would be expected from a simple theoretical model, so that they cannot be accounted for in terms of differences in the magnetocrystalline anisotropy constants Since the specimens used for the magnetic measurements were produced by mechanical comminution, Knoop hardness measurements were made in an attempt to account for the magnetic behaviour in terms of the crystallographic damage and plastic deformation produced during the grinding process The hardness results show that, within experimental error, the SmCo5 is very nearly isotropic, whereas the LaCo5 is very anisotropic on the {10¯10} planes with a Knoop hardness of 138 in the 〈0001〉 directions and 511 in the 〈12¯10〉 It is concluded that plastic deformation will occur more easily in LaCo5 and that this could, to some extent, explain the comparatively low coercivities

Temperature dependence of magnetic properties of oriented MnBi films

Abstract: The temperature dependence of the Faraday rotation and the coercive field of oriented low-temperature phase and quenched high-temperature phase MnBi films was measured between 20 °C and 400 °C. A dependence of the Curie temperature of low temperature phase films on the film thickness was found and measured.