Showing papers on "Single domain published in 1981"

Kinetic effects in coercivity measurements

Abstract: The coercivity of a magnetic material is a crucial parameter in determining its use in recording. The apparent value of this quantity is found to depend upon the time scale of interest. A method of measurement that observes changes of magnetization on a short time scale will yield a larger coercivity than one that measures changes over a longer time scale. The cause of this phenomenon is thermally assisted reversal of magnetic domains. Thus, for a magnetic recording material, the coercivity relevant to a high-speed writing process is greater than that relevant to transition broadening and long-term storage stability. The difference between the short-and long-term coercivities becomes more pronounced as the size of the magnetic switching unit becomes smaller. Therefore, these kinetic effects will become increasingly important in determining the ultimate limitations of future high-density media. Data are presented that illustrate the variation of coercivity with time scale in presently used and experimental recording materials.

A review of coercivity mechanisms (invited)

Abstract: Mechanisms of magnetization reversal and factors determining coercivity are reviewed for various permanent‐magnet materials.

Co energy and magnetization anisotropies in RCo5 intermetallics between 4.2 K and 300 K

Abstract: The magnetocrystalline anisotropy has been known for a long time to be very large in the RCo5 intermetallics. The recent availability of high D.C. magnetic fields up to 200 kOe has enabled us to carry out an extensive experimental study of the energy and magnetization anisotropies in YCo5 from 4.2 K to 300 K. The magnetization anisotropy reaches 4% at 4.2 K. From a polarized neutron study on NdCo5 we find that this anisotropy of magnetization originates mainly from the 2c site. Previous measurements on YCo5 had shown that the orbital contribution to the 2c moment is large. Hence, we conclude that the Co magnetization anisotropy in RCo5 comes from a large orbital contribution to the magnetic moment. Finally, we account for the observed discontinuity in the magnetization at its reorientation in RCo5 ferrimagnets, and for the absence of a similar discontinuity in NdCo5 which is a ferromagnet.

A deuterium NMR study of the discotic mesophase of hexa-hexyloxytriphenylene

Abstract: The discotic liquid crystalline mesophase of p-n-hexa-hexyloxytriphenylene was studied using deuterium NMR spectroscopy. Spectra of two selectively deuterated isotopic species were recorded : (i) in which all aromatic positions were substituted, and (ii) in which the substitution was made in the α-carbon side-chains. Measurements were also made on a deuterated probe compound (C6D 6) dissolved in normal hexa-hexyloxytriphenylene. Samples prepared by slow cooling of the isotropic liquid in a sufficiently strong magnetic field ( > 20 kgauss) consist of domains whose directors are evenly distributed in a plane perpendicular to the field direction. Once formed, the various domains conserve their original orientation (at least in a field as high as 23 kgauss) thus making it possible to record spectra at different orientations of the magnetic field. The results of such experiments provide information on the distribution of the director and on the order parameter of the liquid crystal. For the rigid aromatic moiety of the molecules the order parameter ranges between 0.90 and 0.95 in the mesophase region and is very weakly temperature dependent even close to the clearing point. The results for the quadrupole splittings of the aliphatic deuterons suggest that the alkyl side chains are quite disordered. This is in agreement with the short effective radius of the discotic columns determined by X-ray studies in certain discotic mesophases. Finally it is demonstrated that a single domain of the discotic mesophase can be obtained by allowing the mesogen to cool slowly from the isotropic liquid in a magnetic field, while spinning the sample about an axis perpendicular to the field direction. The single domain is formed with its director parallel to the spinning axis.

Domains, domain walls and the coercive field of amorphous ferromagnets

Abstract: Magnetic properties of the hysteresis loop of magnetostrictive amorphous ferromagnetic alloys are predominantly determined by the magnetoelastic coupling energy between the spontaneous magnetization and internal stresses. Domain structures as well as the pinning of domain walls are governed by long-range and short-range stresses, respectively. As sources of elastic stresses we have detected quasidislocation dipoles exerting stress fields which vary as 1/r2. The coercive field is shown to be determined by four types of interactions : 1) volume pinning by stress sources; 2) intrinsic fluctuations of material properties; 3) surface irregularities; 4) relaxation phenomena.

Some aspects of magnetic viscosity in subaerial and submarine volcanic rocks

Abstract: Summary. The magnetic viscosity of 334 Upper Tertiary and pre-Bruhnes Quaternary volcanic rocks from the Massif Central (France) and the Steens Mountain (Oregon), and of 40 basaltic cores from DSDP leg 37 has been investigated. Thellier's viscosity index follows a log normal distribution with mean values equal to 6 and 3.5 per cent for subaerial and submarine rocks respectively. For subaerial rocks, the average intensity of the viscous remanent magnetization (VRM) acquired in the Earth's field since the beginning of the Bruhnes polarity epoch (t= 0.7 Myr) is estimated to be equal to one-quarter of the average intensity of the primary remanence. Alternating field demagnetization of VRMs acquired in low fields for acquisition times t ranging from 2 day to 32 month indicates the resistance to alternating fields is quite different from sample to sample and increases linearly with log t. Neel's diagnostic parameter of domain structure of the grains involved in magnetic viscosity shows that hard VRM is carried by single-domain grains and soft VRM carried by multidomain particles. Single domain particles carrying hard VRM in subaerial volcanic rocks are almost equant magnetite intergrowths with size near the superparamagnetism threshold, resulting from high temperature oxidation of titanomagnetite. Soft VRM is carried by low Curie point homogeneous titanomagnetite. Unlike his single domain theory, Neel's multidomain theory of magnetic viscosity does not account quantitatively for the resistance of VRM to alternating fields.

Thin film magnetoresistive head

Abstract: Flux sensitive apparatus, in one form thereof, employs paired thin single domain magnetic film structures, the easy magnetization axes of which are canted with respect to the direction of sense current flow therewithin. The magnetization vectors of the film structures are opposite with respect to each other, thereby to preclude, or at least lessen, demagnetization fields associated with the structures. By special connections between the thin film structures, the respective sense currents therein are provided with the same spatial direction of flow; and mutual biasing of the structures is thereby avoided.

The horizontal magnetic dance of the honeybee is compatible with a single-domain ferromagnetic magnetoreceptor

Abstract: Although honeybees are able to sense the geomagnetic field, very little is known about the method in which they are able to detect it. The recent discovery of biochemically precipitated magnetite (Fe 3 O 4 ) in bees, however, suggests the possibility that they might use a simple compass organelle for magnetoreception. If so, their orientation accuracy ought to be related to the accuracy of the compass, e.g. it should be poor in weak background fields and enhanced in strong fields. When dancing to the magnetic directions on a horizontal honeycomb, bees clearly show this type of alignment behavior. A least-squares fit between the expected alignment of a compass and this horizontal dance data is consistent with this hypothesis, and implies that the receptors have magnetic moments of 5 × 10 −13 emu, or magnetite volumes near 10 −15 cm 3 . Additional considerations suggests that these crystals are slightly sub-spherical and single-domain in size, held symmetrically in their receptors, and have a magnetic orientation energy of approximately to 6 kT in the geomagneticfield. A model of a magnetite-based magnetoreceptor consistent with these constraints is discussed.

On the properties of two-dimensional dipoles and magnetized bodies

Abstract: Several little-known relationships between the two-dimensional magnetic dipole and its external field are discussed in both the spatial and spatial frequency domains. Extensions of the theory to arbitrary magnetization patterns are made, which lead to the familiar Wallace results as a special case. The magnetization conditions yielding one-sided flux patterns are analyzed. Several of the results are expressed in matrix notation, facilitating brevity. Finally some limitations on determining internal magnetization patterns from measurements of the external field alone are discussed.

Gyromagnetic remanence and anisotropy in single-domain particles, rocks, and magnetic recording tape

Abstract: Experimental results are described for the gyromagnetic remanence acquired by two rock samples and a sample of magnetic tape arbitrarily orientated in a non-magnetic holder. The remanence (gyroremanent magnetization—GRM) is produced in standard ‘demagnetization’ equipment when the sample is exposed without rotation to a strong alternating field in the absence of any other fields. This effect has already been qualitatively explained (Stephenson 1980 c) in terms of an effective transient biasing field resulting from the intrinsic angular momentum associated with the moments of single-domain particles as they are forced to undergo irreversible flips in the strong applied field. By constructing a theoretical model of a three-dimensional anisotropic distribution of uniaxial particles, a consideration of the average flip of the assembly using this transient field approach enables expressions for the dependence of GRM on field-axis orientation to be derived. Use of these expressions in conjunction with ...

Magnetization process and magnetostriction of a four percent Si-Fe single crystal close to

Abstract: The magnetization process in a cube-on-edge Si-Fe single crystal whose tilt angle θ of the [001] axis to a specimen surface is small is experimentally clarified and explained from the viewpoint of the magnetic energy. The relation between the magnetostriction and the domain structure varying with induction is made clear. It is shown that the magnetostriction in the specimen used is quantitatively explained from consideration of the variation of volume of the 90° domain within a lancet structure, which expands or contracts accompanied with a displacement of the main 180° walls in the region of 0 \leq B and also with variation of the magnetizing force that directly acts upon the lancet domains in the region of B_{merge} \leq B .

Toward a better theory of thermal remanent magnetization

Abstract: The major results of this paper are the pointing out of deficiencies in existing thermal remanent magnetization theories for nonuniformly magnetized grains and the suggestion of ways in which some of these deficiencies might be removed. Most models of pseudo single domain behavior fail to predict certain critical experimental observations of TRM, particularly those dealing with magnetic stability. At least part of the problem with understanding TRM in small nonuniformly magnetized grains is that the size and role of the demagnetization field has been grossly overestimated. TRM in small multidomain grains is probably acquired by domain wall movement, except for the smallest (submicron) grains in which pseudo single domain behavior associated with wall moments probably dominates. A collective response of domain walls seems to be required to explain magnetic stability observations, although the degree and the precise nature of this collective response has not yet been determined.

Coercivity and unit particle size of metal pigment

Abstract: Iron based metal pigments exhibiting coercivity as high as 1000-1400 Oe, and iron nitride (Fe 4 N) pigments exhibiting coercivity as high as 500- 800 Oe for magnetic recording media was prepared from acicular particles of iron-cobalt oxalate, iron oxalate, iron oxides or iron oxyhydrates by reduction in hydrogen or following nitridation in ammonia. The particles of the pigment maintained the characteristic original shape of the oxalates, oxides or oxyhydrates. In addition, the particles contained a number of pores in them, and the particles consisted of small unit particles linked together to form a stereo-network structure. The coercivity of the pigment was related to the pore size, the specific surface area and the crystallite size. A study of the unit particle which is essentially related to the single domain particle was made. The size of the unit particle was determined by an electron microscope (D1), calculated from the pore size (D2), calculated from the specific surface area (D3) and determined by X-ray diffractometry (Lc). At the higher coercivity level of the metal pigments, D2, D3 and Lc are all in good agreement, that is, they all have a nearly spherical single crystallite as large as 200-300 A which plays the role of a single unit particle. In the case of iron nitride (Fe 4 N) pigment, D2 and Lc are in good agreement, at the value of 300-400 A.

Fine metal particles having super high coercivity

Abstract: Fine iron-cobalt particles with coercivities as high as 2200 Oe have been obtained. The particles were precipitated from an aqueous solution using sodium borohydride. The coercivity was studied as a function of composition with the maximum coercivity found at 65:35 (Fe:Co ratio). The coercivity was found to arise from two effects. The first is shape anisotropy due to the chain of spheres structure of the particles. The second is a high magnetic surface anisotropy. No evidence of a contribution from magnetocrystalline anisotropy was found.

Ferromagnetic liquids - Their magnetic properties and applications

Abstract: Ferromagnetic liquids are here defined as colloidal suspensions of single domain particles with diameters between 40-200 A. The magnetic properties are best described as superparamagnetic and the classical theory of Langevin paramagnetism, modified to include a particle size distribution, applies to these systems. Liquids have been prepared with saturation magnetisations between 50 G and 1000 G, their viscosity increasing from approximately 1 cp to 104cp as the magnetisation increases. The colloidal particles have magnetic, hydrodynamic and physical sizes which are different. The mode of magnetisation may, therefore, be complex arising either through physical rotation of the particle (Brownian rotation) or a rotation of the magnetisation within an essentially static particle (Neel rotation). The actual process can have a profound effect on the properties of the ferromagnetic liquids. Ferromagnetic liquids have a viscosity which is anisotropic in a magnetic field and a magnetic birefringence which produces interesting domain observations in polarised white light. The use of ferromagnetic liquids in devices is very closely related to their stability in magnetic field gradients and a criterion in terms of the change in particle concentration in the field gradient ∇/n <1cm-1can be defined. Although stable ferromagnetic liquids are currently used in rotating shaft seals and as damping fluids the use of the unique properties of ferromagnetic liquids in devices has not yet been fully exploited.

The dependence of the coercivity on the volume packing fraction for magnetic particles

Abstract: The advanced method of the dilution of magnetic particles with α-FeOOH particles was studied. This method permits the determination of the dependence of coercivity on the packing fraction and the evaluation of Hc(0) has become possible. The coercivity of Co-adsorbed γ-Fe 2 O 3 or CrO 2 particles decreases rather slowly with the packing fraction compared with γ-Fe 2 O 3 or Co-doped γ-Fe 2 O 3 particles. This behavior is interpreted by the existence of some nonmagnetic surface layers on the particles. These layers are thought to make the magnetic particle interactions weak to prevent the apparent coercivity from decreasing steeply.

Magnetization curve of zero‐anisotropy sphere

Abstract: A specific Ritz model is suggested, in which magnetization curls around the outer part of a sphere, while an inner core is saturated. This model is used to calculate the magnetic field dependence of the magnetization in perfect ferromagnetic spheres, with no anisotropy. For sufficiently small radii, the results indicate an infinite permeability at zero applied field, which brings the average magnetization to an appreciable part of its saturation value. This is then followed by an almost linear dependence of the average magnetization on the applied field, practically up to saturation.

A new colloid technique enabling domain observations of Si--Fe sheets with coating at zero field

Abstract: A new method using a wetting agent in addition to a water-based magnetic ink allows domain observations of coated Si-Fe sheets without applying a normal field. No electrical apparatus whatever is needed to produce line, spike, tree, and other patterns on conventional g.o. or HI-B sheets of arbitrary size and shape.

Method for making large area stable domains

Abstract: A method for producing a large area of single domain magnetic bistability is shown including the steps of selecting the properties of films of magnetic materials such that the saturation field (H s ) that is less than the value of the anisotropy field (H k ) reduced by the value of a demagnetization factor (4π) times the magnetization value (M s ) as stated in the equation: H.sub.s

Principles of a three-dimensional recording model for short wavelength magnetic recording

Abstract: Principles are provided for a method of evaluating the magnetization of a magnetic recording tape element running in front of a recording gap. The distributions of switching fields P(Hc) and magnetization axes P(θ, φ) used in the calculations are derived from simple experimental results. The output level is calculated by means of the reciprocity principle. The first application of the method concerns certain phenomenological aspects of magnetic recording. As a practical example, a qualitative explanation of the output level curve versus the recording current for short wavelength signals is given; the advantage of microgaps for recording are demonstrated and finally, the circular Bitter patterns discovered by S. Iwasaki are shown to be predicted by the model. This study is a first step toward a more general solution including the demagnetizing field.

Prediction and measurement of rotation of magnetization in an anisotropic polycrystalline ferromagnetic material

Abstract: Many polycrystalline ferromagnetic materials are anisotropic; as a result, flux density B and field strength H are not in general parallel to each other. A simple analysis of this effect is given for two-dimensional magnetization. The spatial angle between B and H is predicted from the shapes of the B versus H loops in directions of maximum and minimum permeabilities. Experimental results are given that verify the analysis and show that in some cases B and H rotate in opposite directions, each through 180°, as the B versus H loop is traversed.

A model of magnetic bubble propagation patterns using surface charge embedded in a minimum domain configuration

Abstract: A magnetostatic model of thin film polygon shapes is developed as a design aid for conventional field access magnetic bubble devices. A finite surface element treatment of a general polygon shape represents the magnetization by surface charge residing on domain walls and polygon sides. Domain walls are located according to an algorithm which partitions an arbitrary polygon into a minimum number of domains. The system energy is expressed in terms of magnetic charge density on the surface elements, and minimization of the energy produces the solution for the linear magnetization problem. The accuracy of the two-dimensional analysis is improved by including an approximate treatment of charge distribution on the thin film surfaces. The relatively small number of unknowns permits the analysis of several interacting polygons in device configurations. The normal magnetic field is averaged over the thickness of the bubble material, and contour plots of this field are made for each applied field orientation. The field plots are shown to be useful for initial screening of new device ideas and for predicting performance differences among similar designs.

Effects of different rare earths on the temperature dependence of rare earth cobalt magnets

Abstract: In many applications, it is necessary to have permanent magnets whose characteristics vary little around a temperature T r . We have developed a calculation to describe the thermal variation of the magnetization of the rare earth (RE) in such magnets including the effects of crystalline field. The exchange field of cobalt has been deduced at all temperatures from magnetization measurements on a single crystal of YCo 5 . From the calculation we can deduce the concentration x 0 , which gives dM/dT = 0 around room temperature, the resulting magnetization and the corresponding d^{2}M/dT^{2} .