Showing papers on "Single domain published in 1969"

Magnetoviscosity of Magnetic Colloids

Abstract: We have observed that the viscosity of a dilute (≈ 1015 particles cm−3) colloidal suspension of single domain ferromagnetic particles is a marked function of both the magnitude and direction of an externally applied magnetic field. For particles with dimensions of a few hundred angstroms, the viscosity saturates in a field of order 1 kG. The additional viscosity is greater by a factor of 2 for the field parallel to the flow than with it perpendicular. This magnetoviscous effect is qualitatively accounted for by the hindrance to particle rotation caused by the magnetic torque.

An experimental evaluation of single domain theories

Abstract: The Neel theory of magnetization relaxation has been systematically and quantitatively tested for six samples containing synthetic and natural single domain grains. The results of a wide range of experiments involving time, temperature, and direct and alternating magnetic fields were compared with theoretical results predicted by using the Neel theory and the experimental grain size-coercivity distribution of each sample. Good agreement was found for many experiments involving small field remanences, where grain interaction effects are negligible. Most of the discordant results could be explained by using either the Preisach theory or an extended form of the Neel theory, which takes account of interactions. The two methods are shown to be equivalent, but the Neel theory is more useful in analyzing thermal processes that are intractable by the Preisach method. The similarity of the behavior of synthetic and natural samples suggests that monodomain material is important in many rocks, but experiments on a wide variety of rock types are needed for verification.

Effect of a Magnetic Field on the Superparamagnetic Relaxation Time

Abstract: The relaxation time for the fluctuations in the direction of the magnetization vector is calculated for very fine, single-domain ferromagnetic particles which have a uniaxial (shape or crystalline) anisotropy. A magnetic field is assumed to be applied parallel to the easy magnetization axis, and the relaxation time is computed for various magnitudes of this field. It is found that the commonly used approximation for high-energy barriers is about as justified as in the case of zero magnetic field, down to barriers of the order of $\mathrm{kT}$.

Material requirements for circular magnetic domain devices

Abstract: The material requirements of circular cylindrical domain devices are discussed. The current status of the search for circular domain materials is summarized with respect to the anisotropy field H k and the saturation magnetization M s by a chart showing the influence of these quantities on the size, stability and mobility of the circular domains. A preferred region is defined on the chart and representative materials are plotted. At the present time, the preferred region is most closely approached by certain mixed rare-earth orthoferrites and the aluminum substituted hexagonal ferrites.

Observations of magnetization reversal in cobalt-rare-earth particles

Abstract: A new technique for the separation of strongly magnetic powders by particle size has led to observations of discontinuous magnetization changes in samples of cobalt-rare-earth particles. These magnetization jumps were investigated in a single particle, and it was found that they occurred at various reproducible and strongly quantized fields, whose value depended on the value of the field previously applied to saturate the sample. This suggested that the coercive force of these materials might be very sensitive to the nature of the surface, which was verified experimentally. These results add support to the interpretation of the magnetic properties of high-crystal-anisotropy permanent magnet materials in terms of domain boundary nucleation and motion.

Magnetic Properties of Ferromagnetic Metal Alloy Fine Particles Prepared by Evaporation in Inert Gases

Abstract: Single domain fine particles about 150 A in diameter, of alloys of Fe, Co and Ni were prepared by evaporation in an atmosphere of argon at 3 Torr The particles were coupled with each other like a necklace The X-ray diffraction analysis revealed that the lattice parameters of the alloy particles agreed with those of the original bulk alloys within an experimental error of 03% It was indicated, however, that they might be smaller than the latter within that error The maximum coercive force observed in Fe–Co alloys was about 15 kOe It was found that the oxidation of Fe–Co alloys can be prevented by adding some Ni The magnetic orientation of particles caused by application of a magnetic field during the evaporation was studied In a magnetic field of 05 kOe the necklace structure was aligned parallel to the field and a magnetic anisotropy of 35×105 erg/g appeared in this oriented sample This strong anisotropy is considered to be due to a shape anisotropy induced by alignment

Anhysteretic Magnetization of an Assembly of Single‐Domain Particles

Abstract: A kinetic analysis of anhysteretic magnetization is presented that departs from previous analyses by including the time dependence of particle switching. Each particle is assigned a switching probability which depends upon the applied field and its period. The growth of anhysteretic remanence is shown to occur in a narrow range of fields immediately below the particle coercivity. In contrast to previous analyses, the kinetic analysis predicts a finite anhysteretic susceptibility for noninteracting particles that is large but not infinite as previously supposed.

Behavior of circular domains in GdIG

Abstract: A close-packed array of circular domains was formed by a pulsed magnetic field in a 1-mil-thick slab of

Nucleation of magnetization reversal in ESD magnets

Abstract: The nucleation field, by the curling mode, is calculated for an ideal chain of identical ferromagnetic spheres as a function of the angle between the applied magnetic field and the chain axis, and of the radius of the spheres, neglecting magnetocrystalline anistropy. Computations for an infinite chain using first- and second-order perturbation theory yield practically the same results so that higher order terms can certainly be neglected. Comparison with experiment will be possible only when measurements are carried out on larger radii than those experiments which have been done so far.

Magnetostatic effects in grain-oriented silicon iron

Abstract: The function of the free-surface poles appearing on oriented silicon-iron strips in controlling the domain structure is discussed The energy of a simple 180° domain structure is given as a function of the dimensions, angle of dip, and applied field; this model is shown to give high values of the magnetostatic energy for angles of misorientation greater than 2° On general theoretical grounds, the proportion of 90° domains is calculated as a function of the misorientation and is shown to agree with experiment A detailed model of the 90° domain structure is discussed; its magnetostatic energy is estimated and predictions of the equilibrium domain size are in reasonable agreement with observation

The temperature dependence of the coercivity of a random array of uniaxial single domain particles

Abstract: Calculations of the temperature dependence of the coercivity of a random array of non-interacting single domain particles of uniaxial anisotropy are presented. The extent of the coercivity variation in a given temperature range is found to depend upon the particle volumes, as well as on the anisotropy energy. Under special circumstances, these variations may be regarded as being approximately linear.

Piezomagnetization of Single-Domain Grains: A Graphical Approach

Abstract: The remanent magnetic properties of single-domain grains under uniaxial compression are easily interpreted by a graphical method based on the Neel thermal fluctuation theory, provided the stress σ and the magnetic field H are parallel. The change in remanence resulting from a change in magnetic field, stress, or temperature can be represented by the area swept out by a moving “blocking curve” on a graph of grain volume, v, versus microscopic coercive force, Hc. Applying or removing σ moves the blocking curve parallel to the Hc axis a distance 3λσ/Js. This graphical method is useful for visualizing pressureinduced remanence (PRM) and pressure demagnetization. If the grain distribution n(v, Hc) is known, it is also suitable for quantitative analysis. The theory explains some observed relationships between the intensity of PRM and the order in which σ and H are applied and removed, and predicts that these different types of PRM should have measurably different stabilities against AF, thermal, and pressure demagnetization.

Production of pulse magnetic fields higher than 400 kOe and their applications to various magnetic measurements

Abstract: By using a condenser bank of the capacity 9000 μF and the working voltage 33 kV, magnetic fields higher than 400 kOe have been produced in a pulse form, the duration of which is about 1 ms In order to avoid the mechanical breakdown of the coil, various types of coils including a toroidal transformer-type device have been constructed Using these fields, various experiments have been performed Magnetization parallel to the c axis (hard axis) of the Tb single crystal in the magnetic fields of 200 to 620 kOe at 77°K caused an appreciable lattice elongation parallel to the c axis, which resulted in an appearance of the local anisotropy with its hard axis perpendicular to the c axis Measurements of the magnetoresistance effect of dilute alloys and the high field susceptibility of ferromagnets are also described

Anhysteretic magnetization processes in multidomain crystals and polycrystals

Abstract: The third model is found to be unrealistic, for a polycrystalline material without any second phase at the grain boundaries, and it is shown that the observed magnetization behaviour may be more generally accounted for in terms of the misorientation between grains. The calculations are precise, but the results are dependent on the initial choice of model domain structure and any limitations imposed on the way in which the walls move. 1. INTROtDUCTION Real domain structures vary greatly in character, according to the intrinsic properties and microstructure of the specimen involved, and certain simplifications are necessary for the calculation of domain structure to be practicable. The first is to deal only with equilibrium structures, where the domain walls are in a position of minimum energy, i.e. to ignore any resistance to wall motion. The next simplification is to deal only with 180? domains; uniaxial materials contain only 1800 domains, and oriented cubic materials contain predominately 180? domains, and may be considered to be uniaxial as an approximation. The magnetic materials are supposed to have high anisotropy, so that the It effect is negligible. These restrictions are complied with in many experimental materials. A further restriction of a more fundamental nature, which is unavoidable in domain theory, is the necessity to devise a plausible structure for analysis. Since the analysis proceeds by minimizing the energy, we normally postulate a structure whose geometric form and energy can be described as a function of a limited number of parameters. The total energy of the system is then minimized with respect to these parameters. The form of the solution is thus imposed by the initial choice of domain structure and the freedom of movement. This objection to domain theory is discussed by Brown (I962), who would prefer the entire domain structure to be obtained from first principles as a solution of the fundamental equations. This micromagnetic approach has been used successfully in the treatment of single domain particles, but unfortunately becomes of insuperable complexity when dealing with a multidomain structure. In this paper we consider the domain structure in an applied field of an isolated

A theory of the shape of spike-like magnetic domains

Abstract: Spike-like magnetic domains play several important parts in the magnetization processes occurring in a hysteresis cycle, especially in a material magnetized in an easy direction of magnetization, such as transformer sheet. This theory is applicable to the spike domains observable in bulk magnetic materials but not, unfortunately, to thin magnetic films. A comparison was made between the calculated shape of a spike domain and that of such domains in 3% silicon-iron, the latter being available in the form of photographs. As far as could be determined from the photographs the calculated shape was an exact fit to the observed shape.

Arc and spark extinguishing contacts utilizing single domain magnetic particles

Abstract: This invention relates to contacts which have about 0001 to 96 percent by weight of ferromagnetic or ferrimagnetic permanent magnetic particles embedded in the contacts such that the magnetic particles are distinguishable from the basic contact material The Curie temperature of the magnetic particles used in the contacts are greater than about 250* C and the magnetic particles have an average spontaneous magnetic energy product greater than about 5,000 GaussOersted The magnetic particles having spontaneous magnetic energy can also be incorporated in certain contact configurations to also obtain excellent noise suppression characteristics

Investigation on the Magnetization Structure of Polycrystalline Uniaxial Ferrites with the Torsion Pendulum Method. I. Calculation of the Magnetic Stiffness

Abstract: Using the formulas for the magnetic stiffness (torque per radian) of a ferromagnetic single crystal the stiffness of a polycrystalline sample performing small rotational oscillations in a homogeneous magnetic field with the axis of rotation perpendicular to the field direction is calculated. The sample is assumed to consist of non-interacting crystallites with positive uniaxial magnetic anisotropy. The crystallites may be either in the single domain state or may be splitted up into domains according to the phase theory. The orientation distribution of the crystallites is supposed to have rotational symmetry with respect to a preferred direction.

Hard magnetic two-phase Co-Cu thin films

Abstract: The magnetic properties of sputtered films of 25 Co-75 Cu and 50 Co-50 Cu before and after annealing were investigated. In the as-sputtered state the films exhibit the structure of a metastable fcc solid solution. Annealing at 500 to 700°C causes decomposition into two phases, Cu and fcc 89 Co-11 Cu. The decomposition supposedly occurs by heterogeneous nucleation at the grain boundaries, and growth by grain boundary diffusion. The 89 Co-11 Cu phase exists in the form of small particles with magnetic single domain behavior. The films have coercivities up to 280 Oe. Squareness ratios between 0.7 and 0.9 were found. No strain sensitivity of the magnetic properties could be detected. This material is regarded to be suitable for magnetic recording.

A method for calculating hysteresis loops of interacting single-domain particle systems in a nonmagnetic binder

Abstract: A method is given for calculating the minor and full hysteresis loops of a system of uniaxial single-domain particles with a common direction of easiest magnetization. A probabilistic approach is used in formulating the problem by means of two differential equations which are solved for the simple case of an infinite particle chain. The method is based on a model in which each particle is characterized by a rectangular hysteresis loop with a specific saturation field H s , and the loops are influenced by the (homogeneous) external magnetic field H as well by the magnetization-dependent interaction of the particles among themselves.

On the magnetization distribution about a magnetic particle in an iron film

Abstract: The form of the magnetization distribution in a ferromagnetic material about a single magnetic particle has been calculated and is compared with that observed by transmission electron microscopy near an inclusion in an iron film. It is suggested that regions of the theoretical distribution, where the magnetization direction at neighbouring points changes rapidly, could give rise to domain walls such as those observed in the film.

Ultrasound and magnetic domain structure in polycrystalline materials

Abstract: Ultrasound magnetostrictively stimulates vibrations of Bloch walls and periodic rotations of the magnetic moments of the domains. If the stress amplitudes are small the domain configuration remains unaltered. When compared with the measured reversible susceptibility, measurements of the attenuation and the velocity of sound as functions of magnetization provide information about some aspects such as domain width and the occurrence of different elementary processes during magnetization. Using sound waves with propagation direction normal to the external magnetic field, one gets as a new variable the angle between the field and the oscillation direction. The results of these measurements cannot be discussed without contradicting classical ideas concerning te magnetostrictive excitation process.

The domain structure of alloys YuNDK24, YuNDK35T5, and YuNDK40T7 in the high-coercivity condition

Abstract: 1. After optimal TMT, domains of magnetic interaction appear in the form of a band system only in single crystals of the YuNDK24 alloy. 2. Rearrangement of the domain structure in single crystals of alloy YuNDK24 is observed in fields corresponding to the steep sections of the hysteresis loop. 3. The domain structure depends on the magnitude and the uniformity of the demagnetizing field; the more intense and the less uniform the field, the more distinct the domain structure.