Showing papers on "Single domain 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.

Micromagnetic studies of thin metallic films (invited)

Abstract: A computer simulation model has been developed to conduct micromagnetic studies of thin magnetic films. Thin‐film media are modeled as a planar hexagonal array of hexagonally shaped grains. Each grain is a single domain particle whose magnetization reverses by coherent rotation. The computation utilizes coupled gyromagnetic dynamic equations with phenomenological Landau–Lifshitz damping. In particular, the effects of particle interactions are investigated. The effect of media microstructure on magnetic hysteresis is examined as well as the effect of intergranular exchange coupling. The difference between planar and completely random orientation of the crystalline anisotropy axes is discussed. Recorded transitions are simulated by allowing a pair of perfect transitions to relax. With no intergranular exchange coupling, the transitions show profound irregularity and zig‐zag structure. Intergranular exchange coupling produces more uniform transitions with increased zig‐zag structure amplitude. For a closely ...

Single‐domain particles in rocks and magnetic fabric analysis

Abstract: Magnetic susceptibility anisotropy is commonly used as a guide for determining stress and flow axes in rocks, and for assessing the suitability of anisotropic rocks for palaeomagnetic purposes. We have recently demonstrated however in laboratory produced samples that, as theoretically expected, susceptibility anisotropy is dependent on particle size. Multidomain particles of magnetite, or similar strongly magnetic particles with shape anisotropy, exhibit a maximum susceptibility parallel to their easy (remanent) magnetization axes whereas uniaxial single-domain particles exhibit a maximum susceptibility perpendicular to their easy magnetization axes. Hence susceptibility anisotropy does not yield an unambiguous determination of particle alignment in rocks independent of domain state. By demonstrating uniaxial single-domain anisotropy characteristics for the first time in a rock specimen, we show here that susceptibility measurements by themselves may not correctly identify the degree or type of alignment of ferro- or ferrimagnetic particles in rocks. In particular, apparently foliated rocks can actually be lineated (and vice-versa). Moreover rocks with low anisotropy of susceptibility can have a much higher anisotropy of remanence. Remanence anisotropy methods are shown to give unambiguous determinations of the degree and type of alignment.

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.

Coercivity mechanisms in ferrites and rare earth transition metal sintered magnets (SmCo/sub 5/, Nd-Fe-B)

Abstract: Measurements of the temperature dependence of the coercive field, of the magnetic viscosity, and of the angular dependence of the coercive field are reported for ferrite and rare-earth-transition-metal sintered magnets. A satisfactory explanation for the observed properties is obtained in a simple model, considering that magnetization reversal is initiated in a volume equal to the activation volume and is determined by the formation of a domain wall. From magnetic viscosity measurements, the activation volume is found to be proportional to the cube of the domain wall width, delta . The observed angular dependence of the coercive field reveals that, in the activation volume, the anisotropy is much larger than the coercive field and is not strongly reduced with respect to the bulk. If the coercivity is determined by true nucleation in a fully saturated sample, this is unlike the usual assumption that the magnetocrystalline anisotropy is strongly reduced in the volume of the nucleus. >

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.

Demagnetizing field computation for dynamic simulation of the magnetization reversal process

Abstract: The magnetic-field distribution for a thin magnetic film is computed using the fast Fourier transform technique. The method is quite general and accommodates any two-dimensional magnetization distribution. It allows the computation of fields both inside the film (demagnetizing fields) and outside (stray fields and leakage). >

Thermal demagnetization of partial thermoremanent magnetization

Abstract: Three well-characterized glass-ceramic samples containing magnetite crystals of different domain states have been given partial thermoremanent magnetizations (pTRMs) in the temperature interval 400°–350°C for varying lengths of time (10 min to 5 days) and in magnetic fields from 0.5 to 5 Oe (0.05–0.5 mT). The thermal demagnetization behavior of the pTRMs changes with domain state in the sense that with decreasing coercivity (increasing grain size), demagnetization curves become less steep and may not completely demagnetize until heated to the Curie temperature. Numerical calculations based on Neel's single domain (SD) theory invoking field dependence and experimentally determined as well as theoretical volume distributions based on power laws have been performed to predict viscous magnetization acquisition and demagnetization temperatures. We observe that experimental results do not match any of the theoretical predictions. There is no discernible field dependence of demagnetization curves for the SD and pseudo-single domain samples and even for the “soft” multidomain (MD) sample in the field range investigated. An important consequence for thermally derived paleointensity data is that they may not be entirely reliable, especially if the sample remanence is due to MD grains.

Elongated single‐domain ferromagnetic particles

Abstract: The ‘‘fundamental theorem’’ of Brown on spherical particles is extended to prolate spheroids, whose long axis is also an easy axis for a cubic or uniaxial anisotropy. It is rigorously proven that for such a particle, with a minor semiaxis a, the lowest free‐energy state is that of a uniform magnetization, if a

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. >

Numerical micromagnetics of small particles

Abstract: The results of applying the authors' micromagnetic code to a homogeneous spherical particle that is large enough to support inhomogeneous magnetization are given. For smaller particles, magnetization reversal is by coherent rotation. Larger particles initially exhibit curling as the applied magnetic field is reduced from a saturating value. Then one of two new behaviors is observed. For weak crystalline anisotropy, the axis of the curling state rotates and bends, and the magnetization reversal process is reversible, or nearly so. For strong crystalline anisotropy, a sudden discontinuous transition occurs to a vortex state with axis perpendicular to the anisotropy axis, and the vortex moves across the particle as reversal of the applied magnetic field continues. The formation and disappearance of the vortex are irreversible, but all other aspects of the process are reversible. >

Detection of the magnetization reversal of individual interacting single-domain particles within Co-Cr columnar thin-films

Abstract: The fundamental Barkhausen noise generated by the magnetization reversal of individual particles within a particulate magnetic medium has been observed using the anomalous Hall effect (AHE) as a sensitive magnetization probe. This is the first time the reversal of individual interacting single or nearly single domain particles has been detected. The jumps correspond to magnetic switching volumes of approximately 3*10/sup -15/ cm/sup 3/ with moments around 10/sup -12/ emu. The magnetization in a Co-Cr thin film, a columnar perpendicular magnetic recording medium, is determined by passing a current through the film and measuring the AHE voltage induced within the sample itself. Hall samples as small as 0.6 mu and containing only a few hundred columns have been made via microlithography. For these samples, the voltage change due to the reversal of a single column can be readily detected with a system sensitivity of 4*10/sup -14/ emu. The spatial dependence of the measurement response over the surface of the Hall geometry has been determined by both calculation and experiment, so that the magnitude of magnetization changes detected by jumps in the AHE voltage can be adjusted for geometric effects. >

Magnetoelastic Properties of CexLa1-xB6

Abstract: The thermal expansions and magnetostrictions of Ce x La 1- x B 6 ( x =1.0, 0.5 and 0.25) single crystals in both longitudinal and transverse magnetic fields up to 80 kOe have been performed in the temperature range from 1.39 K and ∼20 K. In the paramagnetic region, the results show the systematic but drastic variations with increasing Ce concentration indicating the importance of the interaction between Ce ions. The magnetic field induced enhancement of the crystal volume shrinkage observed in Phase II indicates that the antiferro-quadrupolar ordering has a magnetic field induced character. The anisotropic expansion of the crystal volume was observed in the single domain state in Phase III.

Cooling rate effects in the magnetization of single-domain grains.

Abstract: The effect of the rate of cooling on the magnetization of assemblies of single-domain grains is solved numerically, and analytically for the specific case of magnetite, with results that can easily be extended to other magnetic compounds. It is shown that to a good approximation the moment produced on cooling from above the transition temperature is proportional to (lnc/q)1/2 where q is the cooling rate constant, and c is a frequency on the order of 108Hz. This result is independent of the grain size and shape distribution.

Theoretical time‐temperature relationships of magnetization for distributions of single domain magnetite grains

Abstract: Experimental studies have demonstrated that demagnetization temperatures for removing thermoviscous magnetizations (TVRMs) in rocks are often higher than predicted by Pullaiah et al.'s time-temperature (t-T) nomogram based on Neel's single domain (SD) theory. Better agreement has been found for t-T relationships based on Walton's calculations invoking volume distributions. It has been pointed out, however, that Walton's formula applies for t-T conditions for acquisition of magnetization only, not to demagnetization. We applied numerical methods for assessing the true influence of distributions in SD volume. As it turns out, for reasonable volume distributions, the differences between our results and Pullaiah et al.'s simpler approach are marginal; the differences in demagnetization temperature are significant only for moderate-term low-temperature acquisition. Discrepancies between experiments and theory appear because rocks contain predominantly pseudo-single and multi-domain grains to which SD theory does not apply.

Magnetization time decay in CrO/sub 2/ tape

Abstract: Measurements are reported of the bulk saturation magnetization, the coercive field, and the rate of magnetization decay per decade of time of IBM 3480 CrO/sub 2/ tape over the temperature range 1.6 K >

A synthetic TRM induction curve for fine particles generated from domain observations

Abstract: A synthetic thermoremanent magnetization (TRM) induction curve is produced from domain observations of an ensemble of pseudosingle domain (PSD) titanomagnetite particles in oceanic basalts. The observations were made after the samples were given a TRM in fields of 0.1, 2, 4, 6, and 8mT. An observation was also made of the domain state for saturation isothermal remanent magnetization (IRMs). The number and maximum size of grains which remain in a metastable single domain (SD) state increases with increasing field. This in turn gives rise to a field dependence of the total moment carried by such particles in the ensemble. By observing the number of metastable SD grains and estimating their volume one can construct a TRM induction curve. This curve approximates the known TRM induction curve for fine particles, demonstrating that the magnitude of TRM in fine particles of titanomagnetites is controlled by nucleation. This nucleation model also explains why weak field TRM is more stable against alternating field (AF) demagnetization than IRMs.

The Status of Domain Theory for an Investigation of Magnetostriction and Magnetization Processes in Grain-Oriented Si-Fe Sheets

Abstract: Magnetostriction in modern (110)[001] grain-oriented Si-Fe sheets can be understood by considering magnetic domain structures and their variation with magnetization on the basis of magnetic domain theory. Magnetostrictive elongation in g.o. sheets is dominated by the behavior of 90? domain walls included in subdomains which appear in the main domains carrying the magnetization of the [001] direction. The relationship among magnetostriction, the static domain structure and process of magnetization is reviewed with respect to g.o. sheets with conventional thickness and ones with extremely reduced thickness. Losses are reviewed in relation to the domain structure and its variation during magnetization.

Thermal cleaning of viscous magnetic-moments

Abstract: We calculate the demagnetizing temperature necessary to remove viscous magnetic moments from assemblies of single domain magnetite grains. It is shown that this cleaning temperature may be the same as that predicted by Pullaiah et al [1975] depending on the fraction of the viscous remanence removed. Experimental data by Williams ]1986] are shown to be in good agreement with the numerical calculations.

A study of the magnetic domains of isolated fine particles of Ba ferrite

Abstract: Isolated Ba ferrite particles are prepared by the conventional powder metallurgical method, and the magnetic domains of about 200 particles 0.5 to 5 µm in size are investigated by the colloid-SEM method (the Bitter method using a scanning electron microscope). The relationship between the three-dimensional sizes of Ba ferrite particles and their magnetic domain states is clarified. It is found that the critical size of isolated single-domain particles of Ba ferrite is somewhat larger than the value calculated using the magnetic domain theory.

Domain theory model for magnetic thin films

Abstract: A domain-theoretic model for external-field-driven magnetization processes in thin-film microelements is described. The film is mathematically described by a dynamic grid of quadrilateral domains whose magnetization directions and vertex locations are variables with which to minimize the total free energy. The magnetostatic interaction between uniformly (in-plane) magnetized polygonal domains is essentially treated exactly. Predicted results for reversal mechanisms and field-induced metastable remanent domain configurations in Permalloy microelements are in good qualitative agreement with Bitter pattern observations. >

Phase analysis and single domain detection in hexaferrite powders for magnetic recording

Abstract: SrFe/sub 12/O/sub 19/, SrInFe/sub 11/O/sub 19/, and SrCo/sub 0.9/Ti/sub 0.9/Fe/sub 10.2/O/sub 19/ hexagonal ferrites were prepared by the metallorganic precursor method to test their characteristics as particulate media for magnetic recording applications. The chemical reactivity and the magnetic characteristics were analyzed as a function of thermal treatment. Reversible transverse susceptibility curves were used to verify the single- or polydomain state of the particles. All the magnetic data are explained on the basis of the phase diagram. It is concluded that the substitution of iron in hexaferrite results in a decreased reactivity and that the more drastic thermal treatment required for optimizing the magnetic properties deteriorates the morphological characteristics of the powders. It is possible, however, to define for each compound a suitable thermal treatment allowing single-domain particles with good magnetic properties. >

Charging on the metastable domain states of small NiFe rectangles

Abstract: The effect of charging on the domain structure in thin NiFe shapes is studied experimentally. Satisfaction of the boundary condition leads to some rather energetic metastable domain structures with a high degree of charging, which often plays an important role in determination of the final domain shape. It is shown that the magnetic history of a rectangle has a strong influence on how much field is necessary to reverse the domain structure. Practical implications of the above results for magnetoresistive sensor design is discussed. >

Effects of magnetic history on the domain structure of small NiFe shapes

Abstract: Buckled domain structures, usually with charge on both surfaces and domain walls, are created after applying a large field (historic field Hh) to rectangles and other odd‐shaped elements and returning to the remnant state. The type of domain structure can be predicted by imposing the boundary condition that Hh⋅ Medge ≥0, where Medge is the magnetization at the edge of the shape. The origin of the boundary condition comes from the high demagnetizing energy necessary to reverse the magnetization at the edges of a thin film. Details of the metastable state are determined by wall and anisotropy energies as well as the demagnetizing energy from surface and domain wall charging.

Application of domain wall stabilization to magnetic sensors

Abstract: The influence of different stresses and annealing conditions is studied to improve the induced helical anisotropy. The possibility of developing a magnetic sensor based on the domain wall stabilization effect to detect low magnetic fields is pointed out. >