Showing papers on "Single domain published in 1987"

Detection of singularities in the reversible transverse susceptibility of an uniaxial ferromagnet

Abstract: The reversible transverse susceptibility χt of a ferromagnet having a positive uniaxial anisotropy has been measured as a function of the applied magnetic field. The presence of peaks in the χt (H) curve of a polycrystal was predicted by theoretical calculations based on the Stoner and Wohlfarth model [A. Aharoni, E. M. Frei, S. Shtrikman, and D. Treves, Bull. Res. Counc. Isr. A 6, 215 (1957)]. Unlike previous measurements, our data verify the predictions of such a theory. An explanation for the failure of previous attempts to detect the peaks in the χt (H) curve is given. It is also experimentally shown that the χt (H) measurement allows a precise determination of both coercive and anisotropy fields in a uniaxial ferromagnet provided it is made by single domain particles.

A micromagnetic study of pseudo single‐domain remanence in magnetite

Abstract: The magnetic structure in fine grains of magnetite has been studied theoretically using a micromagnetic model which is unconstrained except for requiring Bloch-type domain walls. Stable structures were found by second-order numerical minimization of exchange, crystalline anisotropy, and demagnetizing energies. Single- (SD), two- (2D) and three-domain (3D) structures were studied for particles at room temperature and in zero applied field. Two-domain structures with relatively large domains have very small net magnetizations because of “skirts” which oppose wall moments. In particles almost filled with domain walls the net magnetization can be quite high because wall moments are uncompensated. Three-domain structures always have a relatively large remanence due to unbalanced domains. The critical SD-2D equilibrium size d0 was determined to be 0.084 ± 0.012 μm. The size range over which single-domain structures can exist is smaller than previously predicted for perfect crystals. Experimental saturation remanence data on submicron precipitated magnetite crystals can be reasonably well fit with the present results, assuming that magnetic structures assume the lowest energy state. A technique for studying the transition mechanism between stable states is developed which demonstrates that the SD reversal mechanism is coherent rotation in particles around the critical superparamagnetic size, while domain wall nucleation, propagation, and denucleation become favored at larger sizes.

Distinctive characteristics of barium ferrite media

Abstract: Barium ferrite magnetic recording media have demonstrated the capability of achieving much higher recording densities than other particulate media. Even though the best performance is obtained with perpendicularly oriented media, longitudinally oriented and non-oriented Ba-ferrite media also exhibit exceptional recording characteristics. These media have very distinctive magnetic and switching field (SFD) characteristics, exhibiting very small SFD values, almost rectangular remanence loops (indicating no irreversible losses over a wide range of reverse fields), and minor loops very close to the major loop until the driving field becomes almost equal to the coercivity. These unique magnetic and switching field characteristics can explain the very low or negligible demagnetization losses (including recording demagnetization) which we observe in Ba-ferrite media and which result in their outstanding recording performance. The fundamental reasons for these unique magnetic and switching field properties are the predominant high uniaxial crystalline anisotropy of the platelets and their single domain character. On the negative side, the high anisotropy field is directly implicated as the main reason behind the marginal overwrite modulation which characterizes these media.

First‐order field‐induced magnetization transitions in single‐crystal Nd2Fe14B

Abstract: Magnetization measurements in high‐pulsed magnetic fields have revealed the presence of magnetic anisotropy in the hard basal plane of a single crystal of Nd2Fe14B. A discontinuity in the magnetization occurs when the magnetic field is applied along the [100] direction in the hard basal plane below 220 K while there is a regular approach to saturation magnetization along the [110] direction. This discontinuity has been interpreted as a first‐order magnetization process. High‐order terms in the free energy are required to account for the spin reorientation transition from easy axis to easy cone and first‐order magnetization process. The temperature dependence of the uniaxial and planar anisotropy energy constants has been determined.

Magnetization models of Co-Cr film for the computer simulation of perpendicular magnetic recording process

Abstract: The microscopic magnetization model of medium and the statistical calculation of the vectorial magnetization are proposed in order to give a reliable computer simulation of magnetic recording process. The curling incoherent magnetization reversal mode is introduced for the magnetization model of crystalline particles, because the coherent magnetization reversal model is insufficent for the restoration of M-H loop of Co-Cr film having lower coercive force than the anisotropy field strength. Using the newly proposed method, the vectorial M-H loops of Co-Cr film can be restored well by computer. It will be very useful for the computer simulation of perpendicular magnetic recording.

Method and apparatus for non-destructive materials testing and magnetostructural materials investigations

Abstract: A method and apparatus for generating and detecting magnetization responses from ferromagnetic, ferrimagnetic, paramagnetic, or diamagnetic materials comprises a means for generating an alternating magnetic field. The cycle of the alternating magnetic field includes a first magnetization pulse and a second demagnetization pulse, the energy content of the magnetization pulse and the demagnetization pulse being approximately equal, the time duration of the magnetization pulse being substantially longer than the time period of the magnetization pulse, the demagnetization pulse having a higher magnetic field strength than the magnetization pulse magnetic field strength. The effect of the alternating magnetic field on the material to be tested is measured by magnetic field detectors. The field detectors generate electrical signals which are fed to an oscilloscope, a comparator, or a computer for analysis. Characteristics of the material to be tested which are measured by the device include rotation of the magnetic domains, alignment of magnetic domains, alignment of magnetic moments, the Barkhausen effect, coercive force, remanence, material structure, stress fields, and defects.

Surface effect on saturation magnetization of Co and Ti substituted Ba-Ferrite fine particles

Abstract: The surface effect on saturation magnetization of Co and Ti substituted Ba-ferrite platelet fine particles is reported. The saturation magnetization decreases linearly with increasing hexagonal c-plane surface area of the particles, and the effect of the side plane surface area on the saturation magnetization is negligible. This result indicates that a low saturation magnetization layer exists predominantly on the c-plane surfaces. The saturation magnetization of the c-plane surface layers increases noticeably with decreasing temperature, and approaches the saturation magnetization value for the bulk Ba-ferrite. The low saturation magnetization at the surface layer is explained by a thermal fluctuation effect on the magnetic ions at the surface layer which have a weaker exchange field than the ions inside the particles. Therefore, particles with small diameter to thickness ratio values are beneficial in obtaining higher saturation magnetization under a given particle volume.

Cylindrical permanent magnet to produce a transversal and uniform induction field

Abstract: Industrial problems concerning the production of a field of uniform and traverse induction are solved by providing a cylindrical magnet with magnetic blocks of which the internal magnetization is either radial or tangential to the cylinder. Such an arrangement facilitates the fabrication and the magnetization of the blocks. A radial or tangential magnetization distribution produces a transverses field only if the magnetization modulus varies respectively with the cosine or the sine of the localization angle of the block with respect to the direction of induction to be produced. By combining both structures, it is even possible to obtain a magnet having a zero bipolar moment. The variation of the magnetization modulus is obtained by providing composite magnetic bricks. They comprise plates of magnetic materials which are coupled to non-magnetic plates in a proportion dependeing on the reduction of magnetization to be achieved. Such a magnet applies to imaging by nuclear magnetic resonance in the medical field.

Magnetization reversal for barium ferrite particulate media

Abstract: A perpendicularly oriented Ba ferrite particulate medium was investigated with regard to the magnetization reversal process. Coercivity and remanence coercivity for the medium were found to be decreasing and increasing functions, respectively, of the angle between the easy axis and the applied field. The angular dependence of remanence coercivity, being supported by the rotational hysteresis integral value, implies that the magnetization reversal process for the medium is incoherent. A fanning model for the two aligned particles with uniaxial crystalline anisotropy, was developed. This model yields variable angular dependencies of coercivity for varying ratios of a crystalline anisotropy field (H k to a dipole-dipole interaction field (H m ). Taking an appropriate H k /H m value, the model can predict the angular dependence of coercivity and remanence coercivity for the medium.

Hexagonal ferrite particles for perpendicular recording prepared by the precursor method

Abstract: Single domain MFe 12 O 19 (M=Ba Sr) hexagonal ferrite particles have been prepared by the precursor or liquid mix technique. It is shown that single phase samples with different mean diameter and diameter to thickness ratio may be prepared by modifications of thermal treatments. It is shown as well that surface spin canting has been formed and that the magnetocrystalline anisotropy is reduced by the platelet-like shape anisotropy.

Fine iron particles having super high coercivity

Abstract: Fine iron particles having coercivity higher than 2000 Oe were produced by the conventional method which is conmercially used,i.e, the reduction of acicular goethite particles with hydrogen gas. This could be done by adsorption of Nd3+or Ce3+ions together with B3+ions on the surface of the goethite particles. Electron micrographs show that the particles have very smooth surface and few micropores, and are isolated from each other. The optimum size of the iron particles is around 0.13 μm in length and 0.02 μm in width. The specific surface area of this powder is about 38 m2/g, which is much lower than the ordinary ones. This super high coercivity may be due to the peculiar microstructure of the amorphous iron oxide layer which is coupled with the iron core, together with the well defined single domain behavior of the iron particles.

On the origin of K U in amorphous RE-TM magnetooptic recording materials

Abstract: Exchange anisotropy was recently proposed as the origin of uniaxial perpendicular anisotropy in some RE-TM thin films. This magnetic anisotropy arises from the exchange interaction between the ferrimagnetic matrix and acicular shaped single domain ferromagnetic regions. Rotational hysteresis measurements on GdCo films show a non-vanishing value of the rotational magnetic hysteresis for magnetic fields greater than the magnetic anisotropy field (H k ). Such a behavior, first discovered in Co-CoO, is a necessary but not a sufficient condition for exchange anisotropy. The origin of the intrinsic uniaxial magnetic anisotropy energy density (K u ) in amorphous rare earth-transition metal (RE-TM) thin films has been attributed to pair ordering, single ion anisotropy, columnar structures that occur in the sputtering process, stress between the film and the substrate, and exchange anisotropy.

Magnetization process in RE-TM alloys: Wall mobility and nucleation

Abstract: Magnetization process on GdFe and TbGdFe samples is studied It is seen that reverse domains nucleation and domain wall motion are thermally activated phenomena As a result new expressions for coercivity based on thermal activation are proposed If the magnetization process occurs mainly by nucleation, it is suggested that coercivity is only a function of temperature, saturation magnetization, uniaxial anisotropy and Curie temperature Predicted values of the coercivity are compared with experiments and available data as found in the literature

Control of parameters in rare earth-transition metal alloys for magneto-optical recording media

Abstract: The techniques which can be used to control the magnetic and magneto-optical recording properties of sputtered amorphous thin films containing Fe,Co,Tb and Gd are reviewed. It is shown that by proper selection of composition one may independently vary the Curie temperature, the compensation temperature, and coercivity. On-the-other-hand it is shown that the uniaxial anisotropy perpendicular to the film plane and the magneto-optical Kerr rotation are dependent upon the substrate bias and argon pressure used during deposition. A procedure for optimizing the magnetic and magneto-optical properties of these materials for magneto-optical recording applications is outlined. Magneto-optic recording technology today employs amorphous thin films of rare earth-transition metal alloys. The films exhibit a preferred axis of magnetization perpendicular to the plane of the film. At room temperature the coercivity is high so the magnetization remains saturated perpendicular to the film plane, but at high temperatures (> 100°C) the coercivity is low, allowing the magnetization to reverse its direction in local submicrometer regions addressed by a focussed laser beam. To read out the recorded magnetization pattern, the kerr magneto-optic effect is used. This technology allows order of magnitude higher data densities than present rigid disk magnetic recording technology, freedom from head crashes, and removable media. The magnetization of the rare earth-transition metal thin films is determined by antiferromagnetic coupling between the rare earth and transition metal subnetwork magnetizations. Typically at low temperatures the rare earth magnetization dominates, whereas at high temperatures the transition metal magnetization is largest. At some intermediate temperature the two subnetwork magnetizations are equal and opposite resulting in zero net magnetization. At this so-called compensation temperature the coercivity tends toward infinity since there is no net magnetic moment to be acted upon by a magnetic field. At a sufficiently high temperature, the Curie temperature, the net magnetization and coercive force both tend toward zero. In this paper the factors which control the magnetic properties of sputtered GdTbFeCo amorphous films are discussed. Methods to independently control compensation point, Curie temperature, uniaxial anisotropy, and coercivity are explained.

Magnetic remanence measurements of single particles and the nature of domain patterns in titanomagnetites

Abstract: In addition to classical body domain patterns, a profusion of complex maze like domain structures have been observed using the Bitter technique on titanomagnetites. Controversy persists over how important a role such complicated domain patterns play in remanent magnetization. We have compared the magnetic properties of single particles, which exhibit complex patterns with those that exhibit simple ones. IRM acquisition and DC and AF demagnetization of IRMs of single particles were measured. We found that both types of grains exhibit similar bulk magnetic properties. The simple patterns reorganize with changing magnetization state while complicated patterns apparently remain constant. We suggest that the remanent magnetization of grains with complicated patterns is carried by the bulk of the particle within which a more simple domain structure controls magnetic behavior.

Anisotropic Magnetization in Single Crystal YBa2Cu3O7

Abstract: Magnetization measurements on single crystals of YBa2Cu3O7 are used to study the anisotropy of the critical current density and magnetic properties of this superconducting oxide with a transition temperature of 88 K. Anisotropy of the magnetization versus applied field hysteresis loops is pronounced, with the effect increasing as the temperature approaches the superconducting critical temperature.

Calculation of the magnetization in an exchange coupled layer

Abstract: Exchange coupled thin films of a soft magnetic film and an anti-ferromagnet are of interest as a way of providing bias for magneto-resistive recording heads. In this paper the influence of magnetic and geometrical parameters of the soft magnetic film on the magnetization curve and the permeability were derived from micromagnetic theory. A comparison with experimental data showed, that the exchange coupling at the interface of the soft magnetic layer to the antiferromagnet is very weak. It was found that the average of the exchange stiffness constant across the boundary layer appears to be only \cong1/2000 of that in the bulk of the soft magnetic layer for the system permalloy/FeMn. This suggests that the magnitude of exchange anisotropy could be considerably increased, if the interface were improved.

A study of coercivity in Ca-Ge substituted epitaxial garnets

Abstract: This paper is an analysis of two models on the origin of coercivity in certain crystals. The first model, based upon the work of Kersten. suggested that coercivity in soft magnetic materials is due to the variation in domain wall energy throughout the crystal. The second model attributes coercivity to magnetostatic interactions between domain walls and crystal defects. Recent work on calcium and germanium substituted YIG has shown a defect structure similar to the one assumed in both of these models. Introducing the experimentally observed parameters into the models yields calculated values for the coercivity that are very close to the observed value for the magnetostatic model and very poor for the wall energy variation model.

The dependence of coercivity on domain structure and domain wall energy of LPE garnet films

Abstract: The coercivity of epitaxial magnetic garnet films has been measured on both stripe and bubble domain structures by the domain wall oscillation method. The bubble coercivity versus bubble array density relation has been determined experimentally. It was found that on the array reaching a certain density the apparent coercivity was strongly influenced by the neighbouring domains interaction. Also, the coercivity of bubbles was found to be a function of domain wall structure (domain wall energy); the presence of Bloch lines increased the experimentally measured coercivity value.