A mechanism of magnetic hysteresis in heterogeneous alloys
TL;DR: In this paper, it was suggested that in many ferromagnetic materials there may occur particles distinct in magnetic character from the general matrix, and below the critical size, depending on shape, for which domain boundary formation is energetically possible.
Abstract: It is suggested that in many ferromagnetic materials there may occur particles distinct in magnetic character from the general matrix, and below the critical size, depending on shape, for which domain boundary formation is energetically possible. For such single-domain particles, change of magnetization can take place only by rotation of the magnetization vector. As the field changes continuously, the resolved magnetization may change discontinuously at critical values of the field. The character of the magnetization curves depends on the degree of magnetic anisotropy of the particle and on the orientation of easy axes with respect to the field. The magnetic anisotropy may arise from the shape of the particle, from magnetocrystalline effects, and from strain. A detailed quantitative treatment is given of the effect of shape anisotropy when the particles have the form of ellipsoids of revolution, along with a less detailed treatment for the general ellipsoidal form.
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TL;DR: In this paper, a reformulation of the phenomenological theory of the magnetization field was proposed to take large non-eddy-current damping into account in thin Permalloy sheets.
Abstract: In 1955, a phenomenological theory of ferromagnetism was well established and had been corroborated by a considerable amount of experimental data. However, there were problems in the phenomenological theory of the dynamics of the magnetization field. The Landau-Lifshitz equation for damping of the motion of the magnetization field could not account for the large noneddy-current damping in thin Permalloy sheets. The problem undertaken herein is a reformulation of the theory in a way that is more consistent with the theory of damping in other physical systems in order to be able to take large damping into account.
2,181 citations
IBM1
TL;DR: In this article, the authors discuss thermal effects in the framework of basic Arrhenius-Neel statistical switching models and reveal the onset of thermal decay at "stability ratios" (k/sub u/V/K/sub B/T)/sub 0//spl sime/35 /spl plusmn/ 2.
Abstract: In current longitudinal magnetic recording media, high areal density and low noise are achieved by statistical averaging over several hundred weakly coupled ferromagnetic grains per bit cell. Continued scaling to smaller bit and grain sizes, however, may prompt spontaneous magnetization reversal processes when the stored energy per particle starts competing with thermal energy, thereby limiting the achievable areal density. Charap et al. have predicted this to occur at about 40 Gbits/in/sup 2/. This paper discusses thermal effects in the framework of basic Arrhenius-Neel statistical switching models. It is emphasized that magnetization decay is intimately related to high-speed-switching phenomena. Thickness-, temperature- and bit-density dependent recording experiments reveal the onset of thermal decay at "stability ratios" (K/sub u/V/K/sub B/T)/sub 0//spl sime/35 /spl plusmn/ 2. The stability requirement is grain size dispersion dependent and shifts to about 60 for projected 40 Gbits/in/sup 2/ conditions and ten-year storage times. Higher anisotropy and coercivity media with reduced grain sizes are logical extensions of the current technology until write field limitations are reached. Future advancements will rely on deviations from traditional scaling. Squarer bits may reduce destabilizing stray fields inside the bit transitions. Perpendicular recording may shift the onset of thermal effects to higher bit densities. Enhanced signal processing may allow signal retrieval with fewer grains per bit. Finally, single grain per bit recording may be envisioned in patterned media, with lithographically defined bits.
1,223 citations
TL;DR: In this paper, the specific loss power of magnetic nanoparticles for hyperthermia was investigated with respect to optimization of the SLP for application in tumour hyper-thermia and the dependence of the loss power on the mean particle size was studied over a broad size range from superparamagnetic up to multidomain particles.
Abstract: Loss processes in magnetic nanoparticles are discussed with respect to optimization of the specific loss power (SLP) for application in tumour hyperthermia. Several types of magnetic iron oxide nanoparticles representative for different preparation methods (wet chemical precipitation, grinding, bacterial synthesis, magnetic size fractionation) are the subject of a comparative study of structural and magnetic properties. Since the specific loss power useful for hyperthermia is restricted by serious limitations of the alternating field amplitude and frequency, the effects of the latter are investigated experimentally in detail. The dependence of the SLP on the mean particle size is studied over a broad size range from superparamagnetic up to multidomain particles, and guidelines for achieving large SLP under the constraints valid for the field parameters are derived. Particles with the mean size of 18 nm having a narrow size distribution proved particularly useful. In particular, very high heating power may be delivered by bacterial magnetosomes, the best sample of which showed nearly 1 kW g −1 at 410 kHz and 10 kA m −1 . This value may even be exceeded by metallic magnetic particles, as indicated by measurements on cobalt particles.
919 citations
HGST1
TL;DR: Nanofabrication of magnetic storage media, where servo marks, discrete tracks or individual islands are defined, offers the prospect for improved performance and increased areal density as discussed by the authors.
Abstract: Nanofabrication of magnetic storage media, where servo marks, discrete tracks or individual islands are defined, offer the prospect for improved performance and increased areal density. However, this increase in performance will require that new and additional processes be introduced into disk manufacturing. We review here the fundamental patterning and fabrication processes that have been proposed, along with their respective strengths and weaknesses and the potential advantages they may offer for magnetic recording. The increase in data density afforded by nanofabrication may have added significance as more conventional approaches to ever increasing density will encounter physical limitations set by the thermal stability of the recorded bits.
824 citations
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01 Jan 1935
TL;DR: This chapter examines the distribution of magnetic moments in a ferromagnetic crystal and finds that if the crystal is placed in an external magnetic field, the boundaries between the layers begin to move so that the layers with magnetic moments parallel to the field become wider.
Abstract: Publisher Summary This chapter examines the distribution of magnetic moments in a ferromagnetic crystal. When the crystal is magnetized, the boundaries between the oppositely magnetized layers move so that the layers with one direction of magnetic moment grow at the cost of the layers with moments in the opposite direction. The presence of separate elementary regions, magnetized in opposite directions, is due only to the demagnetizing effect of the surface, and the number and dimensions of these regions are entirely determined by the dimensions of the body. The analysis of the preceding section gives only the distribution of the directions of the magnetic moments in the intermediate regions but gives nothing for determining the width of the layers. If the crystal is placed in an external magnetic field, which is directed parallel to the axis of easiest magnetization, the boundaries between the layers begin to move so that the layers with magnetic moments parallel to the field become wider.
1,866 citations
TL;DR: In this article, the authors present a review of the Physical review, vol. 70, nos. 11 and 12, 965-971, Dec. 1 and 15, 1946.
Abstract: "Reprinted from the Physical review, vol. 70, nos. 11 and 12, 965-971, Dec. 1 and 15, 1946."
1,132 citations
403 citations
313 citations
TL;DR: The shape of the particle actually formed is influenced by this energy, by the degree of supersaturation of the solid solution, and by surface effects; the calculated shape is shown to agree reasonably well with that observed experimentally as discussed by the authors.
Abstract: If the process of diffusion in metals results simply in the interchange of pairs of atoms large strains must be set up when a new phase precipitates in an alloy. The strain energy involved is calculated for particles of precipitate of various shapes, and is found to be least if the precipitate forms thin plates. The shape of the particle actually formed is influenced by this energy, by the degree of supersaturation of the solid solution, and by surface effects; the calculated shape is shown to agree reasonably well with that observed experimentally.
217 citations