Magnetic Domain Patterns R.M

Magnetic domain patterns R.M. Bozorth To cite this version: R.M. Bozorth. Magnetic domain patterns. J. Phys. Radium, 1951, 12 (3), pp.308-321. 10.1051/jphysrad:01951001203030800. jpa-00234388 HAL Id: jpa-00234388 https://hal.archives-ouvertes.fr/jpa-00234388 Submitted on 1 Jan 1951 HAL is a multi-disciplinary open access L’archive ouverte pluridisciplinaire HAL, est archive for the deposit and dissemination of sci- destinée au dépôt et à la diffusion de documents entific research documents, whether they are pub- scientifiques de niveau recherche, publiés ou non, lished or not. The documents may come from émanant des établissements d’enseignement et de teaching and research institutions in France or recherche français ou étrangers, des laboratoires abroad, or from public or private research centers. publics ou privés. LE JOURNAL DE PHYSIQUE ET LE RADIUM. TOME 12, MARS 19~1, PAGE 308. MAGNETIC DOMAIN PATTERNS By R. M. BOZORTH, Bell Telephone Laboratories, Murray Hill (New Jersey). Sommaire. 2014 La technique et l’interprétation des diagrammes de poudres magnétiques est briè- vement passée en revue, d’un point de vue historique. Les diagrammes les plus simples observés sont ensuite décrits et expliqués dans la mesure du possible. Dans la troisième Partie, on décrit et discute de nouveaux diagrammes relatifs : a. à un monocristal dont la direction (1 11) est celle de facile aiman- tation (60 pour 100 Co, 40 pour 100 Ni); b. à un monocristal de cobalt; c. à un alliage polycristallin fer-silicium et d. à un alliage polycristallin pour aimants permanents (Alnico 5). Brief revievsr. --- For many decades iron filings considerably our knowledge of the processes of have been used to portray the directions of lines magnetization. They used single crystals contai- of magnetic force in air and to detect flaws or inho- ning 3.8 weight per cent silicon and having surfaces mogeneitics in magnetic materials. In 1931 it cut nearly parallel to crystallographic planes. The occurred to von Hamos and Thiessen [ 1] to use specimens were annealed and polished carefully, magnetic powder to detect the local inhomogeneities first mechanically and then electrolytically. After irr magnetization that the domain theory predicts. mechanical polishing the powder pattern on a Independently Bitter [2] applied a suspension surface almost parallel to (100) is the cc maze » of siderac (Fe,O,), having particles about 10-4 cm pattern of figure 2 (a), similar to that of figure i. in diameter, to a polished magnetized surface and observed under the microscope that the powder formed parallel lines regularly spaced about o. mm apart and approximately perpendicular to the direction of magnetization. The technique and interpretation of such patterns .was then the subject of study of a number of workers [3]. The preparation of colloid for these studies has been described in some detail by Elmore [4] who recommends a suspension of magne- tite, ground to colloidal dimensions, peptized with hydrochloric acid and protected by one per cent of soap; an improvement on his technique has recently been developed and will be published soon. Elec- trolytic polishing [4] overcomes the objectionable mechanical polishing which disturbs the surfaces. A notable advance was made by McKeehan and Elmore [5] who first observed a well-defined on a i pattern demagnetized single crystal. Figure - Fig. I. n Maze » pattern observed on polished surface of such a also shows pattern (b) and those patterns single crystal of iron; (b) demagnetized, (a) and (c) magne- observed when the magnetization is directed (a) tized in opposite directions. into, or (c) out of, the same portion of the surface as that shown in (b). The suspension used for the experiments was a true colloid of Fe,O, particles After electrolytically polishing and reapplying the small enough to show Brownian movement, and a powder to the same area the result is the « tree » change in magnetization of the magnetic specimen pattern of figure 2 (b). It is evident from this was accompanied by a movement of the lines and other experiments that the maze pattern is immediately visible to the eye. characteristic of a strained surface and that the More recent work, reported in various articles by tree pattern shows the domain boundaries of strain- Williams, Bozorth and Shockley [6], has made free material. visible for the first time the domain boundaries The directions of magnetization in the domains characteristic of unstrained iron, and has improved can be determined in several ways, using techniques Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphysrad:01951001203030800 309 described in the original paper. The result for a is always parallel to one of the crystal axes, and the portion of one tree pattern is shown in figure 2 (c). boundaries separate domains magnetized at goo or The local magnetization in unmagnetized material at 180° to each other. Fig. 2. - Maze and a tree » patterns, (a) and (b), observed on the same portion of a single crystal after mechanical and eliectrolytic polishing, respectively. Directions of magnetization in the tree pattern are shown at (c). Fig. 3. - Effect of increasing tension (a) to (d), on the tree pattern. In (fi tension has been released. Visible movement of domain boundaries takes at the expense of domains oriented at right angles, place upon application of field or stress. The so that the latter domains disappear almost entirely effect of uniform tension is shown in figure 3. In when the tension is sufficiently large. With release this material tension increases the magnetization of tension the original kind of tree pattern forms, in the direction of the tension, and the mechanism but the details of the pattern are not the same. by which this is accomplished is here apparent : This shows that the boundaries are not fixed to the domains oriented parallel to the axis of tension are structure of the crystal in the way that they are enlarged by displacement of domain boundaries, in the maze pattern, where the local stresses always 310 cause the return of the powder lines to the same places When the surfaces are not parallel or nearly after they have been disturbed temporarily by parallel to simple crystallographic planes, the field or uniform stress. patterns are likely to be more complicated. Figure 4 Fig. 4. - Complicated patterns observed on (1 10) and (i I i) planes. Fig. 5. -° Patterns on cobalt surfaces cut parallel and perpendicular to the hexagonal axis. shows two examples of such patterns. Although Bitter [7] observed two types of patterns on poly- the simple patterns are well understood, it has not crystalline material and Elmore [8], working with yet been possible to understand in detail the more single crystals, found the hexagonal lace-like patterns elaborate ones. It is believed, however, that the on surfaces parallel to the hexagonal planes (o01 ) basic principles that apply to the simple ones are perpendicular to the crystal axis, and the straight also applicable to the more complex ones. These line patterns on prism planes, as shown in the pho- principles are discussed below. tographs of figure 5, taken by H. J. Williams. Experiments on cobalt have also been instructive. These patterns are in accord with the magnetic 311 properties of cobalt, known to have a direction of are weaker and fall off more slowly with distance, easy magnetization parallel to the crystal axis. in the way that they would expect if the domains are The domains are then expected to be long in the needlelike as assumed. direction of the axis and packed together like a The domain structure around cavities and inclu- bundle of needles (or sheets). The boundaries of sions was investigated theoretically by Néel [10] such domains thus correspond to the patterns. before any direct observations were made. Obser- Moving pictures of the patterns taken with slowly vations of a number of crystal surfaces under the changing field strength show sudden displacements microscope showed the presence of an occasional of the boundaries corresponding to jumps much hole that had formed accidentally during freezing larger than those usually attributed to the Bark- or etching or polishing of the crystal. The patterns hausen effect. around two holes in (100) surfaces are shown in (a) Germer [9] has measured the strength of magnetic and (b) of figure 6. The structure observed was fields close to the surface of an unmagnetized almost identical with that predicted by N6el, cobalt crystal and found that near a hexagonal and can be interpreted with the help of figure 6 (c). face it is of the order of 104 Oe and falls off with Briefly, the energy is lowered by the formation of distance from the surface so that it is relatively « spikes o which help the magnetic poles to spread weak at mm. The fields near prism faces out over a larger area. Fig. 6. - N6el spikes a around holes in a crystal surface, and their interpretation. The interpretation of the various structures can Others, interpreted more recently, are referred to be carried out in terms of energies of associated in the third part of this paper. with domain walls, magnetic poles (magnetostatic energy), crystal anisotropy, strain and the inter- Plate Pattern. - A typical « plate» pattern, action of the magnetization with the field if any with domains of closure, is shown in figure 7. be present. The theory has been summarized This is a stable configuration in zero applied field, recently by Kittel [11]. In the next section the for reasons illustrated in figure 8. In the possible simpler types of structure will be discussed on this single domain (a), the poles at the end give rise to basis.

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