6. The Relevance of Domains (76 pages)

6.1 Overview

The role of magnetic microstructures varies strongly between different applications of magnetic materials. In some fields, such as in the cores of electrical machinery (Sect. 6.2.1), domains and domain walls are essential. Electrical machines simply would not work without the easily displaceable domain walls providing the necessary permeability. The same is true for most inductive devices at medium and high frequencies (Sect. 6.2.2-6.2.4). But irregularities in the magnetic microstructure are also the origin of losses and noise (electrical and acoustical) in these devices.

In some applications of magnetic materials domains play no role at all, such as microwave components, nucleation-type permanent magnets (Sect. 6.3.1), and particulate recording media (Sect. 6.4.1). Other devices would ideally work without any non-uniform magnetic microstructures, but domains are the origin of irregular behaviour if they cannot be suppressed. In these cases, mostly in the field of small sensor and memory elements (Sect. 6.5), domain studies are necessary to understand the conditions of their occurrence and their control. Finally, there are applications in which domain propagation is or was directly put to technical use. These are discussed in Sect. 6.6.

6.2 Bulk Soft-Magnetic Materials

Fig. 6.13:   Magnetostriction-free metallic glasses are unique soft magnetic materials in being insensitive to elastic deformation. The same domain pattern is observed in the flat state (a) and in a strongly bent state (b). In contrast, regular magnetostrictive materials display a complete domain rearrangement on bending (c, d)

6.3 Permanent Magnets

6.4 Recording Media

Fig. 6.23:   Obliquely evaporated metal evaporated tape represents a high-density recording medium in which strong stripe domains can be observed in suitably prepared samples. This observation indicates a certain degree of exchange coupling between the grains which obviously has no adverse effect in this kind of recording. (Courtesy L. Abelmann)

6.5 Thin-Film Devices

Fig. 6.29:   An unavoidable source of noise in thin film reading heads is the irreversible displacement of domain wall substructures (Bloch lines) which are visible in (a) (before) and (b) (after a field pulse)

6.6 Domain Propagation Devices

Sketch 1 p. 589:   Bubble domains of submicron diameter can be transported by these Permalloy overlay patterns under the action of a rotating field

6.7 Domains and Hysteresis

Fig. 5.80:   The complexity of hysteresis phenomena is demonstrated by reference to this experiment. The final state at zero field (e,i) depends on whether after (c) the field is decreased (d) or further increased (f). How to reconcile such subtle effects with tractable global descriptions of hysteresis phenomena remains an open problem