Bloch domain walls

The wall-width parameter 80 varies from about 1 nm in extremely hard materials to several 100 nm in very soft materials. It determines the thickness Sb = nSa and energy yw = AK S0 of Bloch domain walls [13, 14, 97, 98], and describes the spatial response of the magnetization to local perturbations [95], Essentially, the thickness of the walls is determined by the competition between exchange, which favors extended walls, and anisotropy, which favors narrow transition regions. 

There is another, quite distinct, resonance phenomenon concerned with domain wall movements occurring at approximately one-tenth of the ferrimagnetic resonance frequency. To understand this Bloch wall motion needs to be... 

Domain wall - The transition region between adjacent ferromagnetic domains, generally a layer with a thickness of a few hundred angstrom units. Also called Bloch wall. 

Directional order results in a strong induced anisotropy (and an easy axis) in the field direction this leads to large 180° domains separated by highly mobile domain walls parallel to this axis. Induced anisotropy values in the range. 100-400 J/m have been observed in Ni-Fe alloys (Ferguson, 1958). The application of a field parallel to the induced anisotropy axis results in extremely soft behaviour (Fig. 6.9(c)) saturation is reached for small fields, as a result of domain wall mobility. If the field is instead applied in a transverse direction, completely different behaviour is observed. Fig. 6.9(d). Since domain wall displacement has no net effect on the magnetisation in the field direction (domain walls are 180° Bloch walls, oriented in a direction perpendicular to the field), magnetisation proceeds mainly by spin rotation (see Section 4.5.1). The hysteresis loop in Fig. 6.9(a) was obtained after heat treatment at T > 7 (7 for this composition is around 900 K), and therefore corresponds simply to the stress-free state. 

Figure 7.11. Observation of the VBLs in the ADFM. (a. b) Two main observation configurations, (c, e) Typicai images of domain walls and Bloch lines (marked by arrows) obtained in observation configurations corresponding to (a) and (b), respec-tiveiy (contrast is inverted), (d) Magnetic structure of the sample. The Bloch lines are indicated by arrows.

In solid-state physics there are many examples of domain walls which can move Bloch walls in ferro-magnets, dislocations in crystals, etc. Phase-slip cent in charge-density waves can also move. It could also be conjectured that the conjugational defects of Figures 1.13 and 1.14 are also mobile and perhaps move as solitary waves (the step in the alternation parameter... 

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