Question

What happens to the magnetic domain structure of a ferromagnet when it is placed in an external magnetic field?

- Domains whose magnetisation is at odds with the direction of the external field will vanish spontaneously at the expense of their neighbouring domains.

- Domains will re-orientate their magnetisation parallel with the external field, starting with the domain which is most add odds with the external field direction.

- Magnetic moments near Bloch walls will reorientate themselves in parallel with the external field.

- Magnetic moments near Bloch walls will reorientate themselves in line with the orientation of the adjacent domain if this means they will be closer to parallel alignment with the external field.

Answer 1

Option three is the answer that is;

-magnetic moments near Bloch walls will reorientate themselves in parallel with the external feild

Explanation is as follows:
Magnetic domains are small, randomly oriented and spontaneously magnetized regions in a ferromagnetic material whose spins are locked into orderly arrays.   Application of small external fields moves the walls between them to form larger domains. Higher external fields reorient the domains to face the same direction and increase net magnetization.

Bloch wall is a narrow transition region at the boundary between magnetic domains over which the magnetisation changes from its value in one domain to that in the next.The change in allignment of the magnetic domains is due to the rotation of bloch walls.

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