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Electronic, magnetic and optical materials

When an external magnetic field is applied to a demagnetized ferromagnet and gradually increased, which correctly describes how magnetization proceeds?

  1. AAt weak fields, mainly domain wall motion occurs; at strong fields, the magnetization within the domains rotates, approaching saturation
  2. BFrom the start, the magnetic moment of each atom rotates independently, one by one, toward the field direction
  3. CMagnetization proceeds as domains split finely and increase in number
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Answer: A. At weak fields, mainly domain wall motion occurs; at strong fields, the magnetization within the domains rotates, approaching saturation

A demagnetized ferromagnet is divided internally into many domains with different directions of magnetization, which cancel overall. When a weak field is applied, domain walls, the boundaries between domains, move so that domains oriented close to the field direction grow, and the magnetization increases. As the field is increased further, the magnetization within the remaining domains itself rotates toward the field direction, approaching the saturation magnetization. Within a domain the atomic moments are already aligned, so they do not rotate independently one by one. Nor does the number of domains increase; domains with favorable orientations grow. The more the domain walls are pinned by inclusions and grain boundaries, the larger the coercivity.

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