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What is the main reason a ferromagnet splits into many magnetic domains even without an external field?

  1. ATo lower the energy of the exchange interaction, which tends to align the moments of neighboring atoms parallel
  2. BTo lower the magnetostatic (demagnetizing-field) energy of the poles that appear on the surface when the whole body is uniformly magnetized
  3. CBecause even below the Curie temperature, thermal vibration randomizes the directions of the atomic magnetic moments
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Answer: B. To lower the magnetostatic (demagnetizing-field) energy of the poles that appear on the surface when the whole body is uniformly magnetized

If the whole crystal is magnetized in one direction, large magnetic poles appear on its surface, creating fields outside and inside (the demagnetizing field inside), with a large magnetostatic energy. Splitting into domains with different directions lets the poles cancel and lowers this energy greatly. On the other hand, inside a domain wall at the boundary between domains the moments turn gradually, so the energy rises by the exchange and anisotropy energies. The actual domain size is set by the balance between the two. The exchange interaction acts to align all the moments, so splitting into domains actually increases the exchange energy. Moments randomized by thermal vibration describe the paramagnetic state above the Curie temperature.

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