--- title: "Domain walls and demagnetization process" subject: "Electrical Engineering Material" chapter: 4 key: "semester_3/electrical_engineering_material:domain-walls-demagnetization" source: https://www.bctengineer.com/q/semester_3/electrical_engineering_material/domain-walls-demagnetization site: BCT Engineering --- # Domain walls and demagnetization process Electrical Engineering Material · Chapter 4: Magnetic Materials - Asked in: 2081 Bhadra, 1+5 marks - Video: [The Domain Theory of Magnetism | Application of the Domain Theory](https://www.youtube.com/watch?v=1k4uBAqRHiE) — ZED ONLINE SCHOOL - Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14 - Page: https://www.bctengineer.com/q/semester_3/electrical_engineering_material/domain-walls-demagnetization - Whole chapter: https://www.bctengineer.com/md/semester_3/electrical_engineering_material/chapter-4 ## Question What are domain walls? Explain the demagnetization process of a magnetic material due to formation of magnetic domains in the presence of external magnetic field with necessary diagrams. ## Short answer Domain walls are thin transition layers where atomic magnetic moments gradually rotate between domains. Demagnetization occurs as a crystal splits into domains to lower magnetostatic energy; closure domains further eliminate free poles, yielding zero net magnetization. An alternating decreasing field or heating above Tc restores random domain arrangement. ## Answer #### Domain walls A **domain wall** (Bloch wall) is the thin transition layer, a few hundred atoms thick, between two neighbouring magnetic domains, inside which the direction of the atomic moments rotates gradually from the direction of one domain to that of the other. Its thickness is a balance between exchange energy (prefers a wide wall) and anisotropy energy (prefers a narrow wall). ```text domain 1 | wall (spins rotate) | domain 2 ^ ^ ^ ^ | ^ / -> \ v | v v v v ``` #### Demagnetization due to domain formation A ferromagnetic crystal does not stay as a single domain, because a single domain produces poles at its ends and a large external field, which stores a large **magnetostatic energy**. The crystal lowers this energy by dividing into domains. This splitting is the reason an iron piece can exist in an unmagnetized (demagnetized) state. ```text (a) single (b) two (c) four (d) closure domain domains domains domains +--------+ +--------+ +--+--+--+--+ +--------+ | ----> | | ----> | |^ |v |^ |v | |\ <--- /| | N S | | <---- | | | | | | | v ^ | +--------+ +--------+ +--+--+--+--+ |/ ---> \| large field field halved field small +--------+ no free poles ``` 1. **(a) Single domain**: all moments parallel; N and S poles at the ends; large external field and maximum magnetostatic energy. 2. **(b) Two antiparallel domains**: flux partly returns inside the crystal; magnetostatic energy roughly halves, at the cost of one wall. 3. **(c) N domains**: energy falls to about $1/N$ of (a). Splitting stops when the energy of extra walls equals the energy saved. 4. **(d) Closure domains**: triangular domains at the ends, magnetized at 90°, close the flux path completely inside the crystal. Magnetostatic energy becomes almost zero and net magnetization is zero. #### Role of the external field When a field is applied and then reduced in a controlled way (an AC field decreasing slowly to zero), domain walls move back and forth through smaller and smaller distances. The domains settle back into this low-energy, randomly oriented, flux-closed arrangement, and the material is demagnetized. Heating above the Curie temperature and cooling in zero field also restores random domains. ## Check yourself The answer's key points as multiple-choice questions, with the correct option and why. 1. A domain wall is: A. Thin layer where moment direction rotates gradually B. A boundary of no moment C. Region of random spins D. A crystal defect with zero energy Answer: A. Bloch wall a few hundred atoms thick. (Key point: Domain wall) 2. Wall thickness balances: A. Exchange (wider) and anisotropy (narrower) B. Thermal and magnetostatic C. Electric and magnetic D. Strain and gravity Answer: A. Balance sets thickness. (Key point: Wall thickness) 3. A crystal splits into domains to reduce: A. Anisotropy B. Magnetostatic energy C. Exchange energy D. Entropy Answer: B. Single domain has large external field. (Key point: Why split) 4. Closure domains: A. Add free poles B. Are unrelated C. Close flux inside crystal, nearly zero magnetostatic energy D. Increase external field Answer: C. 90° triangular domains. 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