--- title: "Closure domains and demagnetization" subject: "Electrical Engineering Material" chapter: 4 key: "semester_3/electrical_engineering_material:closure-domains-demagnetization" source: https://www.bctengineer.com/q/semester_3/electrical_engineering_material/closure-domains-demagnetization site: BCT Engineering --- # Closure domains and demagnetization Electrical Engineering Material · Chapter 4: Magnetic Materials - Asked in: 2080 Bhadra, 6 marks - Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14 - Page: https://www.bctengineer.com/q/semester_3/electrical_engineering_material/closure-domains-demagnetization - Whole chapter: https://www.bctengineer.com/md/semester_3/electrical_engineering_material/chapter-4 ## Question What are closure domains? Explain demagnetization of a magnetic material on the basis of magnetic domain formation. ## Short answer Closure domains are triangular domains at crystal ends, magnetized at 90° to main domains, closing flux paths internally. Demagnetization by domain formation: a single domain has high magnetostatic energy; splitting into domains reduces this energy, and closure domains remove free poles, making net magnetization zero. ## Answer #### Closure domains **Closure domains** are small triangular (prism-shaped) domains that form at the ends or surfaces of a ferromagnetic crystal, magnetized at 90° to the main domains, so that the magnetic flux forms a closed loop entirely inside the crystal. They remove free poles at the surface, making the external field and magnetostatic energy nearly zero. #### Demagnetization by domain formation A ferromagnetic crystal tends towards the state of lowest total energy. A single, fully magnetized domain has free poles and a large external field, which stores a large **magnetostatic energy**. The crystal reduces this energy by splitting into domains: ```text (a) single (b) two (c) many (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, poles at the ends, large external field and maximum magnetostatic energy $\left(\tfrac{\mu_0}{2}\int H^2 dV\right)$. 2. **(b) Two antiparallel domains**: part of the flux returns inside the crystal; the magnetostatic energy is roughly halved, at the cost of one domain wall. 3. **(c) Many domains**: with $N$ domains the energy falls to about $1/N$. Splitting stops when the energy of a new wall equals the magnetostatic energy saved. 4. **(d) Closure domains**: triangular domains at the ends, magnetized at 90° to the main domains, complete the flux loop inside the crystal. No free poles remain, the external field and magnetostatic energy are almost zero, and the net magnetization is zero: the crystal is **demagnetized**. #### Energy balance Total energy = exchange + magnetostatic + anisotropy + domain wall + magnetostrictive energy. Domains form only as long as the decrease in magnetostatic energy is larger than the extra wall energy needed. The result is a state in which domains are randomly oriented and flux is closed, which is why a piece of iron is normally unmagnetized. Demagnetizing a magnetized sample (AC field reduced slowly to zero, or heating above $T_C$) lets the material return to this low-energy domain pattern. ## Check yourself The answer's key points as multiple-choice questions, with the correct option and why. 1. Closure domains are: A. Triangular domains at ends at 90° closing flux inside crystal B. Large domains in centre C. Domain walls D. Impurities Answer: A. They remove free poles. (Key point: Closure domains) 2. A single domain has: A. No poles B. Large external field and maximum magnetostatic energy C. Zero energy D. Closed flux Answer: B. Poles at ends. (Key point: Single domain) 3. With N domains, magnetostatic energy is about: A. 1/N of single domain B. N times C. Unchanged D. Zero Answer: A. Splitting continues until wall energy equals saving. (Key point: N domains) 4. Splitting stops when: A. Wall energy of new wall equals magnetostatic energy saved B. Energy is zero C. Temperature is zero D. Moments randomize Answer: A. Balance. 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