--- title: "Demagnetization by domains and deperming" subject: "Electrical Engineering Material" chapter: 4 key: "semester_3/electrical_engineering_material:demagnetization-deperming" source: https://www.bctengineer.com/q/semester_3/electrical_engineering_material/demagnetization-deperming site: BCT Engineering --- # Demagnetization by domains and deperming Electrical Engineering Material · Chapter 4: Magnetic Materials - Asked in: 2080 Baisakh, 8 marks - Video: [Methods of Magnetisation & Demagnetisation | Physics Full Concept | NEET JEE](https://www.youtube.com/watch?v=Dka-cROHxBY) — Educational Videos - Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14 - Page: https://www.bctengineer.com/q/semester_3/electrical_engineering_material/demagnetization-deperming - Whole chapter: https://www.bctengineer.com/md/semester_3/electrical_engineering_material/chapter-4 ## Question Explain demagnetization of magnetic materials with the help of magnetic domain formation. What is deperming method of demagnetization? ## Short answer Single domains reduce high magnetostatic energy by dividing into smaller domains; closure domains eliminate external field, giving zero net magnetization. Deperming uses cables wound around a ship’s hull, passing alternating-polarity DC pulses of decreasing magnitude to cancel permanent magnetism acquired during construction. ## Answer **Demagnetization** is the reduction of the net magnetization of a ferromagnetic material to zero. It happens naturally through the formation of domains, and it can be done deliberately by heating, by a reverse field, or by an alternating field of decreasing amplitude. #### Demagnetization through domain formation A single fully magnetized domain has poles at its ends and a large external field, so its **magnetostatic energy** is high. The crystal lowers its energy by dividing 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**. The number and size of domains are fixed by the balance between the saved magnetostatic energy and the energy of the domain walls (plus anisotropy and magnetostrictive energy). #### Practical methods of demagnetization 1. **Heating** above the Curie temperature and cooling in zero field: domains re-form randomly. 2. **Applying a reverse field** equal to the coercive force $H_c$: brings $B$ to zero (but not stable). 3. **AC demagnetization**: an alternating field whose amplitude is reduced slowly to zero takes the material round smaller and smaller hysteresis loops until it ends at the origin. ```text B ^ ___ | / _ \ ---+-/-(o)-\---> H shrinking loops | \___/ spiral to B = 0 ``` #### Deperming method **Deperming** is a demagnetization process used for large steel structures, mainly **ships and submarines**, to remove their permanent magnetism (acquired from the earth's field, welding and hammering during building) so that they do not trigger magnetic mines or disturb compasses. - Large cables are wound round the hull (or the ship is placed in a deperming facility). - Strong **DC pulses of alternating polarity and decreasing magnitude** are passed through the cables, e.g. +I, −0.8I, +0.6I, …. - Each pulse drives the steel round a smaller hysteresis loop, so the permanent magnetization is reduced almost to zero. - It is repeated periodically. A related method, **degaussing**, uses permanently installed coils carrying current to cancel the ship's field continuously. ## Check yourself The answer's key points as multiple-choice questions, with the correct option and why. 1. Why does a single fully magnetized crystal split into domains? A. To lower the high magnetostatic energy caused by free poles at its ends B. To increase its exchange energy C. To raise its Curie temperature D. To remove all atomic magnetic moments Answer: A. A single domain has free poles and a large external field, so its magnetostatic energy is high; splitting reduces it. (Key point: Why domains form) 2. What is the role of closure domains at the ends of the crystal? A. They make the crystal a single saturated domain B. They complete the flux loop inside the crystal so no free poles remain and net magnetization is zero C. They increase the external field D. They align all moments along the easy axis Answer: B. Triangular closure domains magnetized at 90° close the flux internally, giving zero external field. (Key point: Closure domains) 3. When does the splitting into more domains stop? A. When the crystal reaches the Curie temperature B. When all walls disappear C. When the energy of a new domain wall equals the magnetostatic energy saved D. When the anisotropy energy becomes zero Answer: C. Each extra wall costs energy; splitting stops when that cost equals the magnetostatic saving. (Key point: Limit of splitting) 4. How is deperming carried out on a ship? A. A single large DC current is kept flowing in one direction B. The hull is heated above the Curie temperature C. A steady reverse field equal to Bs is applied D. DC pulses of alternating polarity and decreasing magnitude are passed through cables wound round the hull Answer: D. Decreasing alternating-polarity pulses take the steel round smaller hysteresis loops, reducing permanent magnetism almost to zero. 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