--- title: "Demagnetization and material for digital storage" subject: "Electrical Engineering Material" chapter: 4 key: "semester_3/electrical_engineering_material:demagnetization-digital-storage" source: https://www.bctengineer.com/q/semester_3/electrical_engineering_material/demagnetization-digital-storage site: BCT Engineering --- # Demagnetization and material for digital storage Electrical Engineering Material · Chapter 4: Magnetic Materials - Asked in: 2081 Chaitra (new course), 5 marks - Video: [Magnetizing and Demagnetizing - explained simply and clearly](https://www.youtube.com/watch?v=XPjqWbvErhA) — Digital Physics - 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-digital-storage - 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. Suggest which type of magnetic material is used for storing digital information and why? ## Short answer Demagnetization occurs when a ferromagnet splits into domains to reduce magnetostatic energy; closure domains eliminate external field, yielding zero net magnetization. Digital storage uses hard magnetic materials with square hysteresis loops, such as γ-Fe₂O₃ or Co-Pt films, because they provide two stable remanent states for reliable bit storage. ## Answer **Demagnetization** is the process by which the net magnetization of a ferromagnetic material is reduced to zero. It occurs naturally when the material divides into magnetic domains. #### Demagnetization through domain formation A single domain has free poles and a large external field, i.e. high magnetostatic energy. The crystal lowers this energy by splitting: ```text (a) single (b) two (c) closure +-------+ +-------+ +-------+ | ---> | | ---> | |\ <-- /| |N S | | <--- | |^ v| +-------+ +-------+ |/ --> \| big field half field +-------+ zero field ``` 1. **(a)** One domain: large magnetostatic energy. 2. **(b)** Two antiparallel domains: energy about halved; more domains reduce it further, until the wall energy balances the saving. 3. **(c)** Closure domains at the ends close the flux inside; no free poles, net $M = 0$. In practice a material is demagnetized by an AC field of slowly decreasing amplitude or by heating above $T_C$; the domains then return to this random, flux-closed arrangement. #### Material for storing digital information **Hard (semi-hard) ferrimagnetic or ferromagnetic materials with a square hysteresis loop**, e.g. γ-Fe₂O₃, CrO₂, barium ferrite, Co-Cr/Co-Pt thin films (hard disks). **Why**: the square loop gives two stable remanent states $+B_r$ and $-B_r$ for 1 and 0; high remanence gives a strong read signal; adequate coercivity keeps data safe from stray fields (non-volatile) yet allows writing by the head; sharp switching and fine grains allow fast, dense storage. ## Check yourself The answer's key points as multiple-choice questions, with the correct option and why. 1. Why does a crystal divide into domains? A. To reduce the magnetostatic energy of free poles B. To increase external field C. To raise exchange energy D. To eliminate anisotropy Answer: A. Closure domains give zero external field. (Key point: Why domains) 2. What do closure domains do? A. Create free poles B. Close the flux inside, leaving no free poles and net M = 0 C. Increase external field D. Align all moments Answer: B. The crystal becomes demagnetized. (Key point: Closure) 3. Practical demagnetization is done by: A. Applying a strong constant DC field B. Cooling in a field C. A decaying AC field or heating above Tc D. Hammering in a field Answer: C. Both return domains to a random flux-closed arrangement. (Key point: Practical methods) 4. Which material stores digital data and why? A. Soft material with narrow loop B. Diamagnetic C. Paramagnetic D. Hard material with a square loop, giving two stable states +Br and -Br Answer: D. High remanence and adequate coercivity keep data safe yet writable. (Key point: Storage material) ## Related in Electrical Engineering Material - [Domain walls and demagnetization process](https://www.bctengineer.com/question.md?key=semester_3%2Felectrical_engineering_material%3Adomain-walls-demagnetization) — Electrical Engineering Material, chapter 4 - [Demagnetization by domains and deperming](https://www.bctengineer.com/question.md?key=semester_3%2Felectrical_engineering_material%3Ademagnetization-deperming) — Electrical Engineering Material, chapter 4 - [Demagnetizing via B-H curve; data storage material](https://www.bctengineer.com/question.md?key=semester_3%2Felectrical_engineering_material%3Ademagnetize-bh-curve-storage) — Electrical Engineering Material, chapter 4 - [Domains, domain walls and magnetic losses](https://www.bctengineer.com/question.md?key=semester_3%2Felectrical_engineering_material%3Adomain-walls-magnetic-losses) — Electrical Engineering Material, chapter 4 - [Closure domains and demagnetization](https://www.bctengineer.com/question.md?key=semester_3%2Felectrical_engineering_material%3Aclosure-domains-demagnetization) — Electrical Engineering Material, chapter 4 - [Hard vs soft magnetic materials for relays](https://www.bctengineer.com/question.md?key=semester_3%2Felectrical_engineering_material%3Ahard-soft-relay) — Electrical Engineering Material, chapter 4 ## Related in other subjects - [Storing and stacking of civil engineering materials](https://www.bctengineer.com/question.md?key=semester_8%2Fconstruction_management%3Astoring-stacking-materials) — Construction Management, chapter 3 - [Soft magnetic material, Faraday's laws](https://www.bctengineer.com/question.md?key=semester_4%2Felectrical_machines%3Asoft-magnetic-faraday-laws) — Electrical Machines, chapter 1 - [Storing geometric and attribute information](https://www.bctengineer.com/question.md?key=semester_5%2Fcomputer_graphics%3Ageometric-attribute-info-storage) — Computer Graphics, chapter 6, asked 8× - [Why CRGO is a superior magnetic material](https://www.bctengineer.com/question.md?key=semester_5%2Felectric_machine_design%3Acrgo-superior-material) — Electric Machine Design, chapter 1