--- title: "Demagnetizing via B-H curve; data storage material" subject: "Electrical Engineering Material" chapter: 4 key: "semester_3/electrical_engineering_material:demagnetize-bh-curve-storage" source: https://www.bctengineer.com/q/semester_3/electrical_engineering_material/demagnetize-bh-curve-storage site: BCT Engineering --- # Demagnetizing via B-H curve; data storage material Electrical Engineering Material · Chapter 4: Magnetic Materials - Asked in: 2078 Bhadra, 4+4 marks - Video: [Coercivity & retentivity (Permanent & electromagnets) | Magnetism & matter | Physics | Khan Academy](https://www.youtube.com/watch?v=rGgKK3-wep4) — Khan Academy India - English - Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14 - Page: https://www.bctengineer.com/q/semester_3/electrical_engineering_material/demagnetize-bh-curve-storage - Whole chapter: https://www.bctengineer.com/md/semester_3/electrical_engineering_material/chapter-4 ## Question How can you demagnetize a magnetic material? Explain with the help of its B-H curve. What type of magnetic material would you chose for electronic storage of digital data? Justify. ## Short answer To demagnetize, apply an AC field of decreasing amplitude: the material cycles through shrinking hysteresis loops that spiral to the origin, leaving B=0. For digital storage, a hard magnetic material with a square hysteresis loop (e.g., γ-Fe₂O₃) is chosen; its high remanence and two stable states (+Br, -Br) represent 1 and 0, and sufficient coercivity prevents data loss. ## Answer #### Demagnetizing a magnetic material using the B–H curve After a ferromagnetic material has been magnetized to saturation and the field removed, it keeps a remanent flux density $B_r$ (point b). It can be demagnetized in these ways: **(a) By a reverse field equal to the coercive force**: applying $H = -H_c$ takes the material from $B_r$ to $B = 0$ (point c). This is not stable; small disturbances leave some residual magnetism. **(b) By an alternating field of decreasing amplitude (AC demagnetization)**: the material is taken round a series of hysteresis loops, each smaller than the last. As the amplitude falls to zero, the loops shrink and spiral into the origin, where $B = 0$ and $H = 0$. The domains end up randomly oriented, so this method gives true demagnetization. ```text B ^ a (sat.) b Br | ___ __|__/ / / | __ / / /_|/ // -------c-/--o--/-------> H -Hc| |/ / +Hc /___|_/ | loops shrink to o ``` **(c) By heating above the Curie temperature** and cooling without a field: thermal agitation destroys alignment and domains re-form randomly. #### Material for electronic storage of digital data A **hard (semi-hard) magnetic material with a square (rectangular) hysteresis loop** is chosen, e.g. γ-Fe₂O₃, CrO₂, barium ferrite, or Co-Cr/Co-Pt alloy thin films in hard disks. **Justification** - **Two stable states**: a square loop has high remanence $+B_r$ and $-B_r$ at $H = 0$, which represent binary 1 and 0. - **High remanence**: gives a strong readable signal. - **Sufficient coercivity**: stray fields and neighbouring bits cannot erase stored data, so storage is non-volatile; yet coercivity is not so high that the write head cannot switch it. - **Sharp switching**: a field just above $H_c$ flips the bit quickly and cleanly. - **Fine particles/grains**: allow high storage density. The read/write heads themselves use **soft** materials (permalloy, ferrite) so they follow the signal without retaining it. ## Check yourself The answer's key points as multiple-choice questions, with the correct option and why. 1. Applying H = -Hc to a magnetized material takes it from Br to B = 0. Why is this not a reliable demagnetization? A. It is unstable; small disturbances leave residual magnetism B. It permanently destroys the domains C. It raises the Curie temperature D. It only works above the Curie temperature Answer: A. The domains are not randomized, so some magnetism returns after disturbance. (Key point: Reverse field method) 2. How does an alternating field of decreasing amplitude demagnetize a material? A. The loop grows until saturation B. The B-H loops shrink and spiral into the origin, leaving domains randomly oriented C. The material stays at Br after each cycle D. The field is kept constant while temperature falls Answer: B. Successively smaller loops end at B = 0, H = 0 with random domains, giving true demagnetization. (Key point: AC demagnetization) 3. Which material is chosen for digital data storage? A. A soft material with a narrow loop B. A diamagnetic material C. A hard (semi-hard) magnetic material with a square hysteresis loop D. A paramagnetic material Answer: C. A square loop gives two stable remanent states, +Br and -Br, for binary 1 and 0. (Key point: Storage material) 4. Why must coercivity be sufficient but not too high for storage media? A. High coercivity makes reading impossible B. Low coercivity gives higher remanence C. Coercivity has no effect on storage D. Stray fields must not erase data, yet the write head must be able to switch the bits Answer: D. Adequate Hc keeps data non-volatile while still allowing writing by the head. 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