--- title: "Hard magnets; dia, para and ferromagnetism" subject: "Electrical Engineering Material" chapter: 4 key: "semester_3/electrical_engineering_material:hard-magnet-dia-para-ferro" source: https://www.bctengineer.com/q/semester_3/electrical_engineering_material/hard-magnet-dia-para-ferro site: BCT Engineering --- # Hard magnets; dia, para and ferromagnetism Electrical Engineering Material · Chapter 4: Magnetic Materials - Asked in: 2081 Baisakh, 2+6 marks - Video: [Diamagnetic, Paramagnetic and Ferromagnetic Materials](https://www.youtube.com/watch?v=AwCTDtknYWU) — Najam Academy - Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14 - Page: https://www.bctengineer.com/q/semester_3/electrical_engineering_material/hard-magnet-dia-para-ferro - Whole chapter: https://www.bctengineer.com/md/semester_3/electrical_engineering_material/chapter-4 ## Question Why hard magnetic material is preferred for making permanent magnet? Explain the differences between diamagnetic, paramagnetic and ferromagnetic materials with their suitable examples. ## Short answer Hard magnets retain strong magnetism due to high retentivity, coercivity, and (BH)max, resisting demagnetization. Diamagnetic materials (e.g., Cu, Bi) have negative small susceptibility, weakly repelled; paramagnetic (e.g., Al, Pt) have small positive χ, weakly attracted; ferromagnetic (e.g., Fe, Co, Ni) have very large positive χ, strong attraction, and hysteresis. ## Answer #### Why hard magnetic material for permanent magnets A permanent magnet must keep strong magnetism for years without a supply. Hard materials (Alnico, NdFeB, SmCo, hard ferrite) have: - **High retentivity $B_r$**: strong field left after magnetizing. - **High coercivity $H_c$**: stray fields, vibration and temperature changes cannot easily demagnetize them, because domain walls are pinned by fine precipitates and defects. - **Large $(BH)_{max}$**: more energy stored per unit volume, so a smaller magnet does the job. #### Diamagnetic vs paramagnetic vs ferromagnetic | Property | Diamagnetic | Paramagnetic | Ferromagnetic | |---|---|---|---| | Permanent atomic moment | None (paired electrons) | Present, random | Present, aligned in domains | | Susceptibility $\chi$ | Small, negative (~$-10^{-5}$) | Small, positive ($10^{-5}$–$10^{-3}$) | Very large, positive ($10^2$–$10^5$) | | Relative permeability $\mu_r$ | Slightly < 1 | Slightly > 1 | Very large (100–100000) | | Behaviour in field | Weakly repelled | Weakly attracted | Strongly attracted | | Field lines | Pushed out | Slightly drawn in | Strongly concentrated | | Effect of temperature | Independent | $\chi = C/T$ (Curie law) | $\chi = C/(T - T_C)$ above $T_C$ | | Hysteresis | No | No | Yes | | Magnetization after field removed | None | None | Remains (retentivity) | | Examples | Cu, Bi, Ag, Au, water, graphite | Al, Pt, Mn, O₂, CuSO₄ | Fe, Co, Ni, Gd | ```text Dia: lines bend away Para/Ferro: lines drawn in --- \ /--- ---\ /--- ---- ( ) ---- ----(===)---- --- / \--- ---/ \--- ``` **Mechanisms in brief** - **Diamagnetism**: the field alters electron orbits (Lenz's law) and induces a moment opposite to the field; present in all materials but masked if others exist. - **Paramagnetism**: unpaired spins partially align with the field against thermal agitation. - **Ferromagnetism**: exchange interaction aligns spins parallel within domains; the field just rotates/grows domains, giving very high magnetization. ## Check yourself The answer's key points as multiple-choice questions, with the correct option and why. 1. Hard materials suit permanent magnets due to: A. High Br and Hc, large (BH)max B. Low Hc C. High permeability D. Narrow loop Answer: A. They resist demagnetization. (Key point: Permanent magnet) 2. Diamagnetic χ is: A. Positive, small B. Small, negative, T-independent C. Positive, large D. Zero Answer: B. Weakly repelled. (Key point: Diamagnet) 3. Paramagnets in a field are: A. Strongly attracted B. Unaffected C. Weakly attracted and lose magnetization when H removed D. Repelled Answer: C. No hysteresis. (Key point: Paramagnet) 4. Ferromagnets differ in having: A. Hysteresis and retained magnetization B. Negative χ C. No domains D. Temperature-independent χ Answer: A. Retentivity remains. 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