--- title: "Classification by hysteresis loop and applications" subject: "Electrical Engineering Material" chapter: 4 key: "semester_3/electrical_engineering_material:classify-by-hysteresis-applications" source: https://www.bctengineer.com/q/semester_3/electrical_engineering_material/classify-by-hysteresis-applications site: BCT Engineering --- # Classification by hysteresis loop and applications Electrical Engineering Material · Chapter 4: Magnetic Materials - Asked in: 2068 Shrawan, 8 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/classify-by-hysteresis-applications - Whole chapter: https://www.bctengineer.com/md/semester_3/electrical_engineering_material/chapter-4 ## Question Classify magnetic material based on hysteresis loop. Explain the application of such materials. ## Short answer Based on hysteresis loop, magnetic materials are soft (narrow loop, low Hc, used in transformers, relays), hard (wide loop, high Hc, Br, used in permanent magnets), and square-loop (used for magnetic recording and binary storage). ## Answer Based on the shape and size of the **hysteresis loop**, magnetic materials are classified into **soft** and **hard** magnetic materials (with square-loop materials as a special group). ```text B soft B hard ^ __ ^ _______ | / / | / / -----+-/-/----> H --------+-/------/----> H /-/ | Hc small -Hc / | / Hc large /_/ | /__|_____/ narrow, tall loop wide, fat loop ``` #### 1. Soft magnetic materials - Narrow loop, low coercivity ($H_c$ < $10^3$ A/m), low retentivity, high permeability, small hysteresis loss. - Easily magnetized and demagnetized; domain walls move freely. - Examples: soft iron, silicon steel, permalloy, Mumetal, soft ferrites, amorphous alloys. **Applications** - Cores of power transformers, motors and generators (silicon steel, CRGO): low loss under alternating flux. - Relays and electromagnets (soft iron): quick release after the coil is switched off. - High-frequency transformers, SMPS, RF inductors, antenna rods (soft ferrites): high resistivity, low eddy loss. - Magnetic shielding and current transformers (permalloy, Mumetal): very high permeability. - Recording and reading heads (permalloy, ferrite). #### 2. Hard magnetic materials - Wide loop, high coercivity ($H_c$ > $10^4$ A/m), high retentivity, large $(BH)_{max}$, large hysteresis loss. - Hard to magnetize and demagnetize; domain walls are pinned by impurities and fine particles. - Examples: carbon/tungsten steel, Alnico, NdFeB, SmCo, barium/strontium ferrite. **Applications** - Permanent magnets in loudspeakers, microphones and headphones. - PMDC motors, small generators, magnetos, alternators in bikes. - Moving-coil meters, energy meters (braking magnet). - MRI and magnetic separators, magnetic holders and locks. #### 3. Square-loop (rectangular-loop) materials - Nearly rectangular loop with two stable states $\pm B_r$ and sharp switching at $H_c$. - Examples: γ-Fe₂O₃, CrO₂, Mg-Mn ferrites, Co-alloy thin films. - **Applications**: magnetic tapes, hard disks, magnetic cards, old core memories and magnetic switching devices, where $+B_r$ and $-B_r$ represent binary 1 and 0. | Feature | Soft | Hard | |---|---|---| | Loop | Narrow | Wide | | $H_c$, $B_r$ | Low | High | | Loss | Low | High | | Main use | AC cores, relays | Permanent magnets | ## Check yourself The answer's key points as multiple-choice questions, with the correct option and why. 1. Based on the hysteresis loop, materials are classified as: A. Soft and hard, plus square-loop materials as a special group B. Dia, para and ferro C. Metals and insulators D. Only soft and paramagnetic Answer: A. Classification depends on loop shape and size. (Key point: Classes) 2. Why is soft iron used in relays and electromagnets? A. High retentivity B. Quick release after the coil is switched off C. Large (BH)max D. Pinned walls Answer: B. Low retentivity avoids sticking. (Key point: Soft use) 3. Which are applications of hard materials? A. Transformer cores B. RF inductors C. Loudspeakers, PMDC motors and moving-coil meters D. Relay armatures Answer: C. They need permanent flux. (Key point: Hard use) 4. Why are square-loop materials used in tapes and disks? A. Zero remanence B. Narrow loop for low loss C. Very high permeability D. Two stable states +Br and -Br with sharp switching represent 1 and 0 Answer: D. The square loop gives clean binary states. 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