--- title: "Hard magnets and soft high-frequency materials" subject: "Electrical Engineering Material" chapter: 4 key: "semester_3/electrical_engineering_material:hard-permanent-soft-high-frequency" source: https://www.bctengineer.com/q/semester_3/electrical_engineering_material/hard-permanent-soft-high-frequency site: BCT Engineering --- # Hard magnets and soft high-frequency materials Electrical Engineering Material · Chapter 4: Magnetic Materials - Asked in: 2070 Asar, 6 marks - Video: [Lec-22 SOFT/HARD MAGNET | | EDDY CURRENTS | EFFECTS OF CHANGE IN FREQUENCY CHANGE IN TRANSFORMER](https://www.youtube.com/watch?v=ped0qGs4ho0) — Himanshu Verma - 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-permanent-soft-high-frequency - Whole chapter: https://www.bctengineer.com/md/semester_3/electrical_engineering_material/chapter-4 ## Question Why hard magnetic materials is preferred for making permanent magnet while soft magnetic material is used for high frequency application. Explain with B-H curve. ## Short answer Hard materials have wide hysteresis loop with high Br and Hc, retaining magnetization against demagnetization, ideal for permanent magnets. Soft materials have narrow loop, low Hc and loss, plus high resistivity to reduce eddy currents, suited for high-frequency operation. ## Answer The choice depends on the shape of the B–H (hysteresis) loop. ```text B soft (narrow) B hard (wide) ^ __ ^ ______ | / / | / / -----+-/-/-----> H --------+-/-----/----> H -/-/ | -Hc / | / +Hc /_/ | /__|___/ small Hc, small area large Br, Hc, area ``` #### Hard magnetic material for permanent magnets A permanent magnet must keep strong magnetism indefinitely without a supply and resist demagnetization. - **High retentivity $B_r$**: a large flux density remains after the magnetizing field is removed, so the magnet is strong. - **High coercivity $H_c$**: a large reverse field is needed to destroy the magnetism; stray fields, vibration, shocks and temperature changes cannot easily demagnetize it. - **Large $(BH)_{max}$** (area of the second-quadrant curve): more magnetic energy stored per unit volume, so a smaller magnet is enough. - The wide loop (large hysteresis loss) does not matter because the magnet is magnetized only once. - Examples: Alnico, NdFeB, SmCo, barium/strontium ferrite; used in loudspeakers, PMDC motors, meters. #### Soft magnetic material for high-frequency applications In transformers, inductors and chokes at high frequency, the flux reverses thousands to millions of times per second. - **Narrow loop, low $H_c$**: hysteresis loss per cycle (loop area) is small. Since $P_h = \eta B_{max}^{1.6} f V$ is proportional to $f$, a small loop area is essential at high $f$. - **High permeability**: little magnetizing current is needed and the material follows the rapidly changing field. - **Low remanence**: no residual magnetism to oppose the next half-cycle. - **High resistivity** (soft ferrites, MnZn and NiZn): eddy-current loss $P_e \propto f^2$ is kept small, which is critical at high frequency. - Examples: soft ferrites in SMPS transformers, RF coils, antenna rods; permalloy and amorphous alloys in pulse transformers. | Requirement | Permanent magnet | HF core | |---|---|---| | Loop shape | Wide | Narrow | | $H_c$ | High | Low | | $B_r$ | High | Low | | Loss | Not important | Must be very low | ## Check yourself The answer's key points as multiple-choice questions, with the correct option and why. 1. Why are hard materials used for permanent magnets? A. High Br and high Hc keep strong magnetism and resist demagnetization B. Low Hc makes them easy to demagnetize C. Low Br keeps loss low D. Narrow loops reduce area Answer: A. A magnet must retain flux and resist stray fields. (Key point: Permanent magnets) 2. What does large (BH)max indicate? A. Higher hysteresis loss B. More magnetic energy stored per unit volume, so a smaller magnet suffices C. Higher eddy loss D. Lower retentivity Answer: B. (BH)max is the area of the second-quadrant curve. (Key point: BHmax) 3. Why does the large hysteresis loss of hard materials not matter for permanent magnets? A. It is cooled B. Loss is zero for hard materials C. The magnet is magnetized only once D. It operates at high f Answer: C. No alternating flux means no repeated loop traversal. (Key point: Loss ignorable) 4. Why are soft ferrites preferred at high frequency? A. Wide loop stores more energy B. Low resistivity cuts eddy loss C. High remanence helps reversal D. Narrow loop gives low hysteresis loss and high resistivity keeps eddy loss (proportional to f^2) small Answer: D. Both losses grow with frequency, so they must be small. 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