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Significance of hysteresis loop

Short answer

The hysteresis loop area indicates energy loss per cycle; narrow loops suit soft magnetic cores for low loss, while wide loops with high coercivity and remanence suit permanent magnets. Square loops are used in memory.

Explain the significance of hysteresis loop while selecting materials for preparing magnetic materials.

The hysteresis loop (B–H loop) shows how flux density BB lags behind magnetizing force HH when a ferromagnetic material is taken through a full magnetization cycle. Its shape tells the designer which material suits which job.

            B
            ^      Bs
            |    _____
        Br  |   /    /
            |  /    /
   ---------+-/----/------> H
        -Hc |/    / Hc
           /|    /
     _____/ |
            |

Quantities read from the loop and their significance

QuantityMeaningSignificance in selection
Area of loopEnergy lost per cycle per m³Small area → low hysteresis loss (cores)
Retentivity BrB_rB left when H = 0High BrB_r → strong permanent magnet
Coercivity HcH_cReverse H needed to make B = 0High HcH_c → magnet hard to demagnetize
Saturation BsB_sMaximum BHigh BsB_s → smaller, lighter cores
Slope (permeability)μ=B/H\mu = B/HHigh μ\mu → less magnetizing current
(BH)max(BH)_{max}Max energy productFigure of merit of a permanent magnet

Use in selection

  • Soft magnetic materials (narrow, tall loop): low HcH_c, small loop area, high μ\mu, high BsB_s. Chosen for transformer, motor and generator cores, relays and electromagnets, where the field reverses many times and loss must be low. Examples: silicon steel, permalloy, soft ferrites.
  • Hard magnetic materials (wide, fat loop): high HcH_c, high BrB_r, large (BH)max(BH)_{max}. Chosen for permanent magnets in meters, loudspeakers, PMDC motors. Examples: Alnico, NdFeB, SmCo, hard ferrites.
  • Square loop materials: two stable states ±Br\pm B_r; used in magnetic memory and switching cores.

Hysteresis power loss (Steinmetz): Ph=ηBmax1.6fVP_h = \eta B_{max}^{1.6} f V watts (loop area × frequency × volume), so for AC machines the material with the smallest loop area is selected.

✓ Answer checked by Claude Opus 5.5 (high effort)

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