Step-by-Step Hysteresis Loop for a Ferromagnetic Material

Step 1 — Initial state (unmagnetised) Material has random domain orientations. Net magnetisation M = 0. Point: origin (H=0, B=0).

Step 2 — Increasing H (first magnetisation curve) Domains aligned with H grow at expense of others → B increases non-linearly. Curve rises from origin toward saturation.

Step 3 — Saturation (H = $H_{max}$) All domains aligned with H → maximum B = $B_{sat}$. Further increase of H produces negligible increase in B.

Step 4 — Decreasing H back toward zero Domains don't fully randomise (pinning by defects) → B follows upper curve, stays ABOVE first magnetisation curve. At H = 0: B = $B_r$ (retentivity ≠ 0).

Step 5 — H = 0, B = $B_r$ (Retentivity) Material retains magnetism even with no external field. This is the y-intercept of the hysteresis loop. Permanent magnets have high $B_r$.

Step 6 — Applying reverse H (H negative) B decreases. At H = −$H_c$: B = 0 (Coercivity point). $H_c$ is the x-intercept. Permanent magnets need large $H_c$ (hard to demagnetise).

Step 7 — Reverse saturation Further increase of reverse H → B saturates in negative direction at −$B_{sat}$.

Step 8 — Returning H to zero, then positive Loop completes symmetrically. Full cycle forms a closed loop — the hysteresis loop.

Energy significance: Area enclosed by hysteresis loop = energy dissipated per unit volume per cycle as heat.

NEET relevance:

• Soft iron (transformer core): narrow loop → low energy loss per cycle → efficient
• Steel / Alnico (permanent magnet): wide loop → high retentivity + high coercivity → retains magnetism