Hysteresis: Retentivity and Coercivity

A ferromagnetic material "remembers" its magnetic past. Once magnetised and then subjected to a changing field, it does not respond instantly — its magnetisation lags behind the applied field, much like a rubber band that never fully returns to its original shape after repeated stretching. This lag between magnetisation M and magnetising field H is called hysteresis, and understanding it is essential for designing transformers, motors, and permanent magnets.

The phenomenon in which magnetisation M (or flux density B) lags behind the magnetising field H during a cycle of magnetisation.

Residual magnetisation retained by the material when H is reduced to zero after saturation.

SI Unit: A/m

The reverse magnetising field required to reduce residual magnetisation to zero.

SI Unit: A/m

The point beyond which M does not increase further with increasing H, as all domains are aligned.

Step-by-Step Loop Trace

1. O → A: Starting from M = 0, H = 0, increasing H raises M non-uniformly until saturation is reached at A.
2. A → B: Reducing H to zero does not bring M to zero — a residual magnetisation OB remains. This is retentivity.
3. B → C: Applying a reverse field reduces M further; at C, M = 0. The reverse field OC needed is the coercivity.
4. C → D: Continuing the reverse field drives the material to saturation in the opposite direction at D.
5. D → E → F → A: Reversing the field again retraces a similar path — through negative retentivity (E) and positive coercivity (F) — closing the loop back at A.

• Ferromagnetic materials contain magnetic domains — regions where atomic magnetic moments are already aligned.
• On magnetisation, favourably oriented domains grow in size, and others rotate to align with the applied field.
• When the field is removed, domain boundaries do not fully return to their original configuration — this incomplete recovery is the root cause of hysteresis.

• Energy supplied during magnetisation is not fully recovered during demagnetisation; the difference is dissipated as heat.
• Key Result: Energy lost per unit volume, per complete cycle, equals the area enclosed by the M–H loop.

A stretched rubber band does not fully return to its original length once released — some deformation remains. Similarly, a ferromagnetic material retains some magnetisation even after the external field is removed (retentivity), and resists losing it entirely (coercivity).