B-H Hysteresis Loop

Hysteresis: the B vs H curve of a ferromagnet does NOT retrace itself when H is reversed. It forms a CLOSED LOOP.

Starting from unmagnetised material (B = 0, H = 0): as H increases, B follows the INITIAL MAGNETIZATION CURVE up to saturation B_s.

Reducing H to zero: B does NOT return to zero. It retains REMANENT MAGNETIZATION B_r (residual magnetism).

Reversing H: B decreases through zero at COERCIVE FIELD H_c (the negative H required to demagnetize).

Further reverse H: saturates at −B_s. Then reversing again traces out a closed loop.

Area enclosed by the loop = energy dissipated per unit volume per cycle (heat). Source of transformer core losses.

Soft magnets (Si-iron): narrow loop, small area ⇒ small hysteresis loss. Used in transformers.

Hard magnets (alnico, NdFeB): wide loop ⇒ large H_c ⇒ retain magnetization strongly. Used as permanent magnets.