The hysteresis loop records a complete cycle of magnetisation and demagnetisation in a ferromagnetic material. Retentivity is the residual magnetisation left when H becomes zero, while coercivity is the reverse field needed to bring magnetisation back to zero.

• Area of the hysteresis loop represents energy loss per cycle per unit volume.
• Soft iron has small loop area and low coercivity, while steel has larger loop area and higher coercivity and retentivity.
• Trap: students often swap retentivity and coercivity because both are read from the same loop but refer to different physical steps.

Diamagnetic materials are weakly magnetised opposite to the applied field, paramagnetic materials are weakly magnetised along the field, and ferromagnetic materials are strongly magnetised because of domain alignment. Curie law gives chi proportional to 1/T for paramagnets, while ferromagnets become paramagnetic above Curie temperature.

• Diamagnetic susceptibility is small and negative, paramagnetic susceptibility is small and positive, and ferromagnetic susceptibility is large and positive.
• Ferromagnetism is explained by domains, which align under external magnetic field and lose order above Curie temperature.
• Trap: Curie law does not describe ordinary ferromagnetic behavior below Curie temperature; that temperature marks the phase change to paramagnetic behavior.