Hysteresis is well known in ferromagnetic materials. When an external magnetic field is applied to a ferromagnetic material, it becomes magnetized. When the external field is removed, the magnet will retain some field, labelled retentivity in the diagram below. To demagnetize the material an external field in the opposite direction must be applied, labelled coercivity below.
The practical effects of hysteresis include energy losses – the energy loss per unit volume is equal to the area under the hysteresis loop - and time delays – it takes time to magnetize and demagnetize a material, so for example in an alarm bell an electromagnet may continue to attract the armature when the applied electric current to the operating coil is removed
In order to minimize this effect and the power losses associated with it, ferromagnetic substances like Stelloy are often used. This has a low hysteresis loss.