Ferromagnets: In certain substances like Fe, Co, Ni etc. magnetization not only depends on the applied magnetic field but it also depends on the history of the material. In a phenomenon known as hysteresis , a sample of a ferromagnet may exhibit magnetization even when no magnetic field is present exhibiting a memory effect.
When a sample of such a material which has no initial magnetic moment is subjected to a magnetic field, the magnetization increases with increasing strength of magnetic field and soon saturates when all the atomic moments have got aligned in the direction of the magnetic field. If field strength is now decreased gradually, the magnetization decreases. However, the magnetization curve does not retrace its path. When the field strength has been reduced to zero, the sample still has some magnetization left. In order to bring the sample to a state of zero magnetization, a coercive field must be applied in the reverse direction.