Core of electromagnets are made of ferromagnetic materials which have high permeability and low retentivity.
Soft iron is a suitable material for electromagnets. On placing a soft iron rod in a solenoid and passing a current, we increase the magnetism of the solenoid by a thousand fold. When we switch off the solenoid current, the magnetism is effectively switched off since the soft iron core has a low retentivity. Electromagnets have temporary magnetism & are used in making electromagnets, cores of transformers, motors & generators etc.. e.g. Soft iron, Stalloy