For making a strong electromagnet, the material of the core should be
Soft iron
The soft iron inside the coil makes the magnetic field stronger because it becomes a magnet itself when the current is flowing through it. Soft iron is used because it loses its magnetism as soon as the current stops flowing. Soft iron is said to form a temporary magnet. In this way, the electromagnet can be switched on and off by turning the electricity on and off respectively.