In order for a substance to be magnetized, the atoms in it must line up so that their magnetic moments point in the same direction. In ferromagnetic materials, such as an iron bar, the atoms already tend to line up with their magnetic moments pointing in the same direction, but unless an external magnetic field is applied small errors accumulate which result in the bar as a whole being non-magnetic. If you use a strong external field, however, you can get the atoms throughout the iron bar to line up if you provide some encouragement. You can magnetize a piece of iron by stroking it with a magnet, or by placing it in a strong magnetic field and hitting it repeatedly on one end, or even by heating it red-hot and then cooling it in a strong magnetic field. However, this only works with ferromagnetic materials.
Magnetism is lost when the external field is removed.
The material is not normally magnetic, but when an external magnetic field is applied the atoms line up their magnetic moments with the external field. Magnetism is lost when the external field is removed.