Which of the following materials is ferromagnetic?

• A. Iron
• B. Aluminum
• C. Copper
• D. Gold

Answer: (A)

Ferromagnetism is when a material can be permanently magnetized because its magnetic moments tend to align with each other due to strong exchange interactions, forming regions called domains that can be reoriented by an external field and sometimes stay aligned afterward.

Iron fits this idea. It has unpaired electrons in its 3d orbitals, giving each atom a magnetic moment. The exchange interaction between neighboring iron atoms strongly favors parallel alignment of these moments, so below the Curie temperature the moments lock into a common direction within domains. When you apply a magnetic field, domains align with the field; once the field is removed, some magnetization can remain, which is what we call ferromagnetism.

The other metals don’t show this behavior. Aluminum has only very weak paramagnetic responses and doesn’t maintain magnetization. Copper and gold are diamagnetic, producing only a tiny induced effect opposite to an applied field and also don’t retain magnetization in zero field. So iron is the ferromagnetic material.