soft iron

Soft iron is iron or low-carbon steel that magnetizes easily but doesn't retain magnetism permanently—it becomes magnetic when exposed to a magnetic field and loses that magnetism when the field is removed.

This property makes it ideal for electromagnet cores, transformer cores, and relay armatures, where you need magnetism that can be switched on and off.

The term contrasts with hard iron or steel, which retains magnetism and is used for permanent magnets.