Why does a piece of iron in a current-carrying loop increase the magnetic field strength?

Answer: The magnetic domain in the metal is aligned when a piece of metal is placed in a magnetic coil.

Explanation: When a piece of metal is placed in a magnetic coil the field causes the electrons of the metal to gain some magnetic properties, the magnetic field causes the electrons of the metal to align and move in a similar direction forming domains, this action of the electrons of the metal also add to the magnetic properties of the coil, therefore increasing it.