When a ferromagnetic material is placed in absence of any field different domains have different directions of magnetic moment and hence the material remains unmagnetized.

When a magnetic field is applied the domains towards the direction of magnetic field increases in size whereas domains opposite to the direction of magnetic field decreases in size. This happens because walls of the domain move across the sample.

Therefore when a ferromagnetic material is placed in an external magnetic field the magnetic domains may increase or decrease in size