Powdered-iron and ferrite cores

Ferromagnetic substances can be crushed into dust and then mounded into various shapes, providing core materials which greatly increase the inductance of a coil having a given the number of turns. Depending on mixture used, the increase in flux density can range from the factor of a few times, up through hundreds, and even millions of times. A small coil can therefore be made to have a large inductance.

Powdered-iron cores are common at the radio frequencies. Ferrite has a higher permeability than powdered iron, creating a greater concentration of magnetic flux lines within coil. The ferrite is used at lower radio frequencies and at the audio frequencies, as well as at the medium and high radio frequencies.

The main trouble with the ferromagnetic cores is that, if coil carries more than specific amount of current, the core will get saturate.  This means that the ferromagnetic material can be holding as much flux as it possibly can. Any further increase in the coil current will not produce a corresponding increase in magnetic flux in core. The result is that inductance changes, decreasing with the coil currents which are more than the critical value.

In the extreme cases, ferromagnetic cores can lose considerable power as heat. If a core gets hot enough, it may fracture. This will permanently change inductance of the coil, and will reduce its current handling ability.