Powdered-iron and ferrite cores

Ferromagnetic substances can be crushed into dust and then bound into various shapes, providing core materials that greatly increase the inductance of a coil having a given number of turns. Depending on the mixture used, the increase in flux density can range from a factor of a few times, up through hundreds, thousands, and even millions of times. A small coil can thus be made to have a large inductance.
Powdered-iron cores are common at radio frequencies. Ferrite has a higher permeability than powdered iron, causing a greater concentration of magnetic flux lines within the coil. Ferrite is used at lower radio frequencies and at audio frequencies, as well as at medium and high radio frequencies.
The main trouble with ferromagnetic cores is that, if the coil carries more than a certain amount of current, the core will saturate. This means that the ferromagnetic material is holding as much flux as it possibly can. Any further increase in coil current will not produce a corresponding increase in the magnetic flux in the core. The result is that the inductance changes, decreasing with coil currents that are more than the critical value.
In extreme cases, ferromagnetic cores can waste considerable power as heat. If a core gets hot enough, it might fracture. This will permanently change the inductance of the coil, and will also reduce its current-handling ability.