How ferrite is made

It is a magnetic ceramic made mostly of rust-like iron oxide, cheap enough to stick on your refrigerator.

stations: 6
things upstream: 9
raw sources: 6
steps from the ground: 4

The short version

Iron oxide mixed with zinc, manganese or strontium compounds is baked until the metals combine into a magnetic ceramic. The cooled product is milled very fine, pressed to shape, often in a magnetic field, and sintered again. Hard grades become permanent magnets, and soft grades become the cores inside phone chargers, since they barely conduct electricity.

Blend the oxides

Finely powdered iron oxide is mixed with the other metals. Transformer cores use nickel, zinc or manganese compounds, and permanent magnets use barium or strontium carbonate.

Goes in
Iron ore Zinc oxide Manganese ore
Calcine into ferrite

Heat drives off carbon dioxide from any carbonates, and the oxides combine into the new crystal structure. Co-precipitation from solution is the other route, ending in the same calcining step.
Mill very fine

The cooled solid is milled to particles smaller than 2 μm. At that size each particle holds a single magnetic domain.
Press in a field

Powder is pressed into a core or magnet shape and dried. For permanent magnets an external magnetic field during pressing lines the particles up in one preferred direction.
Sinter again

A second firing turns the pressed powder into a hard, dense ceramic body. Particles fuse together while the shape set in the press holds.
Grind and magnetize

Parts are ground to final size. Soft cores are finished at this point, and hard magnets then get a strong field that gives them their permanent charge.

End of the lineFerrite Next it goes into headphones, phone charger, loudspeaker.

Worth repeating

Yogoro Kato and Takeshi Takei of the Tokyo Institute of Technology made the first ferrite compounds in 1930.
Most ferrites do not conduct electricity, which stops eddy currents from wasting energy in transformer cores.
Refrigerator magnets and many loudspeaker magnets are hard ferrite.