a ferrite
/ FAIR-ite /
A ferrite is a ceramic magnet: an iron-oxide-based ceramic that is magnetic and, at the same time, an electrical insulator. The little black magnet on your fridge, the ring-shaped core inside a phone charger, the rod antenna in an old radio — all are ferrites. What makes them special is that unlike an iron bar magnet, a ferrite barely conducts electricity, so a changing magnetic field cannot set up wasteful swirling currents (eddy currents) inside it. That is why, when magnetism has to work at high frequency, engineers reach for a ceramic rather than a metal.
Ferrites are magnetic by a mechanism called ferrimagnetism. They contain two sets of magnetic ions on different crystal sites whose magnetic moments point in opposite directions but do not cancel, because one set is stronger or more numerous than the other — leaving a net magnetisation. The main structural families are the spinel ferrites, formula M-Fe2O4 with M a divalent metal like manganese, nickel or zinc (the soft magnets); the hexagonal ferrites like barium hexaferrite BaFe12O19 (the hard permanent magnets); and the garnets such as yttrium iron garnet (the microwave materials). Because these are oxides, their electrical resistivity is enormous — from about 10^2 to 10^8 ohm-centimetres, against roughly 10^-5 for metallic iron, a difference of ten billion or more.
That combination — usefully magnetic yet electrically insulating — is why ferrites quietly run the high-frequency world: transformer and inductor cores in power supplies, the coils in wireless chargers, antenna rods, noise-suppression beads on cables, cheap permanent magnets in motors and speakers, and non-reciprocal microwave devices. A crucial disambiguation for anyone coming from metallurgy: in steel science 'ferrite' means the body-centred-cubic alpha-iron phase, a completely different thing. In ceramics, a ferrite is always the iron-oxide magnetic ceramic described here.
Wind a coil around a ferrite ring and you have an inductor that works cleanly at hundreds of kilohertz. Try the same with a solid iron ring and, at that frequency, eddy currents circling inside the metal would heat it up and waste much of the energy. The ferrite's near-insulating oxide body is what lets the magnetism follow the fast-changing current without such losses.
A ferrite is magnetic like a metal but insulating like a ceramic — the best of both for high-frequency work.
Do not confuse the ceramic ferrite with the metallurgical 'ferrite' (alpha-iron) in steel — same word, unrelated material. And note ferrites are ferrimagnetic, not ferromagnetic: their net magnetism comes from two opposed, unequal sublattices, so their saturation magnetisation is lower than that of metallic iron.