a hard ferrite
/ hex-uh-FAIR-ite /
A hard ferrite is a permanent magnet made of ceramic — the cheap, black, brittle magnet that holds notes to your fridge, drives the woofer in a loudspeaker, and spins inside small electric motors. 'Hard' is the magnetic opposite of 'soft': where a soft ferrite lets its magnetisation flip easily, a hard ferrite clamps onto its magnetisation and refuses to let go, so once magnetised it stays magnetised for years. By tonnage, hard ferrites are the most-produced permanent magnets on Earth.
The magnetic stubbornness comes from crystal shape. Hard ferrites are hexagonal ferrites — barium hexaferrite BaFe12O19 or strontium hexaferrite SrFe12O19 — with the magnetoplumbite structure. A hexagonal crystal has a strong magnetocrystalline anisotropy: the magnetisation strongly prefers to lie along one special crystal axis and resists being turned away from it, and that resistance is exactly what gives a high coercivity (a fat, wide hysteresis loop). The trade-off is honest: a hard ferrite has a remanence of only about 0.4 tesla and a maximum energy product (BH)max of roughly 25 to 35 kilojoules per cubic metre — several times weaker than a rare-earth neodymium magnet — but it costs a small fraction as much, does not rust, and keeps working up to a Curie temperature around 450 degrees C.
So hard ferrites win wherever cheapness, corrosion-resistance and temperature stability matter more than raw strength: loudspeaker and motor magnets, magnetic door catches, holding and separation magnets, and the striped magnets on fridge-magnet sheets. To squeeze out more performance, the tiny platelet-shaped crystals are aligned in a magnetic field while the powder is pressed, so their easy axes all point the same way (an anisotropic grade). The limits to remember: they are brittle like all ceramics, and their energy product is far below that of rare-earth magnets, so a ferrite motor is bigger and heavier than a neodymium one of the same power.
A loudspeaker's ring magnet is usually strontium hexaferrite. Its high coercivity means the strong, alternating field of the voice coil cannot demagnetise it over the life of the speaker, and its low cost is why a ferrite magnet, not a rare-earth one, sits in almost every budget loudspeaker.
A hard ferrite grips its magnetisation for years — a permanent magnet that is cheap, rust-proof, and heat-tolerant.
A hard ferrite is 'hard' magnetically, not the hardest ceramic mechanically — and it is far weaker as a magnet than a rare-earth NdFeB. Its dominance rests on price and durability, not field strength; when you need a small, powerful magnet, ferrite is not the answer.