Ferrite

A ferrite is an iron oxide ceramic with other metal ions substituted into it, and its defining trick is holding a strong magnetic field while conducting almost no electricity.

Every metal magnetic material conducts, and a conductor in a changing field develops eddy currents that waste energy as heat, rising with the square of the frequency. Above about twenty kilohertz no lamination is thin enough to control them. A ferrite's resistivity is millions of times higher than steel's, so the currents essentially cannot flow, and it works at frequencies where a metal core would simply cook.

It splits into two families with opposite specifications. Soft ferrites — manganese-zinc and nickel-zinc — magnetise and demagnetise easily and are the cores in every switched-mode power supply, every wireless charger, and the beads clamped around cables to kill interference. Hard ferrites — barium and strontium — stay magnetised, and are the cheapest permanent magnet there is: black, brittle, weak per unit volume, and the majority of all magnets made by unit.

It is also the material of magnetite, which is where the whole idea started: lodestone is a naturally occurring ferrite, and it is why anybody knew magnetism existed.

The soft ferrites have the spinel structure — the same atomic arrangement as magnetite — with iron on some sites and manganese, zinc or nickel on others, and the magnetism comes from an arrangement that is not quite what it looks like.

The metal ions sit on two kinds of site, and their magnetic moments point in opposite directions and do not cancel, because the sites are unequally occupied. That is ferrimagnetism rather than ferromagnetism, and it is why a ferrite is magnetic at all despite being an oxide: the iron atoms are too far apart to interact directly and couple instead through the oxygen between them.

The consequence is a weaker magnetisation than a metal — a ferrite saturates at roughly a tenth to a third of the flux density that iron does — traded for a resistivity that is not merely higher but of a different order entirely.

The hard ferrites are a different structure, hexagonal rather than cubic, and their coercivity comes from crystal anisotropy so strong that the magnetisation is very hard to turn away from one axis. That is what makes them permanent, and it is also why they are made by pressing powder in a magnetic field so that every particle is aligned before firing.

Ceramic processing throughout: oxides milled together, calcined so they react into the ferrite phase, milled again to a fine powder, pressed to shape and sintered at 1,100 to 1,300 °C.

The hard grades add the step that makes them worth having. The powder is pressed in an applied magnetic field so that the anisotropic particles all line up before firing, which roughly doubles the strength of the finished magnet over an unaligned one. The trade is that the magnet then has a single working direction fixed at manufacture.

Because it is a sintered ceramic, a ferrite cannot be machined conventionally, shrinks on firing, and is brittle. Cores are made to shape — pot cores, toroids, E-cores — and ground where a tolerance is needed. Dropping one breaks it, and a hairline crack in a core is a gap in the magnetic circuit, which changes the inductance of the part it is in.

Magnetite has been known as lodestone since antiquity and is the reason magnetism was discovered at all, but engineered ferrites are a twentieth-century material, developed at Philips in the Netherlands by Snoek and colleagues in the 1930s and 1940s and commercialised as Ferroxcube.

They arrived exactly when they were needed. Radio and then television required magnetic cores at frequencies where iron was useless, and ferrite made those circuits possible.

Their most consequential application is now historical. Magnetic core memory — a grid of tiny ferrite rings, each storing one bit by the direction it was magnetised, threaded by hand with wires — was the main working memory of computers from the mid-1950s to the mid-1970s. It is why a memory image is still called a core dump. It was displaced entirely by semiconductor memory, which is one of the few cases of a material being wholly retired from an application rather than gradually eroded.

Hard ferrite is the volume magnet of the world and is almost never discussed, because the argument about magnets is about neodymium.

A neodymium magnet is roughly ten times stronger per unit volume, which is why anything where mass or size matters uses one. Ferrite is a small fraction of the price, uses iron oxide and barium or strontium carbonate rather than a rare earth, and has no supply concentration problem worth the name.

So the two coexist along a clean line: where the magnet must be small, it is neodymium; where it merely has to work, it is ferrite. Loudspeakers, windscreen wiper motors, magnetic separators, holding magnets and every fridge magnet ever made are ferrite.

That line has moved twice on price. When neodymium spiked in 2011, designers moved products back to ferrite within a year, and some stayed. It is a useful corrective to the assumption that a better material wins permanently.