a ferrite

A ferrite is a magnet made of rust-like ceramic rather than metal. Chemically these are iron-oxide compounds, and they give you real, holdable magnetism, the black fridge magnet, the core inside a phone charger, without using any of the expensive metallic iron, cobalt, or nickel. Their magnetism comes from ferrimagnetism, the unequal-tug-of-war ordering of moments on two sublattices.

There are two big families. The soft ferrites are cubic spinels with the formula MFe2O4, where M is a metal like manganese, zinc, or nickel; they are easy to magnetize and demagnetize and are used as cores. The hard ferrites are hexagonal, such as barium ferrite BaFe12O19, and keep their magnetism strongly, making cheap permanent magnets. Because a ferrite is an oxide, it is an electrical insulator, and this is its killer feature: in an alternating field a metal core wastes energy heating itself with induced eddy currents, but an insulating ferrite barely does, so it stays efficient up to megahertz and gigahertz frequencies.

That is why ferrites are everywhere in electronics: the cores of high-frequency transformers and inductors, the beads clamped on cables to kill interference, antenna rods in AM radios, and the read heads and recording layers of older magnetic storage. The tradeoff is honest: ferrites are ceramics, so they are brittle and their saturation magnetization is lower than solid iron's. When you need the strongest possible field at low frequency you still reach for iron; when you need efficiency at high frequency, or a cheap permanent magnet, you reach for a ferrite.