Granular media
The recording layer of a hard disk, built from thousands of microscopic magnetic grains each walled off from its neighbours by a non-magnetic material.
The surface of a modern platter is not a smooth magnetic film but a packing of grains — typically a cobalt-chromium-platinum alloy — with each grain wrapped in an oxide boundary so neighbours don't magnetically contaminate one another. A single bit is not stored in one grain but spread across many, because averaging over a cluster gives the signal-to-noise margin a lone grain lacks.
It is grain size that sets the ceiling on how densely you can write. Smaller grains mean more bits per area, but small grains lose their direction to heat — the superparamagnetic limit. Perpendicular recording uses granular media over a soft underlayer that mirrors the write field, and bit-patterned media is the attempt to replace the random grain packing with an ordered array of islands.