As an example - magnetic disk/platters is a circular aluminum disk, similar in shape to a normal cd or dvd, with surface coated with chrome oxide based coating with the sole exception of the read/write arm-head parking zone. In fact, high precision of disk surfaces and the heads causes them to "stick" to each other because of molecular attraction forces. To prevent that effect, manufacturers often use special laser serrations in the zone of contact between the heads and the platters.
