In hard drives, the write head is the component that converts the electrical data signal into a magnetic field that magnetizes the disk‘s surface in order to store the data. The head “flies” or floats a very small distance above the disk surface — as close as three nanometers above.
Early write heads were very similar to the heads found in tape recorders, consisting of a coil of wire wrapped around ferrite. Technology has advanced (and continues to advance) to where heads can now write extremely dense fields of data to the disk, with high stability and reliability.