I had another thought. In all magnetic recording done up to now the north and south pole of the magnetized spot is in the same plane as the recording medium, it's far easy to make them that way, you don't need to maneuver a magnet above and below a spinning disk, but the proximity of opposite poles partially cancels each other out and limits the density of magnetic spots you can read. I don't know it for a fact but it seems to me that with magnetic-optical technology it would be just as easy to make the north and south magnetic pole perpendicular to the plane of the recording medium. If so then that alone would allow a huge increase in the density of magnetic spots you could detect.
John K Clark firstname.lastname@example.org