Yes. The disc material has some sort of phase transition, where below a certain temperature the magnetic orientation is frozen, and above it, the magnetic orienation can be changed with an external field. So you heat up the bit you want to change, and apply an external magnetic field to flip it, and then cool it, and the magnetic orientation freezes.
I have no idea how this can be done fast enough to write huge amounts of data to the drive, but then again I really don't understand how any kind of magnetical storage can even work these days.