The effort to increase the capacity of hard disks has run up against a wall. Perpendicular recorded media top out at 18TB with 9 play helium assemblies. Hard disk heads and media have a 2nm layer carom overcoat. Reducing the thickness of the overcoat is challenging as corrosion and other problems become problematic. The 1nm lubricant layer has to be considered as well with higher areal density.
HAMR is designed to increase the temperature of the plate above the Curie temperature where the magnetic properties change to be disordered. When an external magnetic field is applied and the plate cools it adopts the alignment of the external field. The temperature of media can reach 700°C to exceed the Curie temperature. The high temperature brings challenges to design.
Long ago in Russia scientists were able to make diamond coated disk plates. The idea was to make the media crash resistant. More recent work has been attempting to coat plates with graphene. Graphene can be applied in a very thin layer which favors improved areal density. Graphene has advantages over alternatives. The lubricant layer is also need to cope with the occasional head and media contact.
HAMR disks have been tested in corporate servers but it remains to be seen how good the reliability is. At present the perpendicular class disks in a helium sealed assembly are the best bet for gamers with larger games and media/libraries. Seagate Exos disks top out at 18TB.