Western Digital is busy with mamr (Microwave Assisted Magnetic Recording), Seagate and Toshiba are working on hamr (Heat Assisted Magnetic Recording). 

For a little whole now 18TB HDDS have been around. Seagate has been talking about HAMR hard drives for years, but so far the company has probably been able to advance the capacities sufficiently without this complex and expensive technology. Seagate is now promising that a HAMR hard drive will finally come onto the market in December. Seagate starts with 20 TB, but further details are not yet known, the company has not yet commented on the price.

Seagate expects a data density of 2 to 6 Tbit per square inch from HAMR, five times the current data density. In the presentation, various Seagate managers mentioned HAMR drives with 50 Tbytes. 

Seagate dropping the HAMR by releasing a 20TB HDD Soon