The disks are often made of surface-treated aluminum alloys. For discs with high data density typically use magnesium alloys, glass or glass composites because these materials exhibit less diffusion. They must be as dimensionally stable (under both mechanical and thermal load) and have a low electrical conductivity. Since the magnetizable layer must be very thin, the material of the washers may, however, have no magnetic properties and serve only as a support of the magnetic layer.
The discs have a cobalt layer or iron oxide applied approximately at one micron in thickness. Today’s hard drives are produced by sputtering high-density storage media (materials for high density) as CoCrPt. The magnetic layer is additionally provided with a layer of diamond-like carbon (carbon overcoat) to avoid mechanical damage.
The future reduction of the magnetic `bits’ requires both the high density storage media as well as alternative concepts, as you slowly approach the superparamagnetic limit. In addition, an increase in data density through better support material and by optimizing the writing process has been reached.
Desktop hard drives from 2000 to 2002 by IBM (DTLA-30xxxx 75GXP/40GV Deskstar, Deskstar 60GXP/120GXP IC35Lxxxx) used glass as a material for the discs used. Newer models of the IBM hard disk drive (2003 taken over by Hitachi) use aluminum.
The hard drive enclosure contains one or more superimposed, rotating disks. HDDs have been built with up to twelve discs. Energy consumption and noise level within a disk increase with the number of disks. It is customary to use all surfaces of the plates (plates n, 2n read-write heads). Some disk sizes (eg 320 GB disks with 250 GB / disk) come up with an odd number of write heads (here: 3) and do not use a surface.
With replacement of the Longitudinal Magnetic Recording by Perpendicular Magnetic Recording (PMR), a well known since the 1970s, but at that time did not master memory principle, managed by intensive research since 2000 to increase the data density.
The first hard drive with this storage technology from Hitachi came in 2005: a 1.8 “hard drive with 60 GB. All hard drives contain the technology developed since 2008 (from 200 GB / disk at 3,5 “).