The Evolution and Revolutions in Disk Drive Recording

Since 1956 the areal density of hard disk drives, HDDs, has increased by eight orders of magnitude through a process of evolution punctuated by a number of important revolutions. The disk evolved for three decades through many generations of painted gamma ferric oxide particulate media with in plain orientation. During this time areal density was increased from 2 kilo-bits/inch2 (2kbpsi) for IBM’s RAMAC to ~20Mbpsi .

The technology has seen a number of revolutions. In the mid 1980s the first (non-magnetic!) revolution was a diamond like carbon over coat for media that is key to its durability. The next revolution was the introduction of read sensors based on Giant Magneto-Resistive films with improved sensitivity. HDD proceeded to evolve up to ~100 Gbpsi on this technology base.

By the mid 1990’s Prof. Stanley Charap of Carnegie Mellon University calculated that longitudinal recording would start to experience thermal decay of the data at densities of ~40 Gbpsi. In response to this impeding crisis, the Ultra-High Density Recording project was initiated by Prof. Mark Kryder (CMU) under the National Storage Industry Consortium umbrella. The UHDR team established the reality of the problem and proposed strategies to delay the crisis to ~100 Gbpsi. Key amongst these was to increase tracks per inch faster than bits per inch.

The UHDR theory team also determined that magnetizing the media perpendicular to the disk could extend magnetic recording by almost an order of magnitude beyond the thermal decay limit of longitudinal recording. Perpendicular HDDs are now being shipped at ~300Gbpsi. Key head innovations in achieving this density are the use of the the Shielded Pole writer invented by the author, and the Tunneling Magneto-Resistive reader with an MR effect approaching 100%.

The 30-40% per year growth in areal density will soon drive perpendicular recording to its thermal decay limit near 1 Tbpsi in demonstrations and less in products. Two revolutionary technologies are being developed to deal with this. Heat Assisted Magnetic Recording will allow high anisotropy media to be written at elevated temperatures thus allowing for finer thermally stable grains to be written. Bit Patterned Media will allow the recording of a bit on a single grain as compared to scores of grains with unpatterned media. The promise and problems of these technologies will be discussed in detail.