A hard disk drive platter (or disk) is the circular disk on which magnetic data is stored in a
hard disk drive
A hard disk drive (HDD), hard disk, hard drive, or fixed disk is an electro-mechanical data storage device that stores and retrieves digital data using magnetic storage with one or more rigid rapidly rotating platters coated with magnet ...
. The rigid nature of the platters in a hard drive is what gives them their name (as opposed to the flexible materials which are used to make
floppy disk
A floppy disk or floppy diskette (casually referred to as a floppy, or a diskette) is an obsolescent type of disk storage composed of a thin and flexible disk of a magnetic storage medium in a square or nearly square plastic enclosure lined w ...
s). Hard drives typically have several platters which are mounted on the same
spindle
Spindle may refer to:
Textiles and manufacturing
* Spindle (textiles), a straight spike to spin fibers into yarn
* Spindle (tool), a rotating axis of a machine tool
Biology
* Common spindle and other species of shrubs and trees in genus ''Euony ...
. A platter can store information on both sides, requiring two heads per platter.
Design
The magnetic surface of each platter is divided into small sub-micrometer-sized magnetic regions, each of which is used to represent a single binary unit of information. A typical magnetic region on a hard-disk platter (as of 2006) is about 200–250 nanometers wide (in the radial direction of the platter) and extends about 25–30 nanometers in the down-track direction (the circumferential direction on the platter), corresponding to about 100 billion bits per square inch of disk area (15.5
Gbit
The bit is the most basic Units of information, unit of information in computing and digital communications. The name is a portmanteau of binary digit. The bit represents a truth value, logical state with one of two possible value (computer sc ...
/cm
2). The material of the main ''magnetic medium'' layer is usually a
cobalt
Cobalt is a chemical element with the symbol Co and atomic number 27. As with nickel, cobalt is found in the Earth's crust only in a chemically combined form, save for small deposits found in alloys of natural meteoric iron. The free element, pr ...
-based alloy. In today's hard drives each of these magnetic regions is composed of a few hundred magnetic grains, which are the base material that gets magnetized. As a whole, each magnetic region will have a magnetization.
One reason magnetic grains are used as opposed to a continuous magnetic medium is that they reduce the space needed for a magnetic region. In continuous magnetic materials, formations called ''Néel spikes'' tend to appear. These are spikes of opposite magnetization, and form for the same reason that bar magnets will tend to align themselves in opposite directions. These cause problems because the spikes cancel each other's
magnetic field
A magnetic field is a vector field that describes the magnetic influence on moving electric charges, electric currents, and magnetic materials. A moving charge in a magnetic field experiences a force perpendicular to its own velocity and to ...
out, so that at region boundaries, the transition from one magnetization to the other will happen over the length of the Néel spikes. This is called the transition width.
Grains help solve this problem because each grain is in theory a single
magnetic domain
A magnetic domain is a region within a magnetic material in which the magnetization is in a uniform direction. This means that the individual magnetic moments of the atoms are aligned with one another and they point in the same direction. When c ...
(though not always in practice). This means that the magnetic domains cannot grow or shrink to form spikes, and therefore the transition width will be on the order of the diameter of the grains. Thus, much of the development in hard drives has been in reduction of
grain size
Grain size (or particle size) is the diameter of individual grains of sediment, or the lithified particles in clastic rocks. The term may also be applied to other granular materials. This is different from the crystallite size, which refer ...
.
Manufacture
Platters are typically made using an
aluminium
Aluminium (aluminum in American and Canadian English) is a chemical element with the symbol Al and atomic number 13. Aluminium has a density lower than those of other common metals, at approximately one third that of steel. I ...
,
glass
Glass is a non-crystalline, often transparent, amorphous solid that has widespread practical, technological, and decorative use in, for example, window panes, tableware, and optics. Glass is most often formed by rapid cooling (quenching) of ...
or ceramic substrate. As of 2015, laptop hard drive platters are made from glass while aluminum platters are often found in desktop computers. In disk manufacturing, a thin coating is deposited on both sides of the substrate, mostly by a
vacuum deposition
Vacuum deposition is a group of processes used to deposit layers of material atom-by-atom or molecule-by-molecule on a solid surface. These processes operate at pressures well below atmospheric pressure (i.e., vacuum). The deposited layers can r ...
process called magnetron
sputtering
In physics, sputtering is a phenomenon in which microscopic particles of a solid material are ejected from its surface, after the material is itself bombarded by energetic particles of a plasma or gas. It occurs naturally in outer space, and can ...
. The coating has a complex layered structure consisting of various metallic (mostly non-magnetic) alloys as underlayers, optimized for the control of the crystallographic orientation and the grain size of the actual magnetic media layer on top of them, i.e. the film storing the bits of information. On top of it a protective carbon-based overcoat is deposited in the same sputtering process. In post-processing a nanometer thin polymeric lubricant layer gets deposited on top of the sputtered structure by dipping the disk into a solvent solution, after which the disk is buffed by various processes to eliminate small defects and verified by a special sensor on a flying head for absence of any remaining asperities or other defects (where the size of the bit given above roughly sets the scale for what constitutes a significant defect size). In the hard-disk drive the
hard-drive heads fly and move radially over the surface of the spinning platters to read or write the data. Extreme smoothness, durability, and perfection of finish are required properties of a hard-disk platter.
In 1990,
Toshiba
, commonly known as Toshiba and stylized as TOSHIBA, is a Japanese multinational conglomerate corporation headquartered in Minato, Tokyo, Japan. Its diversified products and services include power, industrial and social infrastructure system ...
released the MK1122FC, the first hard drive to use a glass substrate, replacing the aluminium alloys used in earlier hard drives. It was originally designed for
laptop
A laptop, laptop computer, or notebook computer is a small, portable personal computer (PC) with a screen and alphanumeric keyboard. Laptops typically have a clam shell form factor with the screen mounted on the inside of the upper li ...
s, for which the greater shock resistance of glass substrates are more suitable. Around 2000, other hard drive manufacturers started transitioning from aluminum to glass platters because glass platters have several advantages over aluminum platters.
[
Scott Mueller.
''"PC Hardware Library Volume I: Hard Drives"''.
Sectio]
"Hard Disk Platters (Disks)"
1998.
In 2005–06, a major shift in technology of hard-disk drives and of magnetic disks/media began. Originally, in-plane magnetized materials were used to store the bits but has now been replaced by
perpendicular recording
Perpendicular recording (or perpendicular magnetic recording, PMR), also known as conventional magnetic recording (CMR), is a technology for data recording on magnetic media, particularly hard disks. It was first proven advantageous in 1976 by Sh ...
.
The reason for this transition is the need to continue the trend of increasing storage densities, with perpendicularly oriented media offering a more stable solution for a decreasing bit size. Orienting the magnetization perpendicular to the disk surface has major implications for the disk's deposited structure and the choice of magnetic materials, as well as for some of the other components of the hard-disk drive (such as the head and the electronic channel).
See also
*
Data recovery hardware
In computing, data recovery is a process of retrieving deleted, inaccessible, lost, corrupted, damaged, or formatted data from secondary storage, removable media or files, when the data stored in them cannot be accessed in a usual way. The da ...
*
Heat-assisted magnetic recording
Heat-assisted magnetic recording (HAMR) (pronounced "''hammer")'' is a magnetic storage technology for greatly increasing the amount of data that can be stored on a magnetic device such as a hard disk drive by temporarily heating the disk materia ...
References
{{Reflist
Hard disk drives