page_title: Hard disk drive platter
section_title: Design
hierarchical_section_title: Hard disk drive platter / Design
caption_attribution_description: Difference of Transition width on a magnetic medium between regular surface and grained surface.
context_page_description: A hard disk drive platter is the circular disk on which magnetic data is stored in a hard disk drive. The rigid nature of the platters in a hard drive is what gives them their name. Hard drives typically have several platters which are mounted on the same spindle. A platter can store information on both sides, requiring two heads per platter.
context_section_description: 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/cm²). The material of the main magnetic medium layer is usually a cobalt-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 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 (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.
caption: Comparison of the transition width caused by Néel Spikes in continuous media and granular media, at a boundary between two magnetic regions of opposite magnetization
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