Editing Hard disk drive platter (section)

== 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),{{Citation needed|date=February 2019}} corresponding to about 100 billion bits per square inch of disk area (15.5&nbsp;[[gigabit|Gbit]]/cm<sup>2</sup>). 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.

Many hard drive platters have a layer of lubricant made of amorphous carbon such as [[diamond-like carbon]], called an overcoat, which is deposited onto the disk using sputtering, or using chemical vapor deposition.<ref>{{cite journal | url=https://www.sciencedirect.com/science/article/abs/pii/S0301679X02002402 | doi=10.1016/S0301-679X(02)00240-2 | title=Amorphous carbon overcoat for thin-film disk | date=2003 | last1=Yamamoto | first1=T. | last2=Hyodo | first2=H. | journal=Tribology International | volume=36 | issue=4–6 | pages=483–487 }}</ref> Silicon Nitride, PFPE<ref>https://escholarship.org/content/qt24w0q2v0/qt24w0q2v0.pdf {{Bare URL PDF|date=August 2024}}</ref><ref>https://www.fujitsu.com/global/documents/about/resources/publications/fstj/archives/vol42-1/paper13.pdf {{Bare URL PDF|date=August 2024}}</ref> and hydrogenated carbon have also been used as overcoats.<ref>{{cite journal | url=https://ieeexplore.ieee.org/document/278743 | title=Silicon nitride overcoats for thin film magnetic recording media  | date=1991 | doi=10.1109/20.278743 | last1=Kovac | first1=Z. | last2=Novotny | first2=V.J. | journal=IEEE Transactions on Magnetics | volume=27 | issue=6 | pages=5070–5072 | bibcode=1991ITM....27.5070K }}</ref><ref>{{cite web | url=https://phys.org/news/2007-12-future-hard.html | title=Protecting Future Hard Drives }}</ref><ref>{{cite journal | url=https://pubs.aip.org/aip/jap/article-abstract/93/10/8704/530804/Coverage-and-properties-of-a-SiNx-hard-disk | doi=10.1063/1.1543136 | title=Coverage and properties of a-SiNx hard disk overcoat | date=2003 | last1=Yen | first1=Bing K. | last2=White | first2=Richard L. | last3=Waltman | first3=Robert J. | last4=Mate | first4=C. Mathew | last5=Sonobe | first5=Yoshiaki | last6=Marchon | first6=Bruno | journal=Journal of Applied Physics | volume=93 | issue=10 | pages=8704–8706 }}</ref> Alternatively PFPE can be used as a lubricant on top of the overcoat.<ref name="Graphene overcoats for ultra-high s">{{cite journal | doi=10.1038/s41467-021-22687-y | title=Graphene overcoats for ultra-high storage density magnetic media | date=2021 | last1=Dwivedi | first1=N. | last2=Ott | first2=A. K. | last3=Sasikumar | first3=K. | last4=Dou | first4=C. | last5=Yeo | first5=R. J. | last6=Narayanan | first6=B. | last7=Sassi | first7=U. | last8=Fazio | first8=D. De | last9=Soavi | first9=G. | last10=Dutta | first10=T. | last11=Balci | first11=O. | last12=Shinde | first12=S. | last13=Zhang | first13=J. | last14=Katiyar | first14=A. K. | last15=Keatley | first15=P. S. | last16=Srivastava | first16=A. K. | last17=Sankaranarayanan | first17=S. K. R. S. | last18=Ferrari | first18=A. C. | last19=Bhatia | first19=C. S. | journal=Nature Communications | volume=12 | issue=1 | page=2854 | pmid=34001870 | pmc=8129078 | arxiv=1906.00338 | bibcode=2021NatCo..12.2854D }}</ref>

[[Image:TransitionNeel.png|frame|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]] Granular media is oriented based on whether longitudinal or perpendicular magnetic recording is used.<ref>{{cite book | url=https://books.google.com/books?id=i_OU045pdF4C&dq=granular+hard+drive&pg=PA100 | isbn=978-0-444-56371-2 | title=Handbook of Magnetic Materials | date=2012 | publisher=Elsevier }}</ref> Ordered granular media can allow for higher storage densities than conventional granular media, and bit [[patterned media]] can succeed ordered granular media in storage density.<ref>{{cite web | url=https://www.anandtech.com/show/16544/seagates-roadmap-120-tb-hdds | title=Seagate's Roadmap: The Path to 120 TB Hard Drives }}</ref>

Grains help solve this problem because each grain is in theory a single [[Weiss domains|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]].<ref>{{cite book | url=https://books.google.com/books?id=NgDmEAAAQBAJ&dq=granular+hard+drive&pg=PA71 | isbn=978-0-19-287311-8 | title=Particulate and Granular Magnetism: Nanoparticles and Thin Films | date=20 February 2024 | publisher=Oxford University Press }}</ref><ref>{{cite book | url=https://books.google.com/books?id=IsbtYtqXCi8C&dq=ordered+granular+media+hard+drive&pg=PA237 | isbn=978-0-470-50100-9 | title=Developments in Data Storage: Materials Perspective | date=8 November 2011 | publisher=John Wiley & Sons }}</ref>