Hard disk drive

A hard disk drive (often shortened as "hard disk", "hard drive", or "HDD"), is a non-volatile storage device which stores digitally encoded data on rapidly rotating platters with magnetic surfaces. Strictly speaking, "drive" refers to a device distinct from its medium, such as a tape drive and its tape, or a floppy disk drive and its floppy disk. Early HDDs had removable media; however, an HDD today is typically a sealed unit (except for a filtered vent hole to equalize air pressure) with fixed media.

The platters are spun at very high speeds. Information is written to a platter as it rotates past devices called read-and-write heads that operate very close (tens of nanometers in new drives) over the magnetic surface. The read-and-write head is used to detect and modify the magnetization of the material immediately under it. There is one head for each magnetic platter surface on the spindle, mounted on a common arm. An actuator arm (or access arm) moves the heads on an arc (roughly radically) across the platters as they spin, allowing each head to access almost the entire surface of the platter as it spins. The arm is moved using a voice coil actuator or in some older designs a stepper motor.

The magnetic recording media are CoCrPt-based magnetic thin films of about 10-20 nm in thickness. The thin films are normally deposited on glass/ceramic/metal substrate and covered by thin carbon layer for protection. The Co-based alloy thin films are polycrystalline and the size of grains has an order of 10 nm. Because the sizes of each grain are tiny, they are typical single domain magnets. The media are magnetically hard (coercively is about 0.3T) so that a stable remnant magnetization can be achieved. The grain boundaries turn out to be very important. The reason is that, the grains are very small and close to each other, so the coupling between each grain is very strong. When one grain is magnetized, the adjacent grains tend to be aligned parallel to it or demagnetized. Then both the stability of the data and signal-to-noise ratio will be sabotaged. A clear grain boundary can weaken the coupling of the grains and subsequently increase the signal-to-noise ratio. During writing process, ideally one grain can store one bit (1/0).

However, current technology cannot reach that far yet. In practice, a group of grains (about 100) are magnetized as one bit. So, in order to increase the data density, smaller grains are required. From microstructure point of view, longitudinal and perpendicular recording are the same. Also, similar Co-based thin films are used in both longitudinal and perpendicular recording. However, the fabrication processes are different to gain different crystal structure and magnetic properties. In longitudinal recording, the single-domain grains have uniaxial anisotropy with easy axes lying in the film plane. The consequence of this arrangement is that adjacent magnets repel each other. Therefore the magneto static energy is so large that it is difficult to increase areal density. Perpendicular recording media, on the other hand, has the easy axis of the grains oriented perpendicular to the disk plane. Adjacent magnets attract to each other and magneto static energy are much lower. So, much higher areal density can be achieved in perpendicular recording. Another unique feature in perpendicular recording is that a soft magnetic under layer is incorporated into the recording disk. This under layer is used to conduct writing magnetic flux so that the writing is more efficient. This will be discussed in writing process. Therefore, a higher anisotropy medium film, such as L10-FePt and rare-earth magnets, can be used.