Magnetic storage technology
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 material during writing, which makes it much more receptive to magnetic effects and allows writing to much smaller regions (and much higher levels of data on a disk).
The technology was initially seen as extremely difficult to achieve, with doubts expressed about its feasibility in 2013. The regions being written must be heated in a tiny area – small enough that diffraction prevents the use of normal laser focused heating – and requires a heating, writing and cooling cycle of less than 1 nanosecond, while also controlling the effects of repeated spot-heating on the drive platters, the drive-to-head contact, and the adjacent magnetic data which must not be affected. These challenges required the development of nano-scale surface plasmons (surface guided laser) instead of direct laser-based heating, new types of glass platters and heat-control coatings that tolerate rapid spot-heating without affecting the contact with the recording head or nearby data, new methods to mount the heating laser onto the drive head, and a wide range of other technical, development and control issues that needed to be overcome.
HAMR's planned successor, known as heated-dot magnetic recording (HDMR), or bit-pattern recording, is also under development, although not expected to be available until at least 2025. HAMR drives have the same form factor (size and layout) as existing traditional hard drives, and do not require any change to the computer or other device in which they are installed; they can be used identically to existing hard drives. 32 TB HAMR drives were shipped to some customers for qualification in 2023.
Overview.
There have been a series of technologies developed to allow hard drives to increase in capacity with little effect on cost. To increase storage capacity within the standard form factor, more data must be stored in a smaller space. New technologies to achieve this, have included perpendicular recording (PMR), helium-filled drives, shingled magnetic recording (SMR); however these all appear to have similar limitations to areal density (the amount of data that can be stored on a magnetic platter of a given size). HAMR is a technique that breaks this limit with magnetic media.
The limitation of traditional as well as perpendicular magnetic recording is due to the competing requirements of readability, writeability and stability (known as the magnetic recording trilemma). The problem is that to store data reliably for very small bit sizes the magnetic medium must be made of a material with a very high coercivity (ability to maintain its magnetic domains and withstand any undesired external magnetic influences). The drive head must then overcome this coercivity when data is written. But as the areal density increases, the size occupied by one bit of data becomes so small that the strongest magnetic field that current technology can create is not strong enough to overcome the coercivity of the platter (or in development terms, to flip the magnetic domain), because it is not feasible to create the required magnetic field within such a tiny region. In effect, a point exists at which it becomes impractical or impossible to make a working disk drive because magnetic writing activity is no longer possible on such a small scale.
The coercivity of many materials is temperature dependent. If the temperature of a magnetized object is temporarily raised above its Curie temperature, its coercivity will become much less, until it has cooled down. (This can be seen by heating a magnetized object such as a needle in a flame: when the object cools down, it will have lost much of its magnetization.) HAMR uses this property of magnetic materials to its advantage. A tiny laser within the hard drive temporarily spot-heats the area being written, so that it briefly reaches a temperature where the disk's material temporarily loses much of its coercivity. Almost immediately, the magnetic head then writes data in a much smaller area than would otherwise be possible. The material quickly cools again and its coercivity returns to prevent the written data being easily changed until it is written again. As only a tiny part of the disk is heated at a time, the heated part cools quickly (under 1 nanosecond), and comparatively little power is needed.
The use of heating presented major technical problems, because as of 2013, there was no clear way to focus the required heat into the tiny area required within the constraints imposed by hard drive usage. The time required for heating, writing, and cooling is about 1 nanosecond, which suggests a laser or similar means of heating, but diffraction limits the use of light at common laser wavelengths because these ordinarily cannot focus into anything like the small region that HAMR requires for its magnetic domains. Traditional plated magnetic platters are also not suitable due to their heat conduction properties, so new drive materials must be developed. Seagate Technology and Showa Denko use an iron-platinum alloy in glass platters for HAMR drives. In addition, a wide range of other technical, development, and control issues must be overcome. Seagate, which has been prominent in the development of HAMR drives, commented that the challenges include "attaching and aligning a semiconductor diode laser to an HDD write head and implementing near-field optics to deliver the heat", along with the scale of use which is far greater than previous near-field optic uses. Industry observer IDC stated in 2013 that "The technology is very, very difficult, and there has been a lot of skepticism if it will ever make it into commercial products", with opinions generally that HAMR is unlikely to be commercially available before 2017.
Seagate stated that they overcame the issue of heating focus by developing nano-scale surface plasmons instead of direct laser-based heating. Based on the idea of a waveguide, the laser "travels" along the surface of a guiding material, which is shaped and positioned in order to lead the beam to the area to be heated (about to be written). Diffraction does not adversely affect this kind of wave-guide based focus, so the heating effect can be targeted to the necessary tiny region. The heating issues also require media that can tolerate rapid spot-heating to over 400 °C in a tiny area without affecting the contact between the recording head and the platter, or affecting the reliability of the platter and its magnetic coating. The platters are made of a special "HAMR glass" with a coating that precisely controls how heat travels within the platter once it reaches the region being heated – crucial to prevent power waste and undesired heating or erasure of nearby data regions. Running costs are not expected to differ significantly from non-HAMR drives, since the laser only uses a small amount of power – initially described in 2013 as a few tens of milliwatts and more recently in 2017 as "under 200mW" (0.2 W). This is less than 2.5% of the 7 to 12 watts used by common 3.5 inch hard drives.
Seagate first demonstrated working HAMR prototypes in continual use during a 3-day event during 2015. In December 2017 Seagate announced that pre-release drives had been undergoing customer trials with over 40,000 HAMR drives and "millions" of HAMR read/write heads already built, and manufacturing capacity was in place for pilot volumes and first sales of production units to be shipped to key customers in 2018 followed by a full market launch of "20 TB+" HAMR drives during 2019, with 40 TB hard drives by 2023, and 100 TB drives by around 2030. At the same time, Seagate also stated that HAMR prototypes had achieved 2 TB per square inch areal density (having grown at 30% per year over 9 years, with a "near-future" target of 10 TBpsi). Single-head transfer reliability was reported to be "over 2 PB" (equivalent to "over 35 PB in a 5 year life on a 12 TB drive", stated to be "far in excess" of typical use), and heating laser power required "under 200mW" (0.2 W), less than 2.5% of the 8 or more watts typically used by a hard drive motor and its head assembly. Some commentators speculated that HAMR drives would also introduce the use of multiple actuators on hard drives (for speed purposes), as this development was also covered in a Seagate announcement and also stated to be expected in a similar time-scale.
Thermomagnetic patterning.
A similar technology to heat-assisted magnetic recording that has been used mainstream other than for magnetic recording is thermomagnetic patterning. Magnetic coercivity is highly dependent on temperature, and this is the aspect that has been explored, using laser beam to irradiate a permanent magnet film so as to lower its coercivity in the presence of a strong external field that has a magnetization direction opposite to that of the permanent magnet film in order to flip its magnetization. Thus producing a magnetic pattern of opposite magnetizations that can be used for various applications.
Setup.
There are different ways in which the setup can be made, but the underlying principle is still the same. A permanent magnetic strip is deposited on a substrate of silicon or glass, and this is irradiated by a laser beam through a pre-designed mask. The mask is designed specifically for this purpose to prevent the laser beam from irradiating some portions on the magnetic film. This is done in the presence of a very strong magnetic field, which can be generated by a Halbach array. The areas that are exposed/irradiated by the laser beam experience a reduction in their coercivity due to heating by the laser beam, and the magnetization of these portions can be easily flipped by the applied external field, creating the desired patterns
References.
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