Hot time for hard disks: why magnetic-recording technology is still going strong

03 Dec 2020 James McKenzie
Taken from the December 2020 issue of Physics World, where it appeared under the headline "Hot time for hard disks". Members of the Institute of Physics can enjoy the full issue via the Physics World app.

James McKenzie looks at the power of incremental improvement and the commercial success of magnetic‑recording technology

When it comes to storing data, you might think that magnetic devices are rapidly becoming a thing of the past. Surely solid-state drives are the future? But when I visited Queen’s University Belfast earlier this year, I was surprised to see lots of exciting R&D going on in magnetic storage. My host was the physicist Robert Bowman, who works closely with Seagate Technology – one of the world’s largest manufacturers of magnetic storage devices and products.

Seagate is among the firms developing new approaches to magnetic storage to allow data to be stored more quickly and with higher density than ever before. Indeed, it sees no end in sight for the technology. The market-intelligence firm IDC, for example, expects the amount of data stored in such devices to rise from 33 zettabytes (33 × 1021 bytes) in 2018 to 175 zettabytes by 2025. If you stored that much information on Blu-ray discs, stacked together they would stretch 23 times the distance to the Moon.

Over 350 million hard disks are shipped every year and by 2029 the global market is expected to surpass $80bn globally. In terms of cost and speed of access, magnetic storage wins hands down. And even if solid-state storage were cheaper (which it isn’t and probably never will be), there just isn’t enough fabrication capacity to physically build the storage we are projected to need in time. Over 350 million hard disks are shipped every year and by 2029 the global market is expected to surpass$80bn globally. Seagate’s technology facility in Northern Ireland alone makes almost 30% of the global supply of read/write heads.

Spinning a yarn

It’s incredible how far we’ve come in magnetic storage since 1888 when the US engineer Oberlin Smith first proposed storing audio in tiny metallic particles on a thread of cotton or silk. Practical difficulties stopped Smith’s ideas in their tracks, but in 1928 the German-Austrian engineer Fritz Pfleumer developed the first magnetic tape recorder – an analogue device for storing sound. Magnetic tape was first used for data storage in 1951 when the UNIVAC I computer was developed.

These days digital “linear-tape” cart-ridges are the cheapest form of storage, costing well below $0.01 per gigabyte. They have an immense storage capacity, with the latest cartridges being able to cram 30 TB (30 × 1012 bytes) of data – a figure that’s expected to grow to 40 TB by the end of the decade. It’s a$5.8bn market that will rise to $6.5bn by 2026 according to market intelligence company PMR, partly driven by the need for offline back-ups and copies for disaster recovery and to counter the growing “ransomware” threat. Linear thinking The trouble with tapes is that they’re linear strips, which means it takes time to go from one part to another. That’s one reason why we have hard disks, which allow data to be accessed more quickly by jumping from one circular ring, or track, to another. The first commercial computer to use a moving-head disk drive was IBM’s RAMAC 305, produced in 1956. Containing 50 disks, each 24 inches in diameter, it offered 5 MB of storage, which was huge back then. These days a hard disk has up to 20 TB of capacity, costs less than$0.02 per gigabyte and fits in the palm of your hand.

Hard disks are marvels of physics and technological innovation. Tiny read/write heads fly above their mirror-like surface (itself a complex multi-layer ferromagnetic coating) with a clearance of as little as 3 nm. Operating in a humidity-controlled air- or helium-filled cavity, the height is controlled by an air bearing etched onto the head’s disk-facing surface and attached to a photo-etched precision-machined slider (itself controlled by an ultra-precise stepper motor).

Each hard-disk track – of which there are about a million per inch – contains bits of information recorded into tiny areas just 42 nm wide (roughly six to eight magnetic grains) and 10 nm long (barely two to three grains). The disks rotate at speeds of up to 15,000 revs per minute (as fast as the engine on a Formula 1 car) while the read/write heads themselves are fabricated on 200 mm wafers using state-of-the-art photolithography and micro- and nano-fabrication processes.

A hard disk’s unique mix of semiconductor technologies, precision machining and ultra-precise stepper motors is a staggeringly impressive piece of engineering.

Modern heads are based on tunnelling magnetoresistance (a quantum effect linked to the 2007 Nobel Prize for Physics), but more advanced designs are being explored to increase the density further still. Each hard disk also has multiple heads and multiple double-sided disks (depending on the storage required), plus lots of precision-control electronics. What a pity that most hard disks are encased in dull-looking metal boxes that live in thousands of dull‑looking “cloud” data centres around the world.

For me, a hard disk’s unique mix of semiconductor technologies, precision machining and ultra-precise stepper motors is a staggeringly impressive piece of engineering. If modern hard disks didn’t exist and you asked a team of engineers to design one from scratch – with a sub-\$50 price tag – they’d think you were crazy and give you a thousand reasons why it couldn’t be done, not at any price. Yet thanks to half a century of hard work and the power of incremental technological development, such devices do clearly exist.

A date with density

And as Bowman explained, thanks to “heat-assisted magnetic recording” (HAMR), Seagate has been able to increase the storage density of its hard disks to more than 2 TB per square inch, with the bits written via a laser and plasmonic near-field transducer integrated into the read/write head. The firm will soon ship its first HAMR-based drives and Seagate is targeting 50 TB drives by 2026. So next time you check social media, browse the Web or look at a photo stored in the cloud, remember your data are on a hard disk on low-power standby, at a data centre somewhere in the world, waiting for your command.

The humble hard disk is one of the least humble things imaginable.