Graphic Designer: Heather
Welcome to Digital Fluencies! Specifically, welcome to my little series on little cherry-picked pieces of technology. Have you ever stopped for a moment –looking at some part of the modern age that's just inconspicuously and insidiously part of our world– and wondered “just how did that come into being? How does it work?”. This series is my attempt to, hopefully, shed a little bit of light on those bits of technology we use and see every day, yet have no real understanding of. For example, by what mechanisms did these very words reach your screen? How do transistors work, and why are they so important? How do cameras work? Or in this first article’s case, how come silicon remembers?
Originally this article was going to be a much longer article spanning across HDDs (both how they work and the history of their development), SSDs (Solid State Drives), as well as the brain and the extent to which it can be viewed as a “biological drive” of sorts. Unfortunately (or fortunately, depending on your view) writing this article has been like standing on quicksand. With every bit of research that I did, and every line that I wrote, I just kept sinking deeper and the scope of the article just kept increasing. Thus, the original monolith will instead be developed over the course of three articles, with the history of hard drives being left for a later date. This first article covers how HDDs work.
Hard Disk Drives (HDDs):
Hard disk drives can be described with four principal components:
Platter: Where the data is physically stored.
Spindle: What rotates the platter.
Read/write arm: What reads and writes on the disk.
Actuator: What controls the movement of the read/write arm.
In essence, the arm (which always hovers a few nanometers above the disk) is moved by the actuator along the width of the disk. In tandem with that the spindle is spinning the platter (disk) at incredibly fast speeds while the arm reads/writes said disk. And just how fast is “incredibly fast”? It varies per drive, but it’ll be in the ballpark of around 7200 rotations per minute (give or take a couple thousand rotations). For context, if you were to rotate at that speed with your arms stretched out you would explode outwards at up to 200 miles per hour. Speed aside though, if the arm never physically touches the disk how exactly can it modify and extract information from it? The answer lies in a clever application of:
Ferromagnetism refers to a property where some materials can become, and stay, magnetized after being put in a magnetic field. Iron, for example, can become a magnet if left under the influence of another magnet for long enough. The way this is applied in HDDs is by coating the disk (which is made of non-ferromagnetic materials like ceramic, glass, or even aluminum) in a thin layer of ferromagnetic material. In the past that used to be iron(III) oxide, though current disks instead use a cobalt-based alloy. Thus, to write on the disk the arm selectively applies a magnetic field on parts of the disk, magnetizing specific –ridiculously minuscule– regions. These serve as a sort of a physical analogue of a computer’s 0s and 1s. So when the arm “writes” on the disk it’s basically encoding these regions with magnetic “moments”, which for our purposes can just be described as whether the North or South pole of the moment (“magnet”) is facing the arm.
So great, now we have a disk with an incredibly finely granulated grid of data, but how do we read and interpret that data? Well, hard drives started off by using a fundamental property of electromagnetism called:
When you have a closed loop of conductive material (think copper wires for example), running a variable magnetic field through that loop creates an electrical current in the material. That’s basically the underlying property of any generator. The amount of current depends on the rate of change of the magnetic flux. Flux is simply how much magnetic field is passing through an area. But what this means is that if you spin the disk underneath the arm, the arm can “read” it by interpreting the changing current it would experience from each individual bit. However, since this method relies on area and magnetic field strength, and components and grains kept getting smaller and smaller, eventually this method threatened to stop working outright from how small things got. You can think of it as the “generator” (the arm and the disk) getting smaller and smaller, and that meant less and less electricity was being produced: to the point it started to become hard to detect.
That’s why HDDs evolved to instead use a property known as “magnetoresistance”. This is a property where the very resistance of the ferromagnetic material changes under a magnetic field. If you don’t know what “resistance” means, it’s basically how energy-intensive it is for current to flow through a given material. A higher resistance will consume a higher amount of energy (voltage) to get the same amount of current across. But what does this mean for our arm here? It means that instead of detecting changes in current induced by the disk, it can instead measure the resistance itself. This is a vastly more sensitive method, making its various applicable forms the standard moving forward. The type of magnetoresistance being used over time (which you are free to read more about, if you're so inclined) went roughly: Magnetoresistance (MR) -> Anisotropic MR (AMR) -> Giant MR (GMR)
Something funny about researching this article is that often the reason people write about how hard drives work is to, well, sell you a hard drive. Now I’m not here to sell you anything –per say– but maybe you’d want to consider bookmarking Digital Fluencies? Or maybe telling your friends about it? I jest, I jest… unless?
If you found this article interesting then you might be interested in looking into Computer Science or Physics. If you were intrigued by the application of magnetic fields and things like magnetoresistance, some courses that fall along those areas would be PHYS 156 (General Physics II), PHYS 325 (Electricity and Magnetism), or even PHYS 451-452 (specialized topics in physics which can include, among other things, semiconductors). Alternatively, if you were more interested in how the data from a hard drive reaches a CPU, or more generally computers in general, CS 310 (Computer Systems Programming) covers exactly those topics. Obviously taking some of those courses (or even any of them) is not feasible for most students given the prerequisites attached to them, as well as the 1 semester commitment. Fortunately one of the advantages of being at a liberal arts college is that faculty are very accessible. Most faculty members’ office hours can be easily found on MyWhitman (just search for their name and view their full profile), or on their door, or by sending an email.