Cramming it all in
Hard disk drives remember the data stored on your computer even when you turn the power off. Most hard drives consist of an actual disk that stores the data magnetically – similar to a tape reel in a cassette. The first hard drive, built by IBM in 1956, worked in a similar way, but since then the amount of data that can fit into the same amount of space has doubled every 2 to 4 years.
Giant magnetoresistance is the concept that has taken data storage to a whole new level, making pocket sized music libraries possible.
A Giant Leap
Giant magnetoresistance (GMR) may sound like comic book science but this method of storing data is very real and in 2007 the discoverers of GMR, Albert Fert and Peter Grunberg, shared the Nobel Prize.
Before GMR, hard disk drives depended on a phenomenon called simply magnetoresistance. Magnetic fields are used to alter the electrical resistance of the disk. This causes measurable changes in the electrical current flowing through it and in turn magnetises regions of the disk in one of two directions.
These specific directions of magnetic field represent a 0 or a 1 and can be used to make up digital information. This information is then read back using sensors but there’s a limit to the size of region they’re capable of detecting and therefore a limit to how small a hard disk drive can be.
Giant magnetoresistance takes advantage of a quantum-mechanical property of electrons called spin. Every electron either has spin ‘up’ or spin ‘down’, and this affects the magnetic properties of not only the electrons themselves but the material they make up. Whether an electron has a spin up or down also affects how the electron moves through a magnetic field, and Fert and Grunberg independently realised that this property could be used to make the hard drive sensors detect tiny changes in magnetic fields. This meant that more data could be stored in smaller spaces and still be measurable.
So why’s it called giant magnetoresistance? Rather counter intuitively, it’s because being able to detect smaller changes in magnet fields allows a much greater, in fact giant, range of regions within a material to be measured.
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