Magnetism makes the world go around, or at least for us hyper-connected types who would suffer brain damage if our data disappeared. But, in spite of the world's reliance on magnetic storage, advances in the technology have been rather uneven. For instance, storage density has scaled quite nicely, and, along with it, the sensitivity of the read/write head. The odd men out in the line are the read and write speeds, which are still quite slow—even ignoring seek times that haven't changed in the last eon.

There is a fundamental reason for this: magnetic storage relies on flipping the orientation of the spins in a magnetic material, and this is subject to the limits of the materials. However, researchers have demonstrated that they can exploit a different type of magnetic material to speed up the write speed, even though the change in magnetic orientation takes the same amount of time.

Normally, when we think of magnetic materials, we think of ferromagnetism. A ferromagnet is made from atoms that each have an unpaired electron. This turns each atom into a tiny magnet and, when these are all lined up, we get a larger magnet. Most hard drive technology is based on flipping the orientation of these sorts of magnets. One applies a magnetic field to the magnet, which starts the electron spins rotating and precessing about the field. The speed of precession, and hence the time it takes to flip the magnet, is proportional to the strength of the magnetic field: larger fields cause spins to precess faster.

So, one might imagine that applying a sufficiently large field would allow you to decrease write times indefinitely. Unfortunately, at this point, the real world reaches out and slaps you in the face. It turns out that if the field is too large, the entire magnetic order of the material is momentarily destroyed and the final orientation of the magnet is randomized. Bummer.

Coming to the rescue is antiferromagnetism. In this case, the material consists of two groups of atoms. The atoms within a group have the same alignment, but the two groups have a different alignment. Furthermore, these atoms are interleaved with each other, like a chess board, so the magnetism that's observed is the summation of the two. Often, the two groups align opposite each other, so the bulk magnetism is generally pretty small.

The flipping behavior of these materials is very different than that of ferromagnetism because the two groups are influenced by each other. Essentially, if you set one group of atoms in motion, that will set the other group in motion, which, in turn—let me interrupt this before it becomes too recursive. The upshot is that antiferromagnets behave as if they had inertia, meaning that after you turn the external magnetic field off, the spins keep oscillating and may still flip.

To explore this behavior, the researchers blasted antiferromagnets with intense pulses of light—light has both electric and magnetic fields, but we are only interested in the magnetic fields—providing a magnetic field pulse just 100 femtoseconds long. After the pulse was gone, a second, weaker laser pulse was shone on the sample. The orientation of the electric field of this second pulse is rotated by the antiferromagnet, and the amount of rotation depends on the orientation of the two groups of atoms in the sample.

The dynamics of the material could be observed by varying the time between the magnetic pulse and the measuring pulse. In doing so, the researchers observed that the magnetic flip takes a few picoseconds, but the magnet continues to oscillate around the new orientation for about 100 picoseconds as it settles down. This is something that is never observed in ferromagnets and could only occur if the spins had some sort of inertia and needed time to settle down.

So, this doesn't make the write process faster, since the oscillations go on for a bit. But the magnetic field only needs to be applied for the first 100 femtoseconds, after which it can move on, meaning that write speeds could be considerably faster.

Sounds good, right? Well, not so fast. Antiferromagnetic materials are quite delicate and most of them lose their antiferromagnetism at high temperatures—and we're talking a physicist's definition of high. For instance, the material used in this research had to be kept at a temperature of around 50K. I don't know if a room temperature antiferromagnetic material with the right properties exists at present, but until it does, this will not end up in a hard drive near you.

Nature Physics, 2009, DOI: 10.1038/nphys1369