Magnetic media, in the form of tapes and disks, have had a long run as the primary means of digital storage. In this hardware, clusters of magnetic atoms are set in a single magnetic orientation, which can be read back to determine whether a bit has a value of one or zero. Advances in capacity mostly come from figuring out how to make those clusters smaller. The ultimate limit, of course, would be a single atom, but here, quantum fluctuations will keep the bit from being stored stably. Single atom magnets have been created, but they have ended up holding a random value within a fraction of a second.

Now, a team of Swiss researchers has identified the two quantum effects that cause most of the problems for these single atom magnets and figured out how to limit them. The result is a device where individual atoms can hold onto their orientation for dozens of minutes. The big downside? It needs to be kept near absolute zero to work.

Magnetism in a bulk material, like a bar magnet, arises from the behavior of individual atoms within the material—more specifically, the behavior of some of the electrons orbiting those atoms. Although it would be possible for individual atoms to flip their orientation, the magnetic field created by all the neighboring atoms makes doing so very improbable. As a result, groups of atoms tend to maintain their orientation indefinitely, allowing us to stably write bits to them.

That sort of group consensus isn't available to a single atom. As a result, its magnetic orientation can be influenced by a variety of quantum phenomena. These include tunneling, as the orientation can simply flip between the two states if the energy barrier is low enough. By carefully choosing the right elements, however, it's possible to have a situation where the energy barrier is high enough that tunneling isn't a significant problem.

But tunneling isn't the only issue. Stray electrons in the surrounding material can hop into the atom, disrupting its magnetic properties. And phonons, the quantum units of vibrational energy, can also bleed in from the surrounding material and randomize the magnetic orientation.

So, the team here worked with holmium, a rare-earth element that doesn't tunnel between magnetic states very readily. They then embedded these atoms on a substrate made of magnesium oxide. The density of holmium atoms was low enough that each should behave as an individual, something that was confirmed using atomic force microscopy.

The magnesium oxide was chosen because it doesn't transmit phonons to the holmium very readily. In addition, the holmium atoms occupied spaces on the material that had a four-fold symmetry. Visually, you can imagine this as the magnesium atoms forming a + symbol with the holmium atom at the center; the oxygen atoms are arranged in an x-shaped pattern. This creates an electronic structure where it takes two electrons to shift the magnetic field of the holmium.

All of that should help keep the magnetic state of individual atoms stable. To test whether it does, the authors magnetized all the atoms using a strong external field, then shut it off and waited to see how long their material retained its internal magnetic field. (Note that this means that they weren't measuring individual atoms, just the collective behavior of individual atoms.)

The primary cause of problems in this material came when light from the environment kicked loose electrons in the material, which made the double-electron randomization process much more likely. When light was limited and the device kept at 10 Kelvin, the lifetime was nearly 1,600 seconds. At 20K, it lasted 675 seconds.

That's not great—you're not going to be buying a disk drive based on this system any time soon. But it could be useful enough for some specialized of purposes—remember that the magnetic orientation of an atom is a quantum state, so each of these atoms represents a potential quantum memory. And, for that purpose, 25 minutes is long-term storage. We would, however, first need to demonstrate that we could successfully read and write the states of these atoms.

The authors also think that the magnesium oxide substrate would work for a variety of other molecules. And they'd like to see if it worked to hold electrons with different spins, which can interact with magnetic fields. This could enable storage for spintronic devices, which many are hoping end up being low-power alternatives to electronics.

Science, 2015. DOI: 10.1126/science.aad9898 (About DOIs).