S. Rittner and R. Hulet, Rice University
Magnetic media, in the form of tapes and discs, has long been the primary means of digital storage. In this hardware, clusters of magnetic atoms are placed in a single magnetic orientation, which can be read back to determine whether a bit has the value of one or zero. Advances in capacity usually come from figuring out how to make those clusters smaller. The ultimate limit would, of course, be a single atom, but here quantum fluctuations cause the bit to not be stored stably. Single-atom magnets have been created, but they ended up holding an arbitrary value within a fraction of a second.
Now a team of Swiss researchers has identified the two quantum effects that cause the most problems for these single-atom magnets and figured out how to mitigate them. The result is a device where individual atoms can hold their orientation for tens of minutes. The big downside? It must be kept near absolute zero to work.
Magnetism in a bulk material, such as a bar magnet, arises from the behavior of individual atoms in the material — more specifically, the behavior of some of the electrons orbiting those atoms. While it would be possible for individual atoms to reverse their orientation, the magnetic field created by all neighboring atoms makes this highly unlikely. As a result, groups of atoms tend to retain their orientation indefinitely, allowing us to stably write bits to them.
That kind of group consensus is not available for a single atom. As a result, its magnetic orientation can be affected by a variety of quantum phenomena. These include tunneling, as the orientation can simply switch between the two states if the energy barrier is low enough. However, by carefully choosing the right elements, it is possible to have a situation where the energy barrier is so high that tunneling is not a major problem.
But tunneling isn’t the only problem. Stray electrons in the surrounding material can jump into the atom and disrupt its magnetic properties. And phonons, the quantum units of vibrational energy, can also penetrate from the surrounding material and randomize the magnetic orientation.
So the team here was working with holmium, a rare-earth element that doesn’t easily tunnel between magnetic states. They then embedded these atoms in a magnesium oxide substrate. The density of holmium atoms was low enough for each to behave like an individual, something confirmed using atomic force microscopy.
Magnesia was chosen because it does not transfer phonons to the holmium as easily. In addition, the holmium atoms occupied spaces on the material that had quadruple symmetry. Visually, you can picture this as the magnesium atoms forming a + symbol with the holmium atom in the middle; the oxygen atoms are arranged in an x-shaped pattern. This creates an electronic structure that requires two electrons to shift the holmium’s magnetic field.
All of that should help keep the magnetic state of individual atoms stable. To test whether this is the case, the authors magnetized all the atoms using a strong external field, then turned them off and waited to see how long their material retained its internal magnetic field. (Note that this means they didn’t measure individual atoms, just the collective behavior of individual atoms.)
The main cause of problems in this material came when light from the environment kicked electrons loose in the material, making the double-electron randomization process much more likely. When the light was limited and the device was kept at 10 Kelvin, the lifespan was almost 1600 seconds. At 20K, it took 675 seconds.
That’s not great – you won’t be buying a disk drive based on this system any time soon. But it may be useful enough for some specialized purposes – remember that an atom’s magnetic orientation is a quantum state, so each of these atoms represents a potential quantum memory. And for that, 25 minutes is long-term storage. However, we would first have to show that we can successfully read and write the states of these atoms.
The authors also think the magnesium oxide substrate would work for a variety of other molecules. And they would like to see if it works to trap electrons with different spins, which can interact with magnetic fields. This could enable storage for spintronic devices, which many hope will become energy-efficient alternatives to electronics.
Science2015. DOI: 10.1126/science.aad9898 (About DOIs).