Magnetic storage reaches the atomic level

The magnetic field of a single atom is read and written, but it's not very stable.

Data storage needs to keep up with our desire to snap pictures, download clips from the Internet, and create new digital documents. Since the early stages of computer technology, magnetic storage has been the method of choice to handle digital data. It has stayed that way because of our ability to continually shrink the area used to hold a single magnetic bit.

But we're closing in on the limits of this approach, as clusters of three to 12 atoms have been used as a functional system. Last week, however, scientists demonstrated the ability to magnetically store data in a single atom.

The basics of magnetic storage

Magnetic storage requires the magnetization of a ferromagnetic material to record data. These materials rely on the atom’s electrons, which themselves behave like tiny magnets. The electrons carry a magnetic dipole moment that is determined by the direction the electron spins and the shape of the path the electron travels (quantum mechanical spin and orbital angular momentum, to be technical). There are only two directions the electron can spin, either “up” or “down.”

Although this system is dynamic, its two stable equilibrium states, “up” or “down,” provide what's often referred to as magnetic bistability. Having a bit stored indefinitely usually involves a cluster of atoms all set to the same state. This provides a bigger signal, ensuring the bit is maintained even if any given atom doesn't behave stably.

So, while there were many advances in the miniaturization of magnetic bistability, there were some obvious questions about the limits it could reach.

A single-atom approach

In this investigation, scientists worked with holmium atoms (Ho) supported on magnesium oxide (MgO). Although many Ho atoms formed clusters on the surface, the researchers identified single atoms located atop oxygen to use as a magnetic storage material.

Using a scanning tunneling microscope, the researchers applied current pulses to the Ho atoms to switch the direction of the magnetic moments, demonstrating the ability to control the magnetic behavior of individual Ho atoms. They were then able to read the magnetic patterns by placing another magnetic material in close proximity, which enabled electrons to tunnel from one magnet to another (tunnel magnetoresistance).

Magnetic storage requires the ability to read and write information, but it also requires information to be retained over time. To gauge the storage retention time, the scientists observed how long a Ho atom would remain in a single state after being switched using a pulse of current. They found that the bits would last for hours before starting to randomize.

The researchers performed a clever trick to test that what they were seeing was truly due to the magnetic moment of the Ho atom. They used the microscope tip to place a single iron atom near the Ho atom. In this experimental set-up, the iron atom functions as a local magnetometer, since it has an external out-of-plane magnetic field that's influenced by nearby magnetic materials. When the scientists applied current pulses to the Ho atom, they saw corresponding shifts in the magnetic field of the iron atom, demonstrating that the individual Ho atoms did in fact possess two distinct magnetic orientations.

Finally, the researchers explored the ability of an array of two Ho atoms to store two bits of information, again using a nearby Fe atom to locally read the magnetic state. They were also able to use other advanced techniques to remotely read the magnetic states.

These experiments demonstrate that high-density magnetic storage at the atomic level is possible, though significant research is still needed to further develop this technology and understand the practical feasibility. After all, something that's only stable for a few hours would require a very different approach than past magnetic media.