Memory Size throughout the Ages
We all know Moore's Law: The number of transistors that can fit on silicon wafers doubles approximately every 18 months. Storage mediums have also increased their capacities thanks to the use of silicon technology. Before the transistor, memory found in computers typically consisted of magnetic drum machines, mercury delay lines, and punched card systems.
Magnetic drum memory was a popular method for storage. Image courtesy of Gregg Tavares (DSC02813) [CC BY 2.0]
As integrated circuits made their debut in 1960s, memory also became increasingly electronic by nature with the eventual introduction of RAM, flip-flops (SRAM), and disk drives. The first hard disks made by IBM could hold 3.75MB, which consisted of 50 24-inch platters. The first DRAM chip, produced by Intel, could hold 1KB of information using PMOS logic.
The limitation of memory during this time was manufacturing processes but, as we continue to create devices on the nanoscale, the quantum world is beginning to present real challenges for semiconductor producers.
Many technological leaps in memory have been a result of work done by IBM. Now, once again, they have achieved a new milestone in memory storage by storing a single bit in one atom reliably.
Achieving Single-Atom Storage
IBM has always pushed the boundaries of technology. IBM has invented many technologies, ranging from the scanning electron microscope to the ATM.
This time, however, they have gone for the smallest memory storage unit by storing a binary digit in a single atom. The researchers at IBM started with an unusual element, holmium, as the atom that would store the bit and then placed the holmium atom on top of a bed of magnesium oxide.
When holmium sits on magnesium oxide, it exhibits a property called magnetic bistability. This essentially means that the atom can be in one of two different spins and is stable in one of these states (such that the atoms will not lose this spin once set). The bistability property of this configuration is due to the many unpaired electrons found in holmium which provide a strong magnetic field (a very important property in magnetic data storage).
IBM's single-atom storage unit (Ho). Image credit of IBM Research and Fabian D. Natterer et al./Nature
The first task set before the researchers was to record information to the holmium atom. This was done by using a voltage potential of 150mV and 10 microamps of current. This pulse of current generates a strong magnetic field and forces the atom's spin to line up with the generated field.
The second task was to then read this data by using the same scanning electron probe, but this time setting the potential to 75mV. The lower voltage is desirable as it prevents the generation of a strong magnetic field which would otherwise destroy the stored information. As it turns out, the holmium configuration has two different resistances when in two magnetic states. These directly correspond to the stored bit.
The last task was to prove that the atom was changing the atom's magnetic state which was done by placing an iron atom nearby. Iron was used as it is ferromagnetic and therefore sensitive to magnetic changes.
The result was that the iron behaved differently depending on the magnetic state of the holmium. This demonstrated that the holmium atom can indeed store an individual bit in just one atom.
IBM researchers used an electron microscope similar to the one used to make this atomic-scale art. Image courtesy of IBM Research
Challenges to Bringing Single-Atom Memory Mainstream
So IBM researchers have demonstrated that information can be stored on one atom. But what about the potential for increasing data density? What about the supposedly large memory units that can be made with this technology?
Well, the concept of all of the world's music on a single holmium data storage unit may be more science fiction and fact.
Could this individual atom enable for larger memory storage? Image credit of IBM
IBM has essentially proved that an atom can store a single bit of information. However, the equipment and conditions needed to make this happen are found only in laboratories with electron scanning microscopes and liquid nitrogen cooling.
Of course, there may be a way to create a microstructure in silicon or some other material to probe individual atoms. This isn't a simple solution, however, as any material will inevitably come with a host of other complications, just as new semiconductor materials also face their own unique challenges.
How much supporting hardware will an individual atom need? It may turn out that each memory unit needs a circuit to read and write information. This could potentially rely on current technology but the resulting memory units may not be that much smaller than DRAM built using 1nm features.
On top of that, quantum effects may pose a real issue with reliability as such small features are subject to phenomena in quantum physics such as electron tunneling.
IBM Research's Dr. Christopher Lutz. Image courtesy of IBM Research - Almaden via IBM
However, not all is doom and gloom. This technology may be “scaled-up” to create magnetic memory units that are hundreds of atoms in size. When on this scale, manufacturing processes can more readily cope with production challenges and may be able to produce the world’s first high-density magnetic nano-storage.
Such technology could exceed the capabilities of solid state drives on two fronts: Firstly, the memory sizes offered could potentially be much larger than what's available now. Secondly, the memory would be non-destructive (unlike FLASH) and therefore provide better read/write cycles.
IBM has a habit of making some impressive strides in the field of electronics and computing and this single atom memory unit is a clear example. Will it take off? Who knows. Will it replace modern memory? Probably not for the next couple of decades—but many felt the same way about transistors! (At first, producers and scientists alike were skeptical about using the three-legged devices, but now they are almost impossible to hide from.)
For now, we know that single atomic magnetic storage is possible and may be a solution for future generations.