Researchers at IBM San Jose have just reported the  experimental demonstration of a magnetic bit made with only 12 atoms (press release).  Sebastian Loth, the first author of the paper, just published in Science  (link to the paper), will visit INL next week and give a seminar on Tuesday 31 January.

Non-volatile storage of digital  information, as recorded in conventional hard-disks, makes use of the  spin orientation of magnetic grains made of more than one million atoms.  The magnetic grains are made of ferromagnetic materials, so that  the  magnetic moment of each atom tends to point parallel to their neighbors,   in the so called ferromagnetic coupling, adding up to form a net  magnetization.  Magnetic grains have a so called easy axis, so that the  collective magnetization points either parallel or anti-parallel  to the  easy axis. These two possible stable orientations define the two  logical states of a classical bit, 0 or 1.

Increasing the storage density  in magnetic recording involves reducing the size of the magnetic grains,  which leads to a number of practical challenges involving the detection  of their magnetization and the loss of stability: the time scale for  unwanted flip of the magnetization  decreases exponentially  as the  volume of the particle is reduced.  This naturally leads to the question  of how small can the magnetic particle be which leads a  sufficiently  large lifetime for the spin orientation.

The experiment of the IBM team  addresses this question but changes the strategy in terms of the type of  magnetic material.  Instead of using  ferromagnetic nanoparticle they  use an  atomically engineered antiferromagnetic system, in which the  magnetic moments of a given atom likes to point anti-parallel to their  neighbors.  Taking advantage of a Scanning Tunneling Microscope (STM) they can move atoms, one by one, deposited on a surface, and design  structures with arbitrary shapes. In this instance they have fabricated  chains of a few (6,8) iron atoms, on top of a copper surface capped with  a single monolayer of an insulating material, Copper Nitride.

The antiferromagnetic coupling  between iron atoms  result in a vanishing total magnetization for a  given chain, which goes completely against the usual approach.  They can  get away with it taking advantage of a readout technique with atomic  scale resolution: the spin polarized scanning tunneling microscope (STM).    By reading the spin orientation of every atom in a given chain  , the IBM team has been able to  observe how the short chains of 4   atoms and less behave as quantum antiferroamgnets: the spin every atom  is in  a quantum superposition of being up and down, which results in an  average null magnetization for each atom, and the impossibility of  using such system so store digital information.

Interestingly, for chains of 6  atoms and higher the chains behave as expected in classical  antiferromagnets and have two stable orientations:  1) Even atoms up,  odd atoms down and 2) Even atoms down, odd atoms up.  Taking advantage  of the atomic scale readout  the IBM team could  trace the thermal  stability of the different chains and found how the stability improved  for larger chains.   In particular,  2 chains with 6 atoms lying  parallel, referred to as (2×6) have been shown to be suitable to store a  bit of classical information.

In addition to the use of STM to  fabricate the chains and to read their magnetization, the IBM team has  also shown how to write the logical state in a bit using electrical  pulses send with the STM.   Combining the 3  capabilities together, the   IBM group has fabricated a byte made with 8 (2×6) chains and has  recorded on it the digital code for the letters S, P, I and N.

Whereas there is a long way  between these experimental breakthroughs and massive commercial  applications, mostly duet  the low temperature and ultra-high vacuum   experimental conditions at which the experiments have been done,  the  IBM team has made it real the vision of  Feynman, back in 1959, in his  inspiring talk “There is Plenty of Room at the bottom” where he prophetically said  “When we get to the very, very small world – say circuits of seven atoms  – we have a lot of new things that would happen that represent  completely new opportunities for design”.   The  group of Theory of  Nanostructures at INL is making progress to understand the  working  principles of all these striking experiments.

Joaquín Fernández-Rossier, Staff Researcher at INL