Credit: © NPG 2009

Information can be stored in the polarization state of a ferroelectric material. At present, ferroelectric memories are read with a capacitive scheme that destroys the information being read. A resistive readout scheme would be non-destructive, and also faster and simpler to implement, but this approach requires thin ferroelectric layers — which are a challenge to manufacture because ferroelectricity becomes weaker as films become thinner.

Now, Manuel Bibes and colleagues have demonstrated resistive readout in ferroelectric films of BaTiO3 as thin as 1 nm. The researchers — from the CNRS, Thales, the University of Paris-Sud, Cambridge University and the University of Evry — used conductive-tip atomic force microscopy (CTAFM) to observe the polarization dependence of the tunnelling current across their films. The current varied by 200% for 1-nm films and by 75,000% for 3-nm films (Nature 460, 81–84; 2009).

Furthermore, Bibes and co-workers were able to write to, and read from, a matrix of 70-nm sized dots. These are pictured in piezoresponse force microscopy phase images (left) and CTAFM resistance maps (right). The centres of the dots are about 200 nm apart, corresponding to a storage density of 16 Gbit inch-2.

In addition to its relevance to next-generation ferroelectric random access memories, the work sheds light on why polarization affects the tunnelling current. The exponential dependence of the current on the thickness of the tunnelling barrier suggests an origin related to physical properties of the barrier, rather than the density of states, as has been suggested previously.