Walls and memory

A domain in a ferroic phase, for example a ferroelectric or a ferromagnetic one, is a well-defined spatial region containing a long-range-ordered configuration for the relevant local order parameter — the ordered electric dipole or magnetic moment for the two examples above, respectively. Different domains are distinguishable because of a different spatial orientation for the order parameter.

Most of the current research on the applications of ferroic materials focuses on the boundaries between neighbouring domains, also known as domain walls, because of their remarkable functional properties. As a prime example, the electrical conductivity value is finite along specific types of domain walls in ferroelectric materials despite the vanishing value inside the domain. There have been various proposals for using domain walls as active elements in electronic devices and, in particular, non-volatile memories. However, in spite of several advances in the manipulation of the ferroelectric domain walls and in the control of their conductivity, proof-of-principle working memory devices are still missing.

Credit: AAAS

Writing in Science Advances (3, e1700512; 2017) , Pankaj Sharma et al. now report on a prototype two-terminal device based on reconfigurable ferroelectric domain walls and acting as non-volatile resistive memory. The researchers use high-quality epitaxial (110)-oriented BiFeO₃ thin films, exploiting their well-known exceptional uniformity — that is, virtually no domains are observed in as-grown samples — and pattern asymmetric coplanar Pt/Ti electrodes on the BiFeO₃ surface. Acting with the tip of a conductive atomic force microscope on one electrode generates in-plane electric fields which, in turn, lead to the generation — or successive deletion — of a pair of ferroelectric domain walls, as visualized by means of piezoresponse force microscopy. The image shows a mapping of the piezoresponsive signal in false colours. Domain walls are imaged as black lines bridging the electrodes, that is, the central dark circle and the dark stripe on the right-hand side, while the arrows denote the local direction of the electrical polarization. To generate and delete domain walls, voltage values around 8 V are required.

Once the walls are generated, the researchers perform a non-destructive, low voltage current mapping of the device by means of conductive atomic force microscopy, confirming a substantial electrical conductivity through the domain walls but negligible contributions through the domain bulk. As a result, confirmed over ∼10³ cycles, conductive bridges between the two electrodes lead to a low resistance ON state, while a high-resistance OFF state is obtained in the absence of domain walls — the characteristic resistance values differing by more than three orders of magnitude. The researchers further demonstrate that the characteristic electrical conductivity value in the ON state can be tuned by varying the distance between the electrodes and, accordingly, the characteristic length of domain walls. Specifically, longer domain walls are characterized by lower conductivity values. These observations demonstrate potential prospects of multilevel data storage going beyond the binary ON/OFF operation.