The so-called phase-separated state in perovskite nickelate materials might be put to good use in nanoscale devices such as switches or memristors. In contrast to some other technologies that exploit these materials, such devices would operate electronically rather than through redox processes or the movement of ions, and could thus be much faster.
Perovskite rare-earth nickelates, which have the formula RNiO3 are correlated oxides that undergo a metal-insulator transition during which their resistivity changes dramatically. This transition occurs at a temperature that depends on the ionic radius of the rare-earth, R. Although researchers are still unsure as to where this transition originates, it can be exploited in a variety of applications, including resistive switching and even neuromorphic computing.
To date, this transition was mainly studied using macroscopic techniques. A team led by Manuel Bibes of the CNRS/Thales/Université Paris-Sud in France has now succeeded in directly imaging the local resistance of a neodymium nickelate thin film as it undergoes this transition using a scanning probe technique called conductive atomic force microscopy (AFM). The resistance maps obtained using this technique show that metallic domains between 100 and 300 nm in size nucleate in the insulating state and then grow and percolate as the temperature increases.
“In NdNiO3, the metal-insulator transition is very sharp, with the resistance dropping by a factor of 1000 or more,” explains Bibes. “What is more, the transition does not occur at the same temperature when measured upon cooling or upon warming – that is, it is hysteretic. In these types of so-called first-order transitions, the material does not continuously transform from one phase to the other, but co-exists in both phases. This is what we call phase separation.”
Pixel by pixel
Bibes and colleagues have directly observed this phase separation. “As with conventional AFM, we use a nanosized tip and scan the film surface to collect local information pixel by pixel and build up an image,” says Bibes. “The difference in our work is that we use a conductive tip (made of doped diamond) and the collected information is the local resistance. We thus construct images of the local resistance, pixel by pixel.”
At each pixel, the researchers measure the series resistance between the intrinsic resistance of the tip itself, the series resistance of the film and the contact resistance between the tip and film. “Here the total resistance is dominated by the contact resistance, which strongly depends on the metallic or insulating nature of the region in contact with the tip,” Bibes tells nanotechweb.org. “This is how we can image insulating or metallic regions.”
The phase-separated state might be harnessed in nanoscale devices behaving as switches or memristors, he adds. And in contrast to some other technologies, these could operate through a purely electronic mechanism rather than through redox processes or the movement of ions, and could thus be much faster.
The team, which includes researchers from ICMAB-CSIC in Barcelona, Spain, and Helmholtz-Zentrum Berlin für Materialien und Energie in Germany, says that it will now be looking into the phase-separated state in nanoscale devices made from NdNiO3. “We will also be imaging the dynamics of the resistance switching effect,” says Bibes.
The research is detailed in Nano Letters 10.1021/acs.nanolett.7b04728.