The nickelate nano-switch

First observation of conductor-insulator transition in rare-earth materials

Recent work undertaken at Diamond Light Source has for the first time demonstrated the application of PhotoEmission Electron Microscopy (PEEM) for observing phase transitions in a class of material known as nickelates.

Rare-earth nickelates are an intriguing class of materials that have attracted attention for their electronic properties. They can undergo a structural phase change that alters their properties from a conductor to an insulator, so that they can perform as a switch. This metal–insulator transition can be driven by the application of temperature and light, but until now, the microscopic mechanism of this phase change has been debated.

For the first time, this transition has been observed at the nanoscale level using the Nanoscience beamline (I06). The results of the study, published in Nature Communications, showed fascinating striped insulating domains formed during cooling that demonstrated how a material’s surface is responsible for guiding the phase transition.

Nanoscale observations
The interest in nickelates stems from their electrical properties. Upon heating, they conduct electricity, but when cooled they undergo a phase transition that turns them into insulators. With the application of light, this change can occur in picoseconds, allowing them to perform as highly sensitive switches. One such member of the family, NdNiO3, has been extensively studied, but although various models have been proposed to describe its electronic structure, no studies have been conducted to explore the phase transition in detail.

A collaboration of scientists from Delft University and Leiden University (both in The Netherlands), University College London, Diamond Light Source, and the University of Geneva sought to understand exactly how this phase transition occurred at the nanoscale level. The lead investigator of the project and PhD candidate at Delft University, Giordano Mattoni, explained the aim of the project: “The material is very interesting as it has a dual electronic and magnetic phase transition. We wanted to understand how the phase transition is related to the material properties, in particular, its surface.”

 

Figure 1: Temperature evolution of insulating domains across the metal–insulator transition. During each thermal cycle the insulating domains nucleate and grow on cooling, while they gradually disappear on warming. Scale bar, 1 µm.

 

Striped domains
The scientists grew a 12 nm-thick NdNiO3 film on a NdGaO3 substrate and applied differing temperatures to the material at beamline I06. PEEM combines a spatial resolution of a few tens of nanometers with real-time imaging, which allowed the team to track the metal–insulator transition at different stages of its evolution during cooling.

The team saw that the nucleation centres, where the phase transitions were occurring, adopted a striped pattern (see Fig. 1 and Supplementary movie). Mr Mattoni described their observations: “The transition did not occur abruptly; we saw striped insulating domains gradually appearing as the material was cooled. For the first time ever we now know where the phase transition originates from.” Moreover, the team also discovered that the shapes of the domains were influenced by the shape of the surface of the material.  “We saw that these domains had a striped form simply because the surface of the material had striped terraces. This tells us that we can shape the surface in different ways to get different shapes of insulating and conducting domains” said Mattoni.

Mr Mattoni highlighted the application of the findings: “Interestingly, the shape of the surface influenced the strain imposed on the material, and it is the strain that drives the transition. Locally straining the material can drive the phase transition within devices, so that we might be able to tune where and at which temperature the phase transition occurs.”

Switch
Mattoni will further the research by studying the effects of light on the nickelates. He hopes to be able to produce a device comprising a single domain that can be switched on and off with light, so that it can be used for multiple applications including neural networks. “The insulating phase is driven by the shape of the surface of the sample, so this tells us that we can control the phase transition by shaping the surface in a certain manner to perhaps even build a network of devices” he concluded.

Principal Beamline Scientist Professor Sarnjeet Dhesi described how the beamline enabled the research: “I06 allows us to explore nanoscale electronic, magnetic, and chemical phenomena, and this work is a remarkable example of how macroscopic measurements in a laboratory can be further understood using nanoscale imaging at the beamline.”

To find out more about the I06 beamline, or to discuss potential applications, please contact the industrial liaison team.

 

Supplementary movie: Temperature evolution of insulating domains across the MIT (Mattoni et al).

 

Related publication: (Open Access)

Mattoni G et al. Striped nanoscale phase separation at the metal–insulator transition of heteroepitaxial nickelates. Nature communications 7, 13141 (2016). DOI: 10.1038/ncomms13141.