Lead author Daniel Sando preparing materials for study at UNSW, Sydney. Image: FLEET.
Lead author Daniel Sando preparing materials for study at UNSW, Sydney. Image: FLEET.

A team led by researchers at the University of New South Wales (UNSW), Sydney in Australia has found a new exotic state in one of the most promising multiferroic materials, BiFeO3 (BFO). Their finding has exciting implications for future technologies.

By combining a careful balance of strain, distortion and thickness, the team has stabilized a new intermediate phase in a BFO thin film, one of the few known room-temperature multiferroic materials. Not only does this new phase have an electromechanical figure of merit over double its usual value, but it can be easily transformed into other phases with an electric field.

As well as providing a valuable new technique to the toolkit of material scientists working with multiferroics and epitaxy, these results shed light on how epitaxial techniques can be used to enhance the functional response of materials for future application in next-generation devices. The researchers report their results in a paper in Nature Materials.

The researchers chose to work with BFO because of its impressive range of multifunctional properties, including piezoelectricity, ferroelectricity, magnetism and optical properties. This is down to BFO being a magnetoelectric material, possessing both magnetic and electrical ordering that can influence each other.

Magnetoelectric materials are highly interesting for spintronics and memory applications since the coupling between magnetism and ferroelectricity promises low-energy technologies (writing data with an electric field is much more efficient than writing with a magnetic field). But not only is BFO magnetoelectric, it is one of the very few materials that is magnetoelectric at room temperature, making it viable for use in applications such as future low-energy electronics, without the requirement for energy-intensive cryo-cooling. Only very few multiferroic materials exhibit these useful properties at room temperature.

In addition to this, BFO is also lead free, giving it a clear advantage against most high-performing piezoelectric materials, which unfortunately contain toxic lead. Piezoelectric materials, which can convert mechanical pressure into electrical energy, have wide applications as ultra-high-sensitivity sensors in devices such as smartphone motion sensors and pacemakers (where obviously avoiding toxic materials is an advantage).

To these already impressive properties, the researchers have now added strain. When stress is applied to crystals, they become strained and can change their structure and physical properties dramatically.

When scientists impose strain on a material, they are usually pushing together or pulling apart along (at least) one axis, creating compressive and tensile strain. When they strain thin films on substrates, the building blocks of the film will deform to match the sizes of the building blocks of the neighbouring substrate.

If the structural units of the substrate are larger than those of the thin film, the film will stretch horizontally (ie, ‘tensile strain’) and compress vertically to fit. On the other hand, a substrate with smaller structural units will cause the film to be compressed horizontally (‘compressive strain’) and stretched vertically.

“In our research, we applied anisotropic strain to our film,” explains first author Oliver Paull at UNSW. “This means that the strain applied is different depending on which direction you’re looking, and this can create complicated strain states that force films into new phases.”

By using highly miscut substrates, the research team were able to push BFO into a new phase that is essentially the link between the well-known rhombohedral-like and tetragonal-like phases. This, coupled with the symmetry-related properties of the phase, allows it to be easily influenced by electric fields.

“We looked through the literature and found that everyone uses fairly standard commercial substrate orientations,” says head investigator Daniel Sando at UNSW. “We asked our providers to custom-make different miscut orientations in between the standard orientations, which led to the discovery of the new phase. We asked ourselves if the reason people hadn’t done this before is that the crystallography involved with these miscuts is rather complex and can be intimidating!”

Together with colleagues at Oak Ridge National Laboratory, the University of Arkansas and Monash University in Australia, the UNSW researchers used theoretical calculations and a suite of experimental techniques to show that this new phase has a much higher electromechanical response than traditional BFO.

“We additionally show strong evidence that this low-symmetry phase can be converted into a higher-symmetry phase using an electric field, and as a result can enhance the electromechanical response even further by a factor of 3,” says Paull.

One of the most appealing aspects of this discovery is its general methodology and applicability to a broad class of materials systems. “We chose to focus on BiFeO3 due to its ferroelectric, magnetic and piezoelectric properties, but the approach is easily applied to other perovskite oxides,” Paull asserts.

“We are currently exploring the effect of these high-index substrates on purely ferroelectric or magnetic systems, but the scope for using this technique is huge,” notes Laurent Bellaiche of the University of Arkansas, the theoretical lead on the study. “We expect to find low symmetry phases of optically interesting materials, as well as novel domain arrangements in ferroelectrics, to name a few.”

“If you’re dealing with epitaxy, then this anisotropic technique could prove very fruitful for your research,” adds Sando.

“This study is just the beginning,” says UNSW lab leader Nagy Valanoor. “We plan to combine this anisotropic epitaxy approach to oxide superlattices [repeating layers of different compositions, i.e. A-B-A-B etc], as well as combining the low symmetry crystal structures with other established routes for improving piezoresponse, including substitution with rare earth elements, for example. Finally, since BFO is multiferroic, we have a raft of magnetic studies planned for this new low-symmetry phase.”

This story is adapted from material from FLEET, with editorial changes made by Materials Today. The views expressed in this article do not necessarily represent those of Elsevier. Link to original source.