Schematics of electrical resistivity switching and reentrant thermal phase change in the novel layered nickelate (Sr2.5Bi0.5NiO5). The top two figures show the crystal structures of Sr2.5Bi0.5NiO5 with different Sr/Bi arrangements. The bottom figure is the crystal structure of double perovskite Sr2BiNiO4.5. The blue value denotes the electrical resistivity of each nickel structure. Image: Hideyuki Kawasoko et al.Scientists have achieved a breakthrough in the development of non-volatile phase change memory – a type of electronic memory that can store data even when the power is turned off – in a material that has never before displayed the sort of characteristics that such memory requires.
Until now, phase change memory has primarily been developed using chalcogenides−−a group of materials known to exhibit reversible electrical changes when they transition between their crystalline and amorphous states. But what if there's an even better material out there?
In a paper in Advanced Science, Japanese researchers report the thermally reversible switching of room-temperature electrical resistivity in a layered nickelate – potentially offering better performance and superior sustainability.
Layered nickelates are a class of complex oxide materials composed of nickel ions, with potential applications in fields such as superconductivity and, in this case, electronics. They exhibit a layered structure, where planes of nickel and oxygen atoms are interspersed with layers containing other elements, often alkaline-earth or rare-earth elements. The unique layered arrangement of these nickelates has drawn the interest of researchers due to the intriguing properties of their electrons.
The researchers' particular layered nickelate is composed of layers of strontium, bismuth and oxygen atoms in a 'rock salt' structural arrangement, interleaved with layers of strontium, nickel and oxygen atoms in a perovskite structure. Perovskites are defined by a specific crystal structure of two positively charged atoms and one negatively charged atom, and have a number of intriguing properties, from superconductivity to ferroelectricity – a spontaneous electric polarization that can be reversed by the application of an electric field.
This latter characteristic is of particular interest for non-volatile phase change memory, which relies on the ability of a material to switch between two states of electrical resistivity in a reversible manner. "We wanted to know if a similar reversibility could be achieved thermally," said Hideyuki Kawasoko, a materials scientist at Tohoku University and co-author of the paper.
Such reversibility has been demonstrated in various chalcogenides – any compound formed with elements from group 16 of the periodic table (such as sulfur, selenium and tellurium). But it has not been demonstrated in transition metal oxides (compounds of oxygen and any elements found in the middle portion of the periodic table, specifically in groups 3 through 12) – at least until now.
This is an important finding because while chalcogenides have already proven effective for many phase-change memory applications, transition metal oxides often exhibit better thermal and chemical stability. It could lead to devices with longer lifetimes that can operate under more challenging conditions.
Many transition metal oxides are also more abundant than chalcogenides, reducing costs and improving sustainability. And transition metal oxides are already widely used in electronics, sensors and related applications. If they can be adapted to new functions, such as phase change memory, it might be easier to integrate them into existing manufacturing processes and devices – further simplifying the supply chain.
The researchers found that their particular layered nickelate exhibited a thermally reentrant crystalline phase change. This is a type of phase change that occurs when a material undergoes a reversible transition between three crystalline phases upon heating and cooling.
"Basically, the material can switch back and forth between the three phases multiple times as it is heated and cooled," says Tomoteru Fukumura, another co-author of the paper from Tohoku University.
This is in contrast to a typical phase change, which is irreversible and occurs only once as the material is heated or cooled. The thermally reentrant phase change observed in the layered nickelate is significant because it permits the reversible switching of electrical resistivity – and, crucially, at room temperature, which could allow for the development of multi-level non-volatile phase change memory using this type of material.
The study also shed light on the specific mechanisms that are involved in the reversible switching of room-temperature electrical resistivity, which could have important implications for any devices that rely on non-volatile memory.
This story is adapted from material from Tohoku University, with editorial changes made by Materials Today. The views expressed in this article do not necessarily represent those of Elsevier. Link to original source.