Scientists develop new phase-change non-volatile memory

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 layered nickelate, potentially offering better performance and sustainability. Until now, phase change memory has primarily been developed using chalcogenides — a group of materials known to exhibit reversible electrical charges when they transition between their crystalline and amorphous states. Now, the researchers have reported a thermally reversible switching of room-temperature electrical resistivity in a layered nickelate. The study was published in the journal Advanced Science.

Layered nickelates are a class of complex oxide materials composed of nickel ions. 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. Their layered arrangement has drawn the interest of researchers due to the properties of their electrons, with potential applications in fields such as superconductivity and electronics.

The researchers’ layered nickelate is composed of layers of strontium, bismuth and oxygen atoms in a ‘rock salt’ structural arrangement, interleaved with layers of molecules 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 one, and have a number of intriguing properties, from superconductivity to ferroelectricity. This latter characteristic is of interest with respect to non-volatile phase change memory, as the latter relies on the ability of a material to switch between two states of electrical resistivity in a reversible manner.

Hideyuki Kawasoko, a materials scientist at Tohoku University in Japan, said the researchers wanted to know if a similar reversibility could be achieved thermally. Such reversibility has been demonstrated in various chalcogenides, but not in transition metal oxides — at least until now. This is important because while chalcogenides are effective for many phase-change memory applications, transition metal oxides often exhibit better thermal and chemical stability compared to some chalcogenides. This could lead to devices with longer lifetimes that can also operate under challenging conditions.

Many transition metal oxides are also more abundant than chalcogenides, which can improve sustainability. The researchers found that their layered nickelate exhibits a thermally re-entrant crystalline phase change. This refers to 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,” said Tomoteru Fukumura, the author of the paper.

This is in contrast to a typical phase change, which is reversible and occurs only once as the material is heated or cooled. The thermally re-entrant phase change observed in the layered nickelate is significant because it enables the reversible switching of electrical resistivity at room temperature, which allows for the development of multi-level non-volatile phase change memory using this type of material, and in everyday applications.

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