To date, phase change memory has been developed primarily using chalcogenides, a group of materials that exhibit reversible electrical changes when transitioning between crystalline and amorphous states. A recently published study reports room temperature thermally reversible electrical resistance switching in layered nickelate, potentially offering better performance and superior stability.
The study was published in the journal by Japanese scientists from Tohoku University Advanced science September 3, 2023. Layered nickelates are a class of complex oxide materials composed of nickel ions. They have a structure in which layers of nickel and oxygen atoms are interspersed with layers of other elements, often alkaline earth or rare earth elements. Their unique structure has piqued the interest of researchers due to the properties of electrons, which can have applications in areas such as superconductivity and microelectronics.
The layered nickelate obtained by the researchers consists of layers of strontium, bismuth and oxygen atoms structurally arranged in a cubic lattice, interspersed with layers of strontium, nickel and oxygen molecules in a perovskite structure. Characterized by a specific crystal structure consisting of two positively charged and one negatively charged atom, perovskites exhibit a range of interesting properties, from superconductivity to ferroelectricity – spontaneous electrical polarization that can be reversed by applying an electric field.
The effect of ferroelectricity seems to be very promising for the development of non-volatile phase change memories, since the principle of operation of such a memory is based on the ability of a material to reversibly switch between two states of electrical resistance.
Previously, scientists knew about similar properties of chalcogenides – binary chemical compounds of chalcogens (elements of the 16th group of the periodic table, which include oxygen, sulfur, selenium, tellurium, polonium and livermorium) with metals. In oxides of transition metals of the periodic table, which include iron, copper, zinc, silver, gold, platinum, molybdenum, cobalt and others, the effect of ferroelectricity has not yet been studied.
Although chalcogenides have proven effective in many phase change memory applications, transition metal oxides often exhibit better thermal and chemical stability. Many transition metal oxides are more common than chalcogenides and are commonly used in electronics manufacturing.
Transition metal oxides can be easily integrated into existing manufacturing processes and equipment to simplify the supply chain and provide additional sustainability benefits. Their use can also help create devices for use in difficult conditions with a longer lifespan.
The researchers found that their layered nickelate exhibited a thermally reversible crystalline phase change, meaning that the material undergoes a reversible transition between three crystalline phases when heated and cooled. “Essentially, the material can switch between three phases multiple times as it heats and cools.”one of the researchers remarked.
This is in contrast to a typical irreversible phase transition, which occurs only once when the material is heated or cooled. The thermally reversible phase transition observed in layered nickelate enables reversible switching of electrical resistance at room temperature, thereby enabling the development of a multistage nonvolatile phase change memory for everyday applications.
The study also highlights the process of reversible phase transition and electrical resistance change at room temperature, which could have important implications for the development of nonvolatile memory, without reference to the specific material used.