What the research is about
Most materials we use in everyday life expand slightly when heated and return to their original size when cooled. In addition to such thermal properties, materials can also have electrical properties or magnetic properties, and traditionally we have used these characteristics separately. However, some materials allow multiple properties to coexist within a single substance. Research on such materials is expected to contribute to the development of next-generation memory devices that can store and retain information while consuming far less energy.
A representative example is a class of materials known as multiferroics, which combine the properties of a capacitor (the ability to store electric charge) and a magnet. Among them, bismuth ferrite (BiFeO₃) is one of the most intensively studied materials in the field. When an external voltage is applied, the direction of its stored electric polarization can be switched, and this change can also influence its magnetic properties.
Because of this unique coupling, bismuth ferrite is considered a promising candidate for next-generation memory materials that can rewrite information using very little electricity and allow that information to be read easily. The reason lies in the complementary strengths of electricity and magnetism: the electric polarization can be switched with very little energy, although it is difficult to read directly, whereas magnetic states are easy to detect but require more energy to switch. Combining these advantages could enable low-power, efficient memory devices.
However, there has been a challenge. Inside bismuth ferrite, many iron (Fe) ions act like tiny magnets. These microscopic magnets do not align in a single direction; instead, they gradually rotate in a wave-like pattern. As a result, their magnetic moments cancel one another out, and almost no overall magnetism can be observed from outside the material. This has made it difficult to fully utilize its magnetic properties.
Previous studies showed that replacing some of the iron with cobalt could alter the arrangement of these tiny magnets, allowing the material to behave as a weak magnet even at room temperature. However, the resulting magnetism was still weak and easily disturbed by external influences.
Why this matters
To overcome these limitations, a research team led by Professor Masaki Azuma at Institute of Science Tokyo (Science Tokyo) reexamined the strategy of element substitution itself. Guided by both experimental results and theoretical predictions, they focused on selecting elements that would strongly influence the behavior of the microscopic magnetic moments inside the material.
Based on theoretical calculations predicting which elements could enhance magnetism, the team replaced some of the iron with heavier elements such as ruthenium (Ru) and iridium (Ir). In bismuth ferrite, both bismuth and iron are normally in a 3+ state, balanced by three oxygen ions each carrying a 2− charge. However, ruthenium and iridium tend to form 4+ ions, which would disrupt this electrical balance if introduced alone. To maintain overall charge neutrality, the researchers simultaneously replaced some of the bismuth (3+) with calcium (2+). Combining theoretical insight with this careful chemical design was a major challenge that determined the success of the study.
As a result, the tiny magnets inside the material no longer fluctuated randomly. Instead, a clear magnetic state emerged that could be detected externally. In particular, materials containing ruthenium or iridium exhibited magnetic stability about four times greater than that of earlier cobalt-substituted materials. This improved stability is crucial for next-generation memory devices, which must retain information reliably over long periods.
During further investigation, the researchers discovered another unexpected property: when heated, the material slightly shrinks-a phenomenon not observed in conventional bismuth ferrite. Remarkably, this behavior occurs within everyday temperature ranges. If such materials are combined with ordinary materials that expand when heated, it may be possible to suppress strain and degradation caused by temperature changes.
What's next
These findings open new possibilities for memory devices that use voltage to write information and magnetism to read it. Stable magnetic states are essential for reliably storing data while consuming very little power. In addition, the unusual property of shrinking upon heating could prove useful in precision instruments, where even small thermal expansions can cause mechanical stress or performance degradation.
The research team has previously worked on developing and commercializing materials with negative thermal expansion, and they hope that this newly designed material will also move toward practical applications in the future.
Comment from the researcher
The behavior of a material can change dramatically depending on how its constituent elements are substituted. In materials research, it is equally important to carefully design element combinations based on predictions and to recognize the significance of unexpected results. In this study, we successfully achieved the magnetic properties we aimed for-and were also fortunate to discover an unexpected phenomenon known as negative thermal expansion, in which the material shrinks when heated.
（Masaki Azuma, Professor, Research Center for Autonomous System Materialogy, Institute of Science Tokyo）
