First explanation for mechanism behind magnetism-driven NTE derived in 40 years

Back in junior high or high school, many of us would have learned in science classes that materials expand upon heating and contract upon cooling. In reality, some materials behave the other way around, i.e. expanding upon cooling and vice-versa, an unusual phenomenon known as negative thermal expansion (NTE). NTE materials have thus come under the spotlight worldwide because by utilizing their characteristics and using them in combination with non-NTE materials, developers could make materials that are even less heat-sensitive than before. Unfortunately, the mechanism behind NTE had remained unknown to scientists and developers over the past 40 years.

However, for the first time, a recent study led by Professor Masahito Mochizuki at Waseda University and graduate student Masaya Kobayashi from Aoyama Gakuin University was able to provide a theoretical explanation to the NTE phenomenon by examining NTE observed in inverse perovskite antiferromagnets Mn3AN (A = Zn, Ga, etc.). The theory could not only help scientists and developers understand the mechanism behind NTE, but also allow them to predict and identify possible candidate materials that would exhibit NTE -- a crucial process in research and development.

An electron has an angular momentum called "spin" originating from its rotation. During cooling, the spin vectors of electrons that orbit around the manganese (Mn) ion present in Mn3AN would align themselves in a specific fashion called "nonplanar antiferromagnetic order." It has been known that as temperature decreases, the Mn3AN material would expand in volume. Believing that there is a close relationship between the electron spin alignment and negative thermal expansion phenomena in Mn3AN, Professor Mochizuki and his team decided to investigate the correlations between the two to understand the NTE mechanism by numerically reproducing the crystal-volume expansion upon cooling triggered by the nonplanar antiferromagnetic order.

"In our study of inverse perovskite antiferromagnets Mn3AN, we have revealed that the mechanism is not specific to the inverse perovskites but might be expected in other crystal structures. Specifically, antiferromagnets in which antiferromagnetic contribution from direct exchange paths and ferromagnetic contribution from indirect 90 degree paths compete severely with each other are potential candidates that could exhibit NTE," says Professor Mochizuki.

Professor Mochizuki and his team believe that the above prediction will be a useful guide to search for new NTE materials because there is no reliable way to seek or identify magnetism-driven NTE materials at present.

"Although the existence of such competition is not a sufficient condition but a necessary condition for emergence of the magnetism-driven NTE, search for compounds that satisfy this condition is a strong strategy to discover new NTE materials with profound effects." adds Professor Mochizuki.