Negative thermal expansion in magnetic materials

By virtue of the unique advantages of compensating and controlling thermal expansion, negative thermal expansion (NTE) materials have attracted great attention and have gained rapid development in the last two decades. The history of NTE can be traced back to the discovery of magnetic Invar alloy in 1897. Due to the complexity and variability of magnetism, studies on the magnetic NTE materials, including the discovery of new materials, the controllable thermal expansion, and the origin of magnetic NTE, have never stopped for more than one century. In this review, we will summarize the progress in the magnetic NTE materials from the prototype Invar alloys to the recently discovered materials such as Mn-based antiperovskites, Laves phases, La(Fe, Si)13, Mn3Ge, R2Fe17, R(Fe,V)12, and R2Fe14B (R = rare earth elements). Based on the variable magnetic transition types, the magnetic NTE materials are reviewed in detail from magnetic and crystal structures, NTE properties, and mechanisms. The main mechanisms of the NTE of magnetic materials are summarized for the first time, including order-to-disorder transition, the change of local moment, metamagnetic transition, short-range magnetic ordering, structural phase transition, and the coexistence of magnetic phases. The NTE properties of magnetic materials can be controlled by adjusting magnetic exchange interaction via the methods of chemical substitution, nanocrystallization, external field, and interstitial atoms. More importantly, by enhancing the negative contribution of the magnetic transition to thermal expansion, the new magnetic NTE materials can be found from the normal positive thermal expansion materials. This review will be helpful for the design of novel magnetic NTE materials, the control of thermal expansion properties, and the understandings of magnetic NTE mechanisms.