0428同步年報-2021-全

Neutron Science 081 Super-Invar Behavior with Ultrawide Temperature Range Super-invar (SIV) metallic materials underpinned by magnetic ordering are of great practical merit for a wide range of high-precision engineering. I t is generally accepted that the volume of most matter increases upon heating; water below 4 o C is a well known textbook exception. Such a phenomenon, so-called thermal expansion, occurs in our daily lives and is a critical issue in an extreme environment. From a technological point of view, the thermal expansion of component devices is crucial for diverse high-precision applications, such as metrology devices, aerospace vehicles, optical instruments etc . Materials with small or even zero thermal expansion (ZTE) are valuable in preventing size deformation under rapid and frequent thermal shock, increasing the accuracy and lifetime as a result. Xianran Xing (University of Science and Technology Beijing, China) and his co-workers are devoted to the study of the negative thermal expansion (NTE) and ZTE materials. RE M 2 intermetallic compounds ( RE = rare-earth element, M = transition metal) display magnetic ordering and an associated magnetovolume effect (MVE), being promising candidates for engineering ZTE materials. The thermal- expansion behavior of RE M 2 can be tuned on either modifying the materials themselves or on applying an external magnetic field or pressure. For example, the thermal-expansion behavior of Lu 2 Fe 17 compounds can be modified with yttrium substitution for lutetium or introducing vacancies to the lutetium sites. 1,2 Because the Fig. 1 : (a) Schematic ferrimagnetic structure of Ho 2 (Fe, Co) 17 determined with NPD refinements. (b) Contour plot of magnetic peaks (110) for Ho 2 Fe 17 , Ho 2 Fe 16 Co and Ho 2 Fe 15 Co 2 . (c) Temperature dependence of magnetic moments of the Fe/Co-sublattice determined from NPD. (d) M(H) curves of Ho 2 Fe 17 , Ho 2 Fe 16 Co and Ho 2 Fe 15 Co 2 measured at 4 K. The inset shows the saturation magnetization. (e)−(g) Temperature dependence of magnetic moments of various atomic sites and sublattices for Ho 2 Fe 17 , Ho 2 Fe 16 Co and Ho 2 Fe 15 Co 2 compounds. [Reproduced from Ref. 3]