Atomic vacancy defect modulated giant magnetocaloric effect in multi-component MnCoNiGeSi based compounds

Recently, the promising multi-component magnetocaloric materials (Mc-MCMs) are found to have a tunable giant magnetocaloric effect (GMCE) near room-temperature and manifest fruitful functionalities like multi-caloric effects, which are candidates for solid-state caloric applications. Introducing vacancy defects is found to be an efficient method to optimize its GMCE property. However, the responsible mechanism and especially the characteristics of the atomic vacancies are far from being elucidated. Here, we produce direct-solidified MnCoNiGeSi-based Mc-MCMs which exhibit the distinct shift in transition temperature (Tt) upon introducing Mn/Ni vacancies. It is found that Tt decreased significantly in the Mn vacancy materials and increased in the Ni vacancy materials. The first-order transition is maintained and the strength of the magnetic entropy change (Δsm) was unchanged without degradation. For the Mn vacancy sample the decreased Mn-Mn atomic distance and strengthened covalent bonding can stabilize the high-temperature hexagonal phase, while for the Ni vacancy sample the decreased interatomic distances among different pairs (Mn-Ge, Mn-Mn and Mn-Ni) promote the stabilization of the low-temperature orthorhombic phase. Additionally, the introduced vacancy defects have directly been observed through HAADF-STEM. Positron annihilation results clarified the mono-vacancy nature for these vacancies, and indicate that the Ni positions around the Ni vacancies could partially be occupied by Mn atoms. Our study reveals that introducing atomic vacancy defects can effectively regulate the magnetocaloric properties and provide important fundamental insights into defect engineering of Mc-MCMs.