Defect Engineering: Electron-Exchange Integral Manipulation to Generate a Large Magnetocaloric Effect in NiMnCoSn Alloys.

A promising magnetocaloric effect has been obtained in Ni-(Co)-Mn-X (X = Sn, In, Sb)-based Heusler alloys, but the low isothermal magnetic entropy change Δ restricts the further promotion of such materials. Defect engineering is a useful method to modulate magnetic performance and shows great potential in improving the magnetocaloric effect. In this work, dense Ni vacancies are introduced in NiMnCoSn alloys by employing high-energy electron irradiation to adjust the magnetic properties. These vacancies bring about intense lattice distortion to change the distance between adjacent magnetic atoms, leading to a significant enhancement of the average magnetic moment. As a result, the saturation magnetization of ferromagnetic austenite is accordingly improved to generate a high isothermal magnetic entropy change Δ of 20.0 J/(kg K) at a very low magnetic field of ∼2 T.