Chemical ordering and non-linear magnetic behavior: enhancing magnetocaloric effect in FeMnNiGeSi high-entropy materials

High-entropy alloys (HEAs) offer opportunities to discover new functional materials with combination of properties not reached in other classes of materials. Here we report a detailed investigation of the structure, magnetic and magnetocaloric properties of some high entropy materials in the FeMnNiGeSi quinary system. Our findings indicate the persistence of partial chemical ordering, with transition metal atoms and metalloids preferentially occupying specific crystallographic sites. This ordering is further supported by ab initio total energy calculation. In turn, the magnetic moment - carried out mainly by Fe and Mn - exhibits a strong dependence on the crystallographic sites occupied by these magnetic atoms. By exploring the chemical landscape, we observed highly non-linear magnetic behavior upon Mn doping, resulting in an isothermal entropy change (ΔSm) as high as 35 Jkg-1K-1 in 5 T, while the transition temperature can be tuned in the range from 160 to 263 K, approaching room temperature. These outstanding performances suggest that rare-earth free high entropy materials can rival conventional magnetocaloric materials. Additionally, we find that the first-order magnetostructural transition can evolve into a second-order magnetic transition through chemical tuning. This opens the possibility of discovering similar materials near the boundary between first- and second-order transitions, with vanishing hysteresis - potentially addressing their mechanical instability issue.