Enhanced magnetocaloric effect in Ni-Mn-Sn alloys by synergistic manipulation of magnetization difference and lattice volume

Ni-Mn-based alloys are renowned for their outstanding magnetocaloric effect, attributed to their significant magnetization difference (triangle M) and low thermal hysteresis (Delta T-hys) during magneto-structural transition. This study employs first-principles calculations to explore the martensitic transformation, magnetic properties, and electronic structure of the Co doped Ni2-xCoxMn1.5Sn0.5 (x = 0-0.42) alloys. Results reveal that Co tends to directly occupy the Ni sublattice, the volumes of ferromagnetic austenite, ferrimagnetic six-layer modulated (6M), and non-modulated (NM) martensites decrease with increasing Co content due to the strong bonding and interaction between Co atoms and surrounding atoms. Fitted equations for the formation energies of austenite and martensites with Co content were also established. The possible phase transformation sequence including the complex structure 6M martensite was clarified. From the perspective of phase transformation driving force, the fundamental reason why the martensitic transformation temperature decreases with the increase of Co content is explained. It also clarifies the composition range of magneto-structural coupling is 0.25 <= x < 0.33. Furthermore, to study the physical properties of Si-doped Ni-Co-Mn-Sn alloy, the Ni1.75Co0.25Mn1.5Sn0.5 alloy with a large triangle M of 4.59 mu(B)/f.u. was selected as the matrix for Si doping. Si atoms preferentially occupy the Mn-Sn sublattice, and Si doping can enhance the phase stability of austenite, 6M and NM martensites, reduce the tetragonal distortion and volume change during the martensitic transformation, and thus may have the benefit to reduce Delta T-hys. These findings provide foundational insights for designing new alloys with improved properties.