Abstract:
MnCoGe-based intermetallics have garnered significant attention owing to their pronounced negative thermal expansion and large magnetocaloric effect (MCE), both of which are highly sensitive to chemical substitutions. Despite extensive studies, the microscopic origin underlying the tunability of these properties remains unclear. In this study, we conducted a comprehensive investigation of the local atomic environments around Mn and Co in MnCoGe sub(1-x)Sn sub(x) (0 less than or equal to x less than or equal to 0.1 using extended x-ray absorption fine structure and other structural and magnetic characterization techniques. Our analysis reveals that the substitutional disorder introduced by Sn atoms occupying the Ge sublattice sites disrupts the cooperative lattice distortions required for the long-range displacive martensitic transformation. This disorder-induced suppression results in a systematic decrease in both the martensitic transition temperature (T sub(M))and the magnetic ordering temperature (T sub(C)). At low doping levels, the convergence of T sub(M) and T sub(C) enhances magnetostructural coupling, thereby amplifying the MCE. However, further Sn substitution ultimately suppresses the martensitic transition entirely, leading to a decoupling of the magnetic and structural degrees of freedom and a consequent degradation of the magnetostructural response.
