Abstract:
The microscopic origin of the martensitic transition (MST) in MnCoGe remains unresolved due to the intertwined roles of lattice distortion, magnetic exchange, and electronic structure. In this work, we employ site-selective substitution - Ni at Mn and Co sites, and Cu at the Co site to decouple these competing effects. Synchrotron X-ray diffraction and X-ray absorption spectroscopy reveal that Ni substitution at the Mn site suppresses the MST by reducing the Mn-Mn interatomic distance, thereby stabilizing the hexagonal phase through enhanced ferromagnetic exchange. In contrast, Cu substitution at the Co site suppresses the MST without significant lattice contraction, instead weakening p-d hybridization, as evidenced by systematic changes in near-edge spectra. Ni substitution at the Co site induces only minor structural and electronic modifications, preserving the MST. These findings demonstrate that the martensitic transition in MnCoGe is governed by a competition between Mn-Mn exchange interactions and the electronic band energy associated with hybridization, both of which are strongly site-dependent.
