Pressure-induced decoupling of the magnetostructural phase transition in MnNiSi-Fe₂Ge alloys

Polycrystalline (MnNiSi)_1-x(Fe₂Ge)_x (0.33<x<0.35) alloys were prepared by the arc-melting technique, and influence of pressure of the magnetostructural response was investigated. All samples demonstrated a coupled first-order magnetostructural transition from a low temperature ferromagnetic (FM)TiNiSi-type orthorhombic phase to a high temperature paramagnetic (PM) Ni₂In-type hexagonal phase. The magnetostructural transition temperatures (T_t) was tuned from 365 K– 200 K via chemical pressure employed by varying x in the range of 0.33–0.36. In the x=0.33 sample, hydrostatic pressure shifts T_t to lower temperatures at a rate of ~75 K/GPa, with the coupled nature of magnetostructural transition unchanged. In the x=0.34, 0.35, and 0.36 samples, application of pressures exceeding ~7.5 kbar, results in decoupling of the magnetic transition (FM-PM) and structural transition (orthogonal-hexagonal). The coupling mechanism underlying the magnetostructural response of the (MnNiSi)_(1-x) (Fe₂Ge)_(x) system will be discussed in the context of temperature- and pressure-dependent changes in the crystal structure. Overall, these features emphasize strong coupling between the magnetic spins and the lattice in MnNiSi-based alloys.