Robustness of helical magnetic structure under external pressure in Fe-doped MnNiGe alloy

We have investigated the detailed magnetic structure of a Fe-doped MnNiGe alloy of nominal composition MnNi$_{0.75}$Fe$_{0.25}$Ge by ambient and high pressure (6 kbar) neutron powder diffraction study. The alloy undergoes a martensitic phase transition between 230 K and 275 K. The low-temperature martensite phase orders antiferromagnetically with helical modulation of Mn-spins. At 1.5 K, the incommensurate propagation vector $\bf k$ is found to be (0.1790(1),0,0), and it remains almost unchanged with temperature under ambient condition. The Application of external pressure ($P$) reduces the martensitic transition temperature and results in a significant change in the incommensurate magnetic propagation vector. At 10 K, with 6 kbar of external pressure, the incommensurate magnetic structure of the alloy stabilizes with $\bf k$ = (0.1569(1),0,0). Interestingly, the strong temperature dependence of $\bf k$ in the presence of external $P$ has also been observed.