Martensitic transition assisted modification of the antiferromagnetic ordering in Ge-site doped MnNiGe alloys

The magnetic equiatomic alloys of the nominal compositions ${\mathrm{MnNiGe}}_0.9{\mathrm{Al}}_0.1$ and ${\mathrm{MnNiGe}}_0.928{\mathrm{Ga}}_0.072$ have been investigated through the neutron powder diffraction (NPD) technique. Both these alloys undergo a martensitic phase transition from the high-temperature hexagonal (${\mathrm{Ni}}₂\mathrm{In}$ type, with space group $P{6}₃/mmc$) austenite phase to the low-temperature orthorhombic (TiNiSi type, with space group $Pnma$) martensite phase. Our NPD analysis confirmed an incommensurate antiferromagnetic ordering of the low-temperature martensitic phase with helical modulation. The incommensurate propagation vector $\mathbf{k}$ is found to be (0.2065(1),0,0) and (0.2088(4),0,0) for Al and Ga doped alloys, respectively, at 1.5 K and shows a monotonic increase with increasing sample temperature. On the other hand, a commensurate antiferromagnetic ordering was observed for the high-temperature austenite phase. The incomplete martensitic transition allowed both these incommensurate and commensurate antiferromagnetic structures to coexist down to the lowest temperature of measurement (1.5 K).