  Tailoring the phase stability and magnetic structure by combining the
antiferromagnetic MnNiGe martensite with ferromagnetic MnFeGe austenite, a
magnetic-field-induced martensitic transformation has been achieved in
MnNi1-xFexGe system. The doped Fe enables the isolated structural and magnetic
transitions of MnNiGe to coincide at room temperature and also establishes
ferromagnetic couplings in six-Mn-centering-one-Fe-atom clusters. The
ferromagnetic clusters transit the antiferromagnetic state in martensite to a
ferromagnetic state and the alloys are further induced to a stronger
ferromagnetic state by external field, resulting in a large magnetization
difference up to 50 emu/g across the transformation. The field-induced
martensitic transformation thus experiences a multiple metamagnetization
process.
