Multiple Magnetic Phase Transitions in Mn-deficient Co_0.4Ni_0.6Mn_1−xGe Compounds

The structural and magnetic transition in Mn-deficient Co_0.4Ni_0.6Mn_1−xGe was investigated by DC magnetization and temperature-dependent X-ray diffraction measurements. By introducing Mn vacancies in the stoichiometric Co_0.4Ni_0.6MnGe compound, multiple magnetic phase transitions and tunable martensitic transitions were observed. A paramagnetic martensite (Ni₂In-type structure) to ferromagnetic austenite (TiNiSi-type structure) transition was realized in Co_0.4Ni_0.6Mn_0.98Ge around 275 K. A large volume expansion of 2.6% accompanied this transition. In addition, a martensitic transition induced by the magnetic field was also observed near room temperature. Furthermore, a significant magnetic entropy variation of − 21.3 J/kg K at 263 K (∆B = 5 T) was obtained in the Co_0.4Ni_0.6Mn_0.98Ge compound.