Combinatorial synthesis and high-throughput characterization of the thin film materials system Co-Mn-Ge: Composition, structure, and magnetic properties

Co-Mn-Ge is a system of interest for magnetocaloric applications as a solid state magnetic refrigerant. A thin film materials library covering a large fraction of the Co-Mn-Ge ternary composition space was fabricated by sputter deposition. After deposition, it was annealed at 600°C for 3 h and quenched subsequently. An energy-dispersive X-ray spectroscopy and X-ray diffraction-based cluster analysis revealed the regions of existence for the CoMnGe and the Co₂MnGe single phase areas. Furthermore, high intensity diffraction peaks revealed the presence of the hexagonal (Co, Mn)₇Ge₆ phase in a region that also featured the CoMnGe phase. In this region, a non-linear, symmetric, and hysteretic shift of the (200) diffraction peak of the (Co, Mn)₇Ge₆ phase was observed by temperature-dependent X-ray diffraction for Co₂₃Mn₃₃Ge₄₄, indicating a structural phase transition taking place between 350 and 375 K upon heating and 325 and 300 K upon cooling. This coincides with a magnetic transition near 325 K from the ferromagnetic to the paramagnetic state. However, no magnetostructural coupling was identified in the temperature range from 330 to 300 K upon cooling. Magnetostriction and an undetected structural transition of the CoMnGe phase were ruled out as probable causes for the non-linear shifts.