Magneto-structural transition in MnCoGe thin films investigated by ferromagnetic resonance

We investigated the relation between magnetic and structural properties of magnetocaloric MnCoGe thin films grown on different substrates. The thin films show a hexagonal structure at room temperature with polycristalline form when grown on Si substrates or a highly textured columnar grains with same orientation when grown on Ge substrates. Their static and dynamic magnetic properties show anomalies at lower temperature, consistent with a gradual mixing of hexagonal and orthorhombic structures. Below a blocking temperature, the thin films show glassy behavior and a decoherence of the FMR mode in the normal direction down to 10K. We relate our experimental results to the the martensitic strain in the sample, due to the hexagonal to orthorhombic structural transition, which affects the magnetocrystalline anisotropy and is known to be sensitive to the morphology (grain size and grain boundaries). We show that the dynamic magnetic properties are strongly dependent on morphology, as the decoherence of the normal FMR mode does not apear for polycrystalline MnCoGe films grown on Si substrates, even though a similar behavior is observed below 10K. Our results suggest a dynamic magnetic ground state in mcg thin films at low temperature.