Impact of magnetic and antisite disorder on the vibrational densities of states in Ni2MnSn Heusler alloys

We have performed a combined experimental and theoretical investigation of the vibrational properties of Ni2MnSn Heusler alloys. Sn-partial vibrational density of states (VDOS) of 119Sn was measured by nuclear resonant inelastic x-ray scattering at temperatures of 15 and 300 K, while magnetism and local environment of Sn was resolved by 119Sn Mossbauer spectroscopy. Using density functional theory, we associate the peaks in the VDOS with particular features in the element-resolved phonon dispersion of L21 ordered Ni2MnSn. The good agreement between theory and experiment in the low-energy region provides the evidence that the inversion of optical modes at I' involving the displacement of Ni and the heavier main group element atoms, which was predicted previously for other Ni-Mn-based Heusler compounds, is also a characteristic property of Ni2MnSn. Introducing different types of magnetic and antisite disorder in our calculations results in a distinctive redistribution and broadening of the Sn-VDOS, suggesting that considering partial disorder further improves the agreement with the experiment in particular at the highest phonon energies.