We employed nuclear inelastic scattering of synchrotron radiation to measure the anisotropy in the phonon density of states of an FePt thin film ordered in the $\text{L}{1}_0$ phase and compared our results with ab initio calculations. We find a strong anisotropy in the phonon density of states along the $a$ and $c$ axes of the crystal, which induces a difference in the calculated thermodynamic and elastic parameters along these axes. Calculations of surface-related effects show that a strong deviation from the bulk vibrational spectrum is observed for an Fe-terminated surface. We deduce by comparison with our surface sensitive measurement that the FePt(100)-oriented surface is Pt terminated. This contrasts with a recent measurement on FePt nanoparticles, where additional low-energy modes are observed. The findings are expected to be representative of the lattice dynamics of $\text{L}{1}_0$ intermetallics.

• Received 14 July 2010

DOI:https://doi.org/10.1103/PhysRevB.82.094109