Interface-related magnetic and vibrational properties in Fe/MgO heterostructures from nuclear resonant spectroscopy and first-principles calculations

TL;DR

This study combines experimental spectroscopy and first-principles calculations to investigate how interfaces in Fe/MgO heterostructures affect magnetic and vibrational properties, revealing enhanced magnetic fields and altered phonon behavior at interfaces.

Contribution

It provides a comprehensive analysis of interface-related magnetic and vibrational properties in Fe/MgO multilayers using combined experimental and theoretical approaches, highlighting the impact of interfaces.

Findings

Enhanced hyperfine magnetic fields at interfaces.

Reduction of longitudinal acoustic phonon peak.

Increased low-energy vibrational density of states.

Abstract

We combine $^{57}$Fe M\"ossbauer spectroscopy and $^{57}$Fe nuclear resonant inelastic x-ray scattering (NRIXS) in nanoscale polycrystalline [bcc-$^{57}$Fe/MgO] multilayers with various Fe layer thicknesses and layer-resolved density-functional-theory (DFT) based first-principles calculations of a (001)-oriented [Fe(8 ML)/MgO(8 ML)](001) heterostructure to unravel the interface-related atomic vibrational properties of a multilayer system. In theory and experiment, we observe consistently enhanced hyperfine magnetic fields compared to bulk which are associated with the Fe/MgO interface layers. NRIXS and DFT both reveal a strong reduction of the longitudinal acoustic (LA) phonon peak in combination with an enhancement of the low-energy vibrational density of states (VDOS) suggesting that the presence of interfaces and the associated increase in the layer-resolved magnetic moments results in drastic changes in the Fe-partial VDOS. From the experimental and calculated VDOS, vibrational thermodynamic properties have been determined as a function of Fe thickness and are found to be in excellent agreement.