Influence of interfaces on the phonon density of states of nanoscale metallic multilayers: Phonon confinement and localization

Abstract

Isotope-selective 57 Fe nuclear resonant inelastic x-ray scattering (NRIXS) measurements and atomic-layer resolved density functional theory (DFT) calculations were used to investigate the effect of interfaces on the vibrational (phonon) density of states (VDOS) of (001)-oriented nanoscale Fe/Ag and Fe/Cr multilayers. The multilayers in the experiment contained isotopically enriched 57 Fe monolayers as probe layers located either at the Fe/Ag or Fe/Cr interfaces or in the center of the Fe films. This allows probing of the vibrational dynamics of Fe sites either at the buried interfaces or in the center of the Fe films. For Fe/Ag multilayers, distinct differences were observed experimentally between the Fe-partial VDOS at the interface and in the center. At the Fe/Ag interface, the high-energy longitudinal-acoustic (LA) phonon peak of Fe near ∼35 meV is suppressed and slightly shifted to lower energy, and the low-energy part of the VDOS below ∼20 meV is drastically enhanced, as compared to the Fe-specific VDOS in the center Fe layers or in bulk Fe. Similar phenomena are found to a less degree in the Fe/Cr multilayers. The measured Fe-partial VDOS was used to determine the Fe site-selective vibrational thermodynamic properties of the multilayers. Our theoretical findings for the layer-dependent VDOS of the multilayers are in qualitative agreement with the experimental results obtained by NRIXS. For Fe/Ag multilayers, which are characterized by a large atomic mass ratio, the experimental and theoretical results demonstrate phonon confinement in the Fe layers and phonon localization at the Fe/Ag interfaces due to the energy mismatch between Ag and Fe LA phonons. These phenomena are reduced or suppressed in the Fe/Cr multilayers with their about equal atomic masses. Moreover, direction-projected Fe VDOS along the (nearly in-plane) incident x-ray beam was computed in order to address the intrinsic vibrational anisotropy of the Fe/Ag multilayer. We have also performed spin-resolved electronic band structure (DFT) calculations, predicting an enhanced magnetic moment (μ Fe = 2.8 μ B ) of the interfacial Fe atoms and a high electron spin polarization (79%) at the Fermi energy for the Fe/Ag interface, as compared to the case of Fe center layers. This is a result of charge transfer from Fe to Ag at the interface. On the contrary, Cr tends to donate electrons to Fe, thus reducing the interfacial Fe moment (μ Fe = 1.9 μ B ). This implies strong chemical bonding at the Fe/Ag and Fe/Cr interfaces, affecting the interfacial VDOS.

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

“…We like to mention that a qualitatively similar reduction and energy shift of the LA phonon peak combined with an enhancement of the low-energy VDOS part have been observed previously by layer selective 57 Fe NRIXS combined with layer-and element-selective DFT-based calculations for Fe/Ag multilayers 45 and were interpreted in terms of high-energy phonon confinement in the Fe layers and phonon localization at the Fe/Ag interfaces, occuring as the result of the large energy mismatch between Ag and Fe LA phonons. For the energetically high Fe LA phonons (at 35.5 meV) there is no counterpart in the g(E) of the phononically soft Ag metal, because the upper phonon cutoff energy of Ag is located at only ∼ 21 meV, much lower than that of bcc Fe (∼40 meV).…”

“…As it has been outlined in Ref. 45 and 47, the modifications of g(E) in nanoscale multilayers are anticipated to influence the phonon transport and the vibrational thermodynamic properties in these systems. Therefore, 57 Fe NRIXS measurements and DFT-based computations of the phononic properties of [Fe/MgO] multilayers are highly desirable, in particular in view of the fact that Fe/MgO is of interest as a prototype system for the study of time-resolved phenomena on the femtosecond scale 47,48 .…”

“…Applying 57 Fe NRIXS to various nanoscale [Fe/metal] multilayers, the observed impact of the Fe film thickness and type of metal on the shape of the partial-Fe g(E) has been interpreted by confinement of high energy Fe phonons in the Fe films 44 . Moreover, the influence of interfaces on the VDOS of nanoscale metallic multilayers in terms of phonon confinement and phonon localization could be determined by 57 Fe probe-layer NRIXS and atomic-layer resolved DFT-based calculations 45 . First-principles calculations for monolayer-scale Fe(001)/Au(001) superlattices predicted drastic variations of g(E) with individual Fe and Au thicknesses 46 .…”

Phonon confinement and spin-phonon coupling in tensile-strained ultrathin EuO films

“…Combined experimental and theoretical studies of strainfree metal/semiconductor heterostructures 42 and metallic multilayers 43 demonstrated that the force constants of the atoms located at the interface are drastically reduced compared to the bulk values. This gives rise to an entirely different PDOS of the interface layers from that of a bulk material.…”

Thermal Characterization and Modelling of AlGaN-GaN Multilayer Structures for HEMT Applications

“…For all interfaces, the PRL and the DMM overestimate the thermal interface conductance. These models, however, do not account for scattering mechanisms from defects at the interface, changes in the interatomic bonds near the interface or interfacial disorder (especially in well-matched materials) [44], that results in a reduction of the thermal conductance [43].…”