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

Abstract

Here, isotope-selective 57Fe 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 57Fe 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 ynamics 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 ofmore » 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 = 2.8μ 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 = 1.9μ B). This implies strong chemical bonding at the Fe/Ag and Fe/Cr interfaces, affecting the interfacial VDOS.« less

Authors:
 [1];  [2];  [3];  [3];  [3];  [3];  [4];  [5];  [6]
  1. Univ. of Duisburg-Essen, Duisburg (Germany)
  2. Univ. of Central Florida, Orlando, FL (United States); Brewton-Parker College, Mount Vernon, GA (United States)
  3. Argonne National Lab. (ANL), Lemont, IL (United States)
  4. California Inst. of Technology (CalTech), Pasadena, CA (United States)
  5. Univ. of Central Florida, Orlando, FL (United States)
  6. Univ. of Central Florida, Orlando, FL (United States); Fritz-Haber Institute of the Max Planck Society, Berlin (Germany)
Publication Date:
Research Org.:
Argonne National Lab. (ANL), Argonne, IL (United States)
Sponsoring Org.:
USDOE Office of Science (SC), Basic Energy Sciences (BES) (SC-22); National Science Foundation (NSF)
OSTI Identifier:
1464638
Alternate Identifier(s):
OSTI ID: 1461030
Grant/Contract Number:  
AC02-06CH11357; FG02-07ER46354
Resource Type:
Journal Article: Accepted Manuscript
Journal Name:
Physical Review B
Additional Journal Information:
Journal Volume: 98; Journal Issue: 2; Journal ID: ISSN 2469-9950
Publisher:
American Physical Society (APS)
Country of Publication:
United States
Language:
English
Subject:
72 PHYSICS OF ELEMENTARY PARTICLES AND FIELDS

Citation Formats

Keune, W., Hong, Sampyo, Hu, M. Y., Zhao, J., Toellner, T. S., Alp, E. E., Sturhahn, W., Rahman, T. S., and Cuenya, B. Roldan. Influence of interfaces on the phonon density of states of nanoscale metallic multilayers: Phonon confinement and localization. United States: N. p., 2018. Web. doi:10.1103/PhysRevB.98.024308.
Keune, W., Hong, Sampyo, Hu, M. Y., Zhao, J., Toellner, T. S., Alp, E. E., Sturhahn, W., Rahman, T. S., & Cuenya, B. Roldan. Influence of interfaces on the phonon density of states of nanoscale metallic multilayers: Phonon confinement and localization. United States. doi:10.1103/PhysRevB.98.024308.
Keune, W., Hong, Sampyo, Hu, M. Y., Zhao, J., Toellner, T. S., Alp, E. E., Sturhahn, W., Rahman, T. S., and Cuenya, B. Roldan. Fri . "Influence of interfaces on the phonon density of states of nanoscale metallic multilayers: Phonon confinement and localization". United States. doi:10.1103/PhysRevB.98.024308. https://www.osti.gov/servlets/purl/1464638.
@article{osti_1464638,
title = {Influence of interfaces on the phonon density of states of nanoscale metallic multilayers: Phonon confinement and localization},
author = {Keune, W. and Hong, Sampyo and Hu, M. Y. and Zhao, J. and Toellner, T. S. and Alp, E. E. and Sturhahn, W. and Rahman, T. S. and Cuenya, B. Roldan},
abstractNote = {Here, isotope-selective 57Fe 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 57Fe 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 ynamics 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 (μFe = 2.8μB) 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 (μFe = 1.9μB). This implies strong chemical bonding at the Fe/Ag and Fe/Cr interfaces, affecting the interfacial VDOS.},
doi = {10.1103/PhysRevB.98.024308},
journal = {Physical Review B},
issn = {2469-9950},
number = 2,
volume = 98,
place = {United States},
year = {2018},
month = {7}
}

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