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

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 Roldan Cuenya, B. (2018) Influence of interfaces on the phonon density of states of nanoscale metallic multilayers: Phonon confinement and localization. Physical Review B, 98 (2). Art. No. 024308. ISSN 2469-9950. doi:10.1103/PhysRevB.98.024308.

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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.

Item Type:Article
Related URLs:
URLURL TypeDescription Information
Hu, M. Y.0000-0002-3718-7169
Zhao, J.0000-0002-0777-3626
Alp, E. E.0000-0002-4803-8863
Sturhahn, W.0000-0002-9606-4740
Additional Information:© 2018 American Physical Society. (Received 31 March 2018; revised manuscript received 17 June 2018; published 20 July 2018) The authors are grateful to Ulrich von Hörsten (University of Duisburg-Essen) for preparing and characterizing the multilayer samples and for his outstanding technical assistance. W.K. appreciates stimulating discussions with Professor Heiko Wende (University of Duisburg-Essen) and his group and their kind hospitality. Theoretical calculations (S.H. and T.S.R.) were supported by US DOE under Grant No. DE-FG02-07ER46354. The experimental work (BRC) was partially funded by the US National Science Foundation (NSF-DMR 1207065). This research used resources of the Advanced Photon Source, a US Department of Energy (DOE) Office of Science User Facility operated for the DOE Office of Science by Argonne National Laboratory under Contract DOE (US), DE-AC02-06CH11357.
Group:Seismological Laboratory
Funding AgencyGrant Number
Department of Energy (DOE)DE-FG02-07ER46354
Department of Energy (DOE)DE-AC02-06CH11357
Issue or Number:2
Record Number:CaltechAUTHORS:20180720-092638056
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Usage Policy:No commercial reproduction, distribution, display or performance rights in this work are provided.
ID Code:88048
Deposited By: George Porter
Deposited On:23 Jul 2018 14:44
Last Modified:16 Nov 2021 00:23

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