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

Reducing the material sizes to the nanometer length scale leads to drastic modifications of the propagating lattice excitations (phonons) and their interactions with electrons and magnons. In EuO, a promising material for spintronic applications in which a giant spin-phonon interaction is present, t...

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Bibliographic Details
Published in:Nanoscale 2019-06, Vol.11 (22), p.1968-1976
Main Authors: Pradip, Ramu, Piekarz, Przemys aw, Merkel, Dániel G, Kalt, Jochen, Waller, Olga, Chumakov, Aleksandr I, Rüffer, Rudolf, Ole, Andrzej M, Parlinski, Krzysztof, Baumbach, Tilo, Stankov, Svetoslav
Format: Article
Language:English
Subjects:
Anomalies
Elastic properties
Epitaxial growth
First principles
Inelastic scattering
Magnons
Phonons
Polarization (spin alignment)
Reduction
Spin dynamics
Tensile strain
Thickness
Thin films
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Summary:Reducing the material sizes to the nanometer length scale leads to drastic modifications of the propagating lattice excitations (phonons) and their interactions with electrons and magnons. In EuO, a promising material for spintronic applications in which a giant spin-phonon interaction is present, this might imply a reduction of the degree of spin polarization in thin films. Therefore, a comprehensive investigation of the lattice dynamics and spin-phonon interaction in EuO films is necessary for practical applications. We report a systematic lattice dynamics study of ultrathin EuO(001) films using nuclear inelastic scattering on the Mössbauer-active isotope 151 Eu and first-principles theory. The films were epitaxially grown on YAlO 3 (110), which induces a tensile strain of ca . 2%. By reducing the EuO layer thickness from 8 nm to a sub-monolayer coverage, the Eu-partial phonon density of states (PDOS) reveals a gradual enhancement of the number of low-energy phonon states and simultaneous broadening and suppression of the peaks. These deviations from bulk features lead to significant anomalies in the vibrational thermodynamic and elastic properties calculated from the PDOS. The experimental results, supported by first-principles theory, unveil a reduction of the strength of the spin-phonon interaction in the tensile-strained EuO by a factor of four compared to a strain-free lattice. Reducing the material sizes to the nanometer length scale leads to drastic modifications of the propagating lattice excitations (phonons) and their interactions with electrons and magnons.
ISSN:2040-3364
2040-3372
DOI:10.1039/c9nr01931f