Abstract
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 151Eu and first-principles theory. The films were epitaxially grown on YAlO3(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.
Highlights
Anomalies in the early Raman and inelastic neutron scattering data obtained across the TC along with a critical behavior of the thermal conductivity and sound velocity, properties mutually related to the lattice dynamics, indicated a strong interaction between spin waves and lattice vibrations in this material
The experimental results, supported by first-principles theory, uncovered a giant and highly anisotropic spin-phonon coupling induced by the spin dynamics slightly above and well below the TC in bulk EuO.[20,21]
In EuO, such phonon anomalies, combined with the strong interaction between spins and lattice vibrations could potentially increase the probability for undesired spin flips reducing the degree of spin polarization along certain crystallographic directions. This fact, combined with the ongoing efforts for epitaxial strain manipulation of the Curie temperature in thin EuO films[28,29] makes lattice dynamics and spin-phonon interaction investigations in strained EuO films exhibiting thicknesses and configurations similar to those envisaged for tunnel junction applications indispensable
Summary
Paper counterparts with the main features being an enhancement of the number of phonon states at low and high energy and suppression and shift of the peaks.[22,23,24,25,26,27] In EuO, such phonon anomalies, combined with the strong interaction between spins and lattice vibrations could potentially increase the probability for undesired spin flips reducing the degree of spin polarization along certain crystallographic directions This fact, combined with the ongoing efforts for epitaxial strain manipulation of the Curie temperature in thin EuO films[28,29] makes lattice dynamics and spin-phonon interaction investigations in strained EuO films exhibiting thicknesses and configurations similar to those envisaged for tunnel junction applications indispensable. The experimental data confirm the presence of spin-phonon interaction in the tensile-strained 8.0 nm thick EuO film being, reduced by a factor of four compared to a strain-free lattice
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