New phenomenon in old material: giant spin-phonon interaction unveiled in EuO

12-07-2016

A remarkably strong and anisotropic spin-phonon coupling was discovered for the ferromagnetic semiconductor europium monoxide through synchrotron experiments, inelastic X-ray scattering and nuclear inelastic scattering, and density functional theory. This discovery paves the way towards the design of spintronics devices with new functionalities.

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Europium monoxide is a very rare ferromagnetic semiconductor that has been known to scientists since the early sixties mainly for its giant magneto-optic Kerr and Faraday effects, metal-to-insulator transition, colossal magnetoresistance, and anomalous Hall effect. Presently, the large exchange splitting of its conduction band along with its integration with Si and GaN has made this fascinating oxide one of the favoured candidates for applications as a spin filter in near-future spintronic devices. It was surprising therefore, that despite the intensive research on this material, its lattice dynamics remained to a large extent unknown until now.

Early Raman and inelastic neutron scattering studies reported the presence of spin waves in the vicinity of the Curie temperatureTc = 69 K with anomalies in their linewidths that remained unexplained. In addition, a reduction of the sound velocity and thermal conductivity have been observed at Tc and attributed to spin-phonon coupling, however, without direct experimental evidence.

We have determined the lattice dynamics of EuO by combining modern X-ray scattering methods with density functional theory. Phonon dispersion relations and Eu-partial density of phonon states have been measured at beamlines ID28 and ID18 by inelastic X-ray scattering and nuclear inelastic scattering, respectively, on a 100 nm thick EuO(001) film epitaxially grown on YSZ(001) substrate and covered by a Nb layer to prevent further oxidation.

Unexpectedly, the analysis unveiled the presence of a remarkably strong and anisotropic spin-phonon coupling induced by the spin dynamics slightly above and well below the Curie temperature that remained undiscovered for decades. More precisely, a 5-fold increase of the full width at half maximum (FWHM) for the transverse acoustic (TA) phonons along the Γ-X direction (Figure 1a) and a 3-fold increase of the FWHM for the longitudinal acoustic (LA) phonons along the Γ-K-X direction towards the boundary of the Brillouin zone (Figure 1d) were discovered.

full width at half maximum for the transverse acoustic phonons along the Γ-X direction and the longitudinal acoustic phonons along the Γ-K-X direction

Figure 1. (a,b) FWHMs as a function of momentum transfer for the TA and LA branches along the Γ-X direction at 295, 90, and 40 K. (c,d) FWHMs for the TA and LA branches along the Γ-K-X direction at 295 and 40 K.

The linewidth broadening observed in the TA modes along the Γ-X direction stems from the fact that the energies of the spin waves (up to 6 meV) are closer to the energies of the TA phonons. Even though these energies do not match exactly, they span a comparable range such that the momentum and energy conservation of the spin-phonon coupling process could be fulfilled. Contrary to the TA modes, the LA phonons show a pronounced broadening along the Γ-K-X direction. This observation arises from the fact that the LA modes modulate the shortest Eu-Eu distance, thus affecting the magnetic exchange interaction between the localised 4f electrons. This effect is particularly strong at the zone boundary, where the nearest neighbour atoms vibrate with opposite phases (Figure 2), which explains the increase of the phonon widths towards the X point.

spin wave in EuO without coupling spin wave in EuO with coupling

Figure 2. Animation of the spin wave in EuO developed in the vicinity of theTc without (left) and with (right) coupling to the longitudinal acoustic (LA) phonons.

Such strong coupling between magnetic (electric) and vibrational properties in materials paves the way towards the design of devices with new functionalities. In the light of spintronics applications, the discovered phenomenon suggests that the choice of single crystal orientation could be crucial for reducing the undesired spin-flips, sustaining the high degree of spin current polarisation in EuO.

 

Principal publication and authors
Lattice dynamics of EuO: evidence for giant spin-phonon coupling, R. Pradip (a,b), P. Piekarz (c), A. Bosak (d), D.G. Merkel (d), O. Waller (a,b), A. Seiler (a,b), A.I. Chumakov (d), R. Rüffer (d), A.M. Oleś (e,f), K. Parlinski (c), M. Krisch (d), T. Baumbach (a,b,g), and S. Stankov (a,b), Phys. Rev. Lett. 116, 185501 (2016); doi: 10.1103/PhysRevLett.116.185501.
(a) Laboratory for Applications of Synchrotron Radiation, Karlsruhe Institute of Technology,  Karlsruhe (Germany)
(b) Institute for Photon Science and Synchrotron Radiation, Karlsruhe Institute of Technology, Eggenstein-Leopoldshafen (Germany)
(c) Institute of Nuclear Physics, Polish Academy of Sciences, Kraków (Poland)
(d) ESRF
(e) Max-Planck-Institut für Festkörperforschung, Stuttgart (Germany)
(f) Marian Smoluchowski Institute of Physics, Jagiellonian University, Kraków (Poland)
(g) ANKA, Karlsruhe Institute of Technology, Eggenstein-Leopoldshafen (Germany)

 

Top image: Mounting the EuO sample in the cryostat at beamline ID28. Credit: S. Stankov, KIT.