Magneto- to Electroactive Transmutation of Spin Waves in ${\mathrm{ErMnO}}_{3}$

Magneto- to Electroactive Transmutation of Spin Waves in ${ErMnO}_{3}$

L. Chaix, S. de Brion, S. Petit, R. Ballou, L.-P. Regnault, J. Ollivier, J.-B. Brubach, P. Roy, J. Debray, P. Lejay, A. Cano, E. Ressouche, and V. Simonet

Phys. Rev. Lett. 112, 137201 – Published 2 April 2014

Abstract

The low-energy dynamical properties of the multiferroic hexagonal perovskite ${ErMnO}_{3}$ have been studied by inelastic neutron scattering as well as terahertz and far infrared spectroscopies on a synchrotron source. From these complementary techniques, we have determined the magnon and crystal field spectra and identified a zone center magnon excitable only by the electric field of an electromagnetic wave. Using a comparison with the isostructural ${YMnO}_{3}$ compound and crystal field calculations, we propose that this dynamical magnetoelectric process is due to the hybridization of a magnon with an electroactive crystal field transition.

Received 6 January 2014

DOI:https://doi.org/10.1103/PhysRevLett.112.137201

Authors & Affiliations

L. Chaix^1,2,3,*, S. de Brion^2,3,†, S. Petit⁴, R. Ballou^2,3, L.-P. Regnault⁵, J. Ollivier¹, J.-B. Brubach⁶, P. Roy⁶, J. Debray^2,3, P. Lejay^2,3, A. Cano^7,8, E. Ressouche⁵, and V. Simonet^2,3

¹Institut Laue-Langevin, 6 rue Jules Horowitz, 38042 Grenoble, France
²Université Grenoble Alpes, Institut Néel, 38042 Grenoble, France
³CNRS, Institut Néel, 38042 Grenoble, France
⁴CEA, Centre de Saclay, DSM/IRAMIS/Laboratoire Léon Brillouin, 91191 Gif-sur-Yvette, France
⁵SPSMS-MDN, UMR-E CEA/UJF-Grenoble, INAC, 38054 Grenoble, France
⁶Synchrotron SOLEIL, L’Orme des Merisiers Saint-Aubin, 91192 Gif-sur-Yvette, France
⁷European Synchrotron Radiation Facility, 6 rue Jules Horowitz, BP 220, 38043 Grenoble, France
⁸CNRS, Université Bordeaux, ICMCB, UPR 9048, F-33600 Pessac, France

^*chaixl@ill.fr
^†sophie.debrion@neel.cnrs.fr

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Vol. 112, Iss. 13 — 4 April 2014

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Figure 1
Schematic illustration of ${ErMnO}_{3}$ crystallographic and magnetic structure in the temperature range $T_{N}^{'} < T < T_{N}$ where Mn magnetic moments are ordered at 120° (a) and Er moments on the $4 b$ site only are polarized in an antiferromagnetic arrangement (b). This multiferroic phase presents an electric polarization $P$ along the $c$ axis. The exchange interactions involved in the Mn magnetic order are also shown.
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Figure 2
THz (a)–(c) and FIR (d)–(f) absorption spectra of ${ErMnO}_{3}$ for the three different orientations of the electromagnetic wave $e$ , $h$ fields with respect to the crystal $c$ axis, in the temperatures range 6–120 K. The gray areas indicate regions where the absorption saturates. The various excitation modes discussed in the text are pointed out. (g)–(i) THz absorption spectra of ${YMnO}_{3}$ measured in the same conditions as ${ErMnO}_{3}$ .
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Figure 3
(a) Left: IN5 inelastic neutron signal spectrum recorded at 1.5 K along (0 $k$ 0); the data were cut and integrated in a reciprocal space part, from $ℓ = - 2$ to $+ 2$ along the $c^{*}$ direction. The calculated Mn spin-wave dispersions (identical for $Γ_{2}$ or $Γ_{4}$ ) are reported (continuous lines). The corresponding Hamiltonian involves three exchange interactions: one in-plane first neighbor $J_{1} = 2.65 meV$ and two out-of-plane interactions $J_{z 1}$ and $J_{z 2}$ with $J_{z 1} - J_{z 2} = - 0.007 meV$ (see Fig. 1), as well as two anisotropy terms at $D = 0.39 meV$ and $h = 0.055 meV$ producing the energy gaps for the spin components in and perpendicular to the ( $a$ , $b$ ) plane, respectively. Right: Corresponding energy map obtained by powder average of all single crystal spectra; all scattering angles were grouped assuming nearly nondispersive CF excitations. (b) IN22 spin-flip and non-spin-flip neutron scattering signals recorded at 20 K at the (0 1 0) zone center and (0 1.5 0) zone boundary. The magnon modes $M_{i}$ and CF transitions $E_{i}$ observed in THz and FIR spectroscopy are also shown.
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Figure 4
Crystal field calculation from a point charge model for the two $4 b$ and $2 a$ ${Er}^{3 +}$ sites. The CF levels are labeled according to the irreducible representations of the double point groups associated with each Er site. The splitting produced by a magnetic field of 0.8 T mimics the effect of the Mn molecular field on $Er (4 b)$ CF levels, which occurs below $T_{N}$ . The CF levels scheme consists in nine doublets for each site. The modes above $200 {cm}^{- 1}$ are not shown. Note that the energy of the levels is very sensitive to the $O^{2 -}$ coordinates, which were obtained from single-crystal neutron diffraction at 10 K [19]. The observed THz and FIR transitions $E_{i}$ are shown (red arrows) as well as the possible excitation involved in the hybridization with a Mn magnon (blue arrow).
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Physical Review Letters

Magneto- to Electroactive Transmutation of Spin Waves in ErMnO3

L. Chaix, S. de Brion, S. Petit, R. Ballou, L.-P. Regnault, J. Ollivier, J.-B. Brubach, P. Roy, J. Debray, P. Lejay, A. Cano, E. Ressouche, and V. Simonet

Phys. Rev. Lett. 112, 137201 – Published 2 April 2014

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Magneto- to Electroactive Transmutation of Spin Waves in ${ErMnO}_{3}$