Magnetically driven ferroelectric atomic displacements in orthorhombic YMnO${}_{3}$

Magnetically driven ferroelectric atomic displacements in orthorhombic YMnO $_{3}$

D. Okuyama, S. Ishiwata, Y. Takahashi, K. Yamauchi, S. Picozzi, K. Sugimoto, H. Sakai, M. Takata, R. Shimano, Y. Taguchi, T. Arima, and Y. Tokura

Phys. Rev. B 84, 054440 – Published 11 August 2011

Abstract

Magnetically driven ferroelectric atomic displacements of the order of 10 $^{- 3}$ Å have been observed in orthorhombic (perovskite like) YMnO $_{3}$ by a single-crystal synchrotron x-ray diffraction. The refined polar structure shows the characteristic bond alternation driven by the exchange striction in staggered Mn-O-Mn arrays with $↑ ↑ ↓ ↓$ type ordering, giving rise to a spontaneous polarization along the $a$ axis. First-principles calculations based on the Berry phase method as well as on the experimentally refined crystal structure can reproduce the observed polarization value.

Received 23 May 2011

DOI:https://doi.org/10.1103/PhysRevB.84.054440

Authors & Affiliations

D. Okuyama¹, S. Ishiwata^1,2, Y. Takahashi³, K. Yamauchi^4,*, S. Picozzi⁴, K. Sugimoto⁵, H. Sakai¹, M. Takata⁶, R. Shimano^3,7, Y. Taguchi¹, T. Arima^6,8, and Y. Tokura^1,2,3

¹Cross-Correlated Materials Research Group (CMRG), and Correlated Electron Research Group (CERG), RIKEN-ASI, Wako 351-0198, Japan
²Department of Applied Physics and Quantum-Phase Electronics Center (QPEC), University of Tokyo, Hongo, Tokyo 113-8656, Japan
³Multiferroics Project, ERATO, Japan Science and Technology Agency (JST), c/o University of Tokyo, Hongo, Tokyo 113-8656, Japan
⁴Consiglio Nazionale del le Ricerche-Superconducting and Innovative materials and device (CNR-SPIN), I-67100, L’Aquila, Italy
⁵JASRI SPring-8, Hyogo 679-5198, Japan
⁶RIKEN SPring-8 Center, Hyogo 679-5148, Japan
⁷Department of Physics, University of Tokyo, Tokyo 113-8656, Japan
⁸Institute of Multidisciplinary Research for Advanced Materials, Tohoku University, Sendai 980-8577, Japan

^*Present address: ISIR-SANKEN, Osaka Univ., Japan.

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Vol. 84, Iss. 5 — 1 August 2011

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Images

Figure 1
The temperature dependence of (a) magnetic susceptibility, (b) polarization ( $P$ ), (c) electric permittivity ( $ɛ$ ), (d) spectral weight of the activated 77 meV oxygen stretching phonon mode [see also Fig. 2b], and (e) integrated intensity of synchrotron x-ray diffraction of a forbidden (0 11 2) reflection and satellites (0 11 $\pm δ$ 2). The inset to (b) shows the poling electric-field ( $E$ ) dependence of the $P$ at 2 K. All the measurements were performed in heating process. The temperature dependence of polarization shows a step-like behavior which would probably arise from the phase coexistence occurring near the first-order phase boundary with the incommensurate cycloidal phase.[23, 34, 38] The gray lines in (d) and (e) are the guide to the eyes.Reuse & Permissions
Figure 2
Infrared reflectivity spectra of the oxygen stretching phonon of (a) polycrystalline and single crystal samples for (b) $E^{ω} ∥ a$ and (c) $E^{ω} ∥ b$ both in the $E$ -type and in the sinusoidal phase. Phonon anomaly is observed only for $E^{ω} ∥ a$ configuration. A dip structure around 77 meV is attributed to an originally infrared inactive but lowered-lattice-symmetry-induced phonon mode. In the inset, the infrared data at 9 K and the fitting spectra with three Lorentz oscillators are shown.Reuse & Permissions
Figure 3
Synchrotron x-ray oscillation photographs in the (a) paramagnetic (50 K), (b) sinusoidal (30 K), and (c) $E$ -type phase (21 K). At 21 K ( $T < T_{N 2}$ ), a (0 11 2) reflection forbidden in the Pbnm symmetry is clearly observed, indicating the disappearance of the $b$ -glide operation. At 30 K ( $T_{N 2} < T < T_{N 1}$ ), (0 11 2) reflection splits along the $b^{*}$ axis, resulting in two incommensurate (0 11 $\pm δ$ 2) superlattice spots with $δ = 0.08$ . Comparison between observed ( $F_{obs}$ ) and calculated ( $F_{cal}$ ) structure factor (d) at 21 K in the ferroelectric phase and (e) at 50 K in the paraelectric phase.Reuse & Permissions
Figure 4
Schematics of the ferroelectric atomic displacements projected onto (a) the $a b$ plane and (b) the $a c$ plane in the ferroelectric $E$ -type phase. Dotted and solid arrows indicate the directions and the relative atomic displacements of Mn and O, respectively. Note that the displacements of O ions are doubled for the purpose of clarify. A unit cell is indicated by a broken line. (c) Changes in the Mn-O-Mn bond angles and the distance between the neighboring Mn ions in the $a b$ plane. In the ferroelectric phase (21 K), the $E$ -type spin configuration is represented by large arrows. Here we assumed the ferromagnetic (antiferromagnetic) spin arrangement for Mn-O-Mn bond with larger (smaller) bond angle.Reuse & Permissions
Figure 5
The absolute value of calculated ferroelectric polarization in the $E$ -type phase by using the Berry phase technique. Solid circles represent the results obtained from the GGA + $U$ method and the horizontal dashed line corresponds to the result from HSE calculations. The simulations were performed on the basis of the experimentally refined atomic coordinates of the ferroelectric-state crystal (at 21 K).Reuse & Permissions

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