Strain-driven structure-ferroelectricity relationship in hexagonal ${\mathrm{TbMnO}}_{3}$ films

Strain-driven structure-ferroelectricity relationship in hexagonal ${TbMnO}_{3}$ films

R. Mandal, M. Hirsbrunner, V. Roddatis, R. Gruhl, L. Schüler, U. Roß, S. Merten, P. Gegenwart, and V. Moshnyaga

Phys. Rev. B 102, 104106 – Published 18 September 2020

Abstract

Thin films and heterostructures of hexagonal manganites as promising multiferroic materials have attracted a considerable interest. We report structural transformations of high-quality strain-stabilized epitaxial hexagonal ${TbMnO}_{3} / yttria$ stabilized zirconia(111) (h-TMO) films, analyzed by means of various characterization techniques. A reversible structural phase transition from $P 6_{3} c m$ to $P 6_{3} / m m c$ structure at $T_{C} \sim 800 K$ was observed in stoichiometric h-TMO films by temperature-dependent Raman spectroscopy and optical ellipsometry. The latter, directly probing the electronic system, indicates its modification at the structural phase transition, likely due to charge transfer from oxygen to Mn. A partially reversible phase transformation and stress relaxation was observed in h-TMO films with Tb excess after temperature cycling (300-1000-300 K) during Raman and ellipsometry. An inhomogeneous microstructure, containing ferroelectric and paraelectric nanodomains, was revealed by transmission electron microscopy in the Tb-rich film after annealing. The results obtained indicate a strong influence of stress, induced by temperature and by constrained sample geometry, onto the structure and ferroelectricity of strain-stabilized h-TMO thin films.

Received 2 June 2020
Revised 19 August 2020
Accepted 2 September 2020

DOI:https://doi.org/10.1103/PhysRevB.102.104106

Physics Subject Headings (PhySH)

Ferroelectric domains Ferroelectricity Phase transitions Stress Structural phase transition

Condensed Matter, Materials & Applied Physics

Authors & Affiliations

R. Mandal ^1,2, M. Hirsbrunner ¹, V. Roddatis ³, R. Gruhl⁴, L. Schüler¹, U. Roß³, S. Merten¹, P. Gegenwart⁴, and V. Moshnyaga^1,*

¹Erstes Physikalisches Institut, Georg-August-Universität Göttingen, Friedrich-Hund-Platz 1, 37077 Göttingen, Germany
²Department of Physics, Indian Institute of Science Education and Research, Pune 411008, India
³Institut für Materialphysik, Georg-August-Universität Göttingen, Friedrich-Hund-Platz 1, 37077 Göttingen, Germany
⁴Experimentalphysik VI, Center for Electronic Correlations and Magnetism, Augsburg University, D-86159 Augsburg, Germany

^*vmosnea@gwdg.de

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Vol. 102, Iss. 10 — 1 September 2020

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Figure 1
Structural characterization of h-TMO/YSZ(111) films: (a) XRD (Θ-2Θ) pattern of a representative film shows the ( $000 l$ ) out-of-plane epitaxy. The inset demonstrates rocking curves of h-TMO(0002) and YSZ(111) peaks with almost the same FWHM∼0.032°; (b) Reciprocal-space mapping of a film (11-22) peak and c) a phis can of the (11-22) film peak evidences the in-plane epitaxy with hexagonal symmetry.
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Figure 2
(a) High resolution cross-section TEM image of a h-TMO/YSZ(111) film measured along the [11-20] crystallographic direction of the film with characteristic buckling (rumpling) of Tb atoms. The arrows show the direction of the ferroelectric polarization within one domain. Atomically smooth and flat film/substrate interface as well as the absence of Tb rumpling can be seen in the very first 4 u.c. of the film. (b) Zoomed part of (a) demonstrates the characteristic regions with the up- (left part) and down rumpling (right part) of Tb ions, separating FE domains with a typical size varying in the range 5–20 nm. (c) HR-STEM image of ${TbMnO}_{3}$ /YSZ interface showing the origin of FE domain at the atomic height step (d) HR-STEM image of the top part of the ${TbMnO}_{3}$ film. A complicated domain structure is shown by dotted lines marking FE domain walls and pointing up and down polarization vectors.
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Figure 3
(a) Unpolarized Raman spectra of a h-TMO/YSZ(111) sample, normalized to the spectrum of the substrate. Inset: room-temperature polarized Raman spectra of a TMO/YSZ(111) film in XX (red) and XY (blue) polarization geometries confirm the hexagonal structure. (b) Evaluated Raman spectra of the h-TMO film. (c) The area (left scale) and the height of the $A_{1}$ peak (right scale) by warming (closed symbols) and cooling (open symbols). Phase transition is indicated by peak vanishing at $T_{C} \sim 800 K$ . The inset in (c) shows the peak position as a function of temperature.
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Figure 4
Time- (a) and temperature (b) dependences of ellipsometric phase shift, $Δ$ , and polarization rotation, $Ψ$ , angles as well as of the calculated refractive index, $n$ . A distinct steplike decrease of $n$ by $6 %$ indicates the phase transition, the end point of which marks $T_{C} \sim 800 K$ . The inset in (b) shows the absorption spectrum of h-TMO/YSZ(111) film measured at room temperature.
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Figure 5
(a)–(d) Electron diffraction, collected at different temperatures upon heating of the TEM lamella up to 410 K. (e), (f) high-resolution STEM images. The initial hexagonal $(P 6_{3} c m)$ structure with Tb shift (e) was irreversibly transformed into a hexagonal $(P 6_{3} / m m c)$ structure with planar Tb planes (f). This irreversible phase transformation is caused by strain relaxation actuated by formation of a system of misfit dislocations as evidenced in (f).
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Figure 6
XRD patterns (a), (b), and original (c), (d) and filtered (e), (f) HRSTEM images of the two ${TbMnO}_{3}$ films, showing the reversible (a), (c), (e) and irreversible (b), (d), (f) behavior after temperature-dependent Raman measurements. Insets in (b) and (c) show two FFT patterns from the selected areas marked with the white line. The spots inside the selected areas in the FFTs (orange contours) were used for the filtering to reveal the distribution of PE and FE phases. The FE phase demonstrates sharper and brighter spots because of pronounced up and down shifts of Tb ions, while the PE phase demonstrates almost flat Tb planes. XRD patterns close to the (0002) peak of the h-TMO for film with reversible (a) thermal behavior shows almost no changes in the $c$ -lattice parameter whereas the film with irreversible (b) behavior demonstrates distinct changes related to the coexistence of FE and PE phases.
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Physical Review B

covering condensed matter and materials physics

Strain-driven structure-ferroelectricity relationship in hexagonal ${TbMnO}_{3}$ films

R. Mandal, M. Hirsbrunner, V. Roddatis, R. Gruhl, L. Schüler, U. Roß, S. Merten, P. Gegenwart, and V. Moshnyaga

Phys. Rev. B 102, 104106 – Published 18 September 2020

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Physical Review B

covering condensed matter and materials physics

Strain-driven structure-ferroelectricity relationship in hexagonal TbMnO3 films

R. Mandal, M. Hirsbrunner, V. Roddatis, R. Gruhl, L. Schüler, U. Roß, S. Merten, P. Gegenwart, and V. Moshnyaga

Phys. Rev. B 102, 104106 – Published 18 September 2020

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Strain-driven structure-ferroelectricity relationship in hexagonal ${TbMnO}_{3}$ films