Communication pubs.acs.org/cm
Epitaxially Constrained Hexagonal Ferroelectricity and Canted Triangular Spin Order in LuFeO3 Thin Films Young Kyu Jeong, Jung-Hoon Lee, Suk-Jin Ahn, and Hyun Myung Jang* Department of Materials Science and Engineering, and Division of Advanced Materials Science, Pohang University of Science and Technology (POSTECH), Pohang 790-784, Republic of Korea. S Supporting Information *
KEYWORDS: hexagonal ferroelectricity, LuFeO3, density-functional theory (DFT)
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In choosing a rare-earth (Re) cation for the purpose of artificially imposing hexagonal ferroelectricity using a ReFeO3type orthoferrite, we considered the fact that the stability of the hexagonal ReMnO3 phase enhances with decreasing radius of Re ion.17,18 Among 15 different lanthanide ions, Lu3+ is known to possess the smallest ionic radius. Considering this, we have fabricated an epitaxially constrained LuFeO3 (LFO) thin film by adopting a suitable hexagonal template. We will show that the heteroepitaxially grown LFO film (60-nm-thick) possesses a 6-fold hexagonal symmetry and surprisingly exhibits c-axisoriented normal ferroelectricity with the Curie temperature as high as 563 K. Theta-2theta (θ−2θ) X-ray diffraction pattern (XRD) indicates that the LFO film grown on a Pt(111)/sapphire (0001) substrate by pulsed laser deposition method is highly caxis oriented (Figure 1a). The heteroepitaxial growth of LFO
ultiferroics exhibit simultaneous ferroic properties with coupled electric, magnetic, and structural orders.1−6 Multiferroic materials have received a great deal of attention because of their potential for enabling new memory devices and information storage.3−5 Among numerous multiferroics currently under investigation, rare-earth manganites have been most extensively studied owing to their tendency toward a strong magnetoelectric coupling, in general. They are classified into two distinct crystal classes depending on the radius of rareearth cation. ReMnO3-type oxides with Re = La−Dy in the lanthanide series belong to orthorhombic manganites1,5,6 whereas ReMnO3-type oxides with Re = Ho−Lu belong to hexagonal manganites.7−9 Unlike perovskite-based orthorhombic manganites, hexagonal manganites possess c-axis-oriented ferroelectricity owing to the disappearance of a mirror image on the a-b plane in a polar P63cm unit cell.7−9 Hexagonal magnetites also exhibit an antiferromagnetic (AFM) order which is governed by the intralayer Mn−O−Mn superexchange interaction. Therefore, the magnetic spins are oriented parallel to the a−b plane with a so-called 120° triangular spin configuration, which is a typical spin order of frustrated magnetic sublattices.8−12 Contrary to the two distinct structural polymorphs in rareearth manganites, all of the ReFeO3-type oxides belong to orthorhombic ferrites (orthoferrites) and are characterized by the corner-linked FeO6 octahedra forming a three-dimensional network in a centrosymmetric Pbnm (or Pnma) perovskite crystal.13,14 Accordingly, all of the rare-earth orthoferrites are known to be nonferroelectric, except for the recently reported improper ferroelectricity in SmFeO3 with a tiny polarization (∼0.01 μC/cm2).15 Considering the c-axis-oriented ferroelectricity observed in hexagonal manganites,7,9,12 we have been exploring the possibility of artificially imposing ferroelectricity by structurally tailoring an AFM orthoferrite, ReFeO3, in a constrained thin−film form. Indeed, Bossak et al.16 reported the fabrication of hexagonal ReFeO3 (Re= Eu∼Lu) thin films on yttria-stabilized ZrO2 (YSZ) substrates. However, their study was limited to structural characterizations. Thus, it is of great importance (i) to clearly demonstrate the existence of the ferroelectric polarization in artificially imposed hexagonal ferrites and (ii) to identify the possible variation of spin configuration arising from the orthorhombic-to-hexagonal transformation. © 2012 American Chemical Society
Figure 1. (a) Theta-2theta (θ−2θ) X-ray diffraction (XRD) pattern of the preferential [0001]-oriented LuFeO3 (LFO) film prepared by pulsed laser deposition method. (b) The heteroepitaxial growth of LFO was confirmed by examining the in-plane XRD phi-scans. These patterns were obtained by keeping the Bragg angle at (112̅2) for LFO and (200) for Pt. (c) Unit-cell crystal structure of the hexagonal LFO with the polar P63cm state.
was confirmed by examining the in-plane XRD phi-scans (Figure 1b). These patterns were obtained by keeping the Bragg angle at (1122̅ ) for LFO and (200) for Pt. The six peaks that are 60° apart each other occur at the same azimuthal phiangles for both LFO and Pt, demonstrating a 6-fold hexagonal symmetry of the LFO layer. Optimized unit-cell crystal structure of the hexagonally constrained LFO (h-LFO) layer was obtained by carrying out Received: March 17, 2012 Revised: June 14, 2012 Published: June 14, 2012 2426
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transition in the h-LFO film is ∼563 K (Curie temperature, Tc). The peak temperature of the relative dielectric permittivity (Figure 2c) as well as that of the loss tangent coincide well with the onset of the para-to-ferroelectric transition (∼563 K), which is essentially independent of the measuring ac frequency. Thus, the onset of the ferroelectricity observed at ∼563 K corresponds to the transition to the ferroelectric P63cm state from the paraelectric P63/mmc state. In the case of hexagonal manganites, the bonding nature associated with the ferroelectricity origin is currently being debated.12 Thus, it is of scientific importance to elucidate the bonding nature of the hexagonal ferroelectricity. To do this for h-LFO, we compared the computed electron density contour of the paraelectric P63/mmc phase with that of the ferroelectric P63cm phase. As shown in Figure 3a, the electron density
first-principles density-functional theory (DFT) calculations using the Vienna ab initio simulation package (VASP).19 The optimized crystal structure of the h-LFO with the noncentrosymmetric P63cm symmetry (Figure 1c) is similar to that of the hexagonal YMnO3, the prototype of the h-RMnO3 family,7,9 in that the h-LFO structure is characterized by (i) the LuO8 unit having trigonal D3d site symmetry and (ii) the FeO5 bipyramid with D3h site symmetry. Oxygen ions surrounding the central Lu ion in the LuO8 unit fall into two distinct groups: two apical (axial) oxygen ions (OA) along the hexagonal c-axis and six oxygen ions (OI) located at two different triangular in planes.12 In this P63cm structure, an asymmetric vertical shift of rare-earth ion with respect to the two neighboring OA ions along the c-axis is known to be the ferroelectricity origin of the hexagonal manganites12 and the hexagonally strained ReFeO3type ferrites as well.20 As presented in Figure 2a, the heteroepitaxially grown h-LFO film (60-nm-thick) on a Pt(111)/Al2O3 (0001) substrate is
Figure 3. (a) Electron localization function (ELF) contour of the paraelectric P63/mmc phase is compared with that of the ferroelectric P63cm phase. (b) Comparison of the orbital-resolved partial density of states for 5dz2 (Lu), 6s (Lu), 2px,y (OA), and 2pz (OA) orbitals of the nonpolar P63/mmc phase with those of the polar P63cm phase.
between the Lutetium (Lu) ion and the axial oxygen (OA) is relatively negligible in the P63/mmc phase, which demonstrates a dominant ionic bonding character in the Lu−OA bond. Upon the transition to the polar P63cm state, however, there occurs a strong asymmetric covalent-bonding interaction between the Lu ion and one of the two OA ions along the c-axis (marked with red arrows in Figure 3a). Thus, one can conclude that the asymmetric Lu−OA covalent-bonding interaction along the caxis is primarily responsible for the development of the offcentering dipoles (polarization) in the ferroelectric P63cm state. To elucidate the orbital interaction responsible for the asymmetric Lu−OA bonding along the c-axis of P63cm, we have considered two distinct possibilities of the Lu−OA bonding interaction: (i) empty 5dz2 (Lu)-2pz (OA) interaction and (ii) empty 6s (Lu)-2pz (OA) interaction (see the Supporting Information). We then examined orbital-resolved partial density of states (PDOS) to identify the covalent bonding mechanism truly responsible for the manifestation of
Figure 2. (a) Polarization-field (P−E) hysteresis loops obtained at 300 K. The 60-nm-thick h-LFO film used in the measurement was epitaxially grown on a Pt(111)/ sapphire (0001) substrate. (b) Temperature-dependent spontaneous polarization curve of the epitaxially grown h-LFO thin film, showing the Curie temperature at ∼563 K. (c) Temperature-dependent relative dielectric permittivity curves of the h-LFO film measured at three different ac probing frequencies.
ferroelectric at room temperature. The c-axis component of the remanent polarization (Pr) obtained from the polarizationelectric field (P−E) hysteresis loop is ∼6.5 μC/cm2 with the coercive field (Ec) of ∼800 kV/cm at 300 K. The temperaturedependent spontaneous polarization was evaluated by measuring the pyroelectric current and subsequently integrating this as a function of temperature. This polarization curve (Figure 2b) clearly indicates that the onset of the para-to-ferroelectric 2427
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ferroelectricity in the h-LFO. As shown in Figure 3b, the degree of the overlapping between the Lu 6s orbital PDOS and the OA 2pz orbital PDOS is little changed upon the transition to the ferroelectric P63cm state. On the contrary, the degree of the overlapping between the Lu 5dz2 orbital PDOS with the OA 2pz orbital PDOS is remarkably enhanced upon the transition to the P63cm state (for the bonding-energy region between −1.4 and −0.8 eV below the valence-band top). Thus, the asymmetric Lu 5dz2−OA 2pz hybridization along the c-axis of P63cm is considered to be the electronic origin of the hexagonal ferroelectricity. The temperature-dependent magnetization of the h-LFO film is presented in Figure 4a for both field-cooling (FC) and
AUTHOR INFORMATION
Corresponding Author
*Fax: +82-54-279-2399. Tel: +82-54-279-2138. E-mail:
[email protected]. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This work was supported by the Brain Korea 21 project 2012 and by the World Class University program through the National Research Foundation of Korea (Grant R31-2008-00010059-0). Computational resources provided by KISTI Supercomputing Centre (Project KSC−2011−C3−16) of Korea are gratefully acknowledged.
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REFERENCES
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Figure 4. (a) Temperature-dependent magnetization curves of the 60nm-thick h-LFO film. (b) Schematic representation of the stable ground−state spin configuration in the h-LFO with the P63cm symmetry. The spins at the Fe3+ ion sublattice are marked with blue arrows, and Mnet denotes the net magnetic moment along the c-axis of P63cm.
zero-field-cooling (ZFC) conditions. For the magnetic measurements, we fabricated the h-LFO film on the hexagonal YSZ (112̅1) substrate. XRD data indicated that the h-LFO layer grown on the YSZ is also epitaxial with a 6-fold hexagonal symmetry. As presented in Figure 4a, both ZFC and FC curves show a rapid increase in the magnetization beginning at ∼120 K. Thus, the magnetic ordering temperature (TN) is considered to be 120 K. Our DFT calculations indicate that Fe3+ spins in the ground-state form a so-called 120° triangular spin structure on the a−b plane. This configuration is largely consistent with a typical spin structure for triangular AFM spins in frustrated magnetic systems such as hexagonal manganites with triangular sublattices.8−12 Thus, one would expect a nearly zero residual moment along the in-plane direction. According to the DFT calculations, however, Fe3+ spins are slightly canted with a nonzero moment of 0.0027 μB per formula unit ( f.u.) along the out-of-plane c-direction, which agrees qualitatively with the experimental ZFC value of 0.0022 μB per f.u. The blue arrows denote these canted Fe3+ spin moments in the P63 cm phase (Figure 4b). In summary, an epitaxially constrained LuFeO3 thin film fabricated by adopting a hexagonal template exhibits roomtemperature ferroelectricity with the remanent polarization of ∼6.5 μC/cm2 at 300 K. It has been shown that the asymmetric Lu 5dz2−OA 2pz hybridization along the c-axis of P63cm is the electronic origin of the observed hexagonal ferroelectricity.
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Communication
ASSOCIATED CONTENT
S Supporting Information *
Experimental and computational details, three distinct LuO8 units in the ferroelectric state, and orbital interaction diagrams. This material is available free of charge via the Internet at http://pubs.acs.org. 2428
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