Modulating Magnetic Properties by Tailoring In-Plane Domain

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Modulating Magnetic Properties by Tailoring In-Plane Domain Structures in Hexagonal YMnO3 Films Shiqing Deng,†,§ Shaobo Cheng,†,§,⊥ Ming Liu,‡ and Jing Zhu*,†,⊥ †

National Center for Electron Microscopy in Beijing, School of Materials Science and Engineering, The State Key laboratory of New Ceramics and Fine Processing, Key Laboratory of Advanced Materials (MOE), Tsinghua University, Beijing 100084, P.R. China ⊥ Center for Nano and Micro Mechanics, Tsinghua University, Beijing 100084, P. R. China ‡ Electronic Materials Research Laboratory, Key Laboratory of the Ministry of Education and International Center for Dielectric Research, Xi’an Jiaotong University, Xi’an 710049, P. R. China

ABSTRACT: Periodic structures and the coupling of multiorder parameters in complex oxides heterojunctions can generate exotic properties, of interest both for fundamental researches and for device applications. Here, we report a self-assembling inplane periodic domain structure, and the resulting rich magnetic states, in a h-YMnO3 thin film fabricated on c-face sapphire substrate. Detailed structural investigations at atomic-level reveal the fashion of alternating domains under tensile or compressive strains separated by a boundary region. Tuned by this in-plane domain structure, the abnormal magnetic properties, such as the ferromagnetic enhancement and the unexpected spin glass state (below ∼38 K), are realized. Moreover, the existence of ferroelectric polarization is confirmed by scanning transmission electron microscopy, which brings in the chances of magnetoelectric coupling effect. These results manifest the close connections between the magnetic properties and such in-plane microstructures, suggesting the possibility of tuning the coupling effects via strain engineering in the hexagonal manganite film. KEYWORDS: h-YMnO3, thin film, strain engineering, domain structure, multiferroic bipyramids (TC ∼ 900 K), which purely depends on ionic size effect, in contrast to the chemical mechanisms for conventional ferroelectric.9,10 This trimerization of MnO5 induces corresponding displacement of the Y ions along the c axis, accompanied by the formation of topological vortexlike domain patterns.10−12 The two-dimensional corner-sharing triangular network of Mn ions contributes to the in-plane frustrated antiferromagnetic ordering below Néel temperature (TN ∼ 70 K).13 Previous works have demonstrated that the magnetic configurations are sensitive to the positions of Mn ions in hYMnO3, thus vary with the different sites of oxygen vacancies,

1. INTRODUCTION Multiferroic materials have become an attractive class of strongly correlated systems with appealing potential for applications in devices.1−3 However, candidate single-phase materials with coexistence of both ferromagnetic and ferroelectric orderings are rare.4−6 Among the known multiferroic systems, BiFeO3 and REMnO3 (RE: rare earth) have been the focus of research efforts due to their various exotic properties.6−8 REMnO3 manganites display hexagonal symmetry when RE ions are relatively small (RE = Y, Sc and Ho− Lu).8,9 The hexagonal unit cell (space group P63cm) of multiferroic h-YMnO3 adopts a layer by layer structure along the c-direction (Figure 1a), in which the yttrium layers are separated by MnO5 bipyramids.9,10 The geometric ferroelectric transition is driven by the tilting and distortion of MnO5 © XXXX American Chemical Society

Received: July 1, 2016 Accepted: September 9, 2016

A

DOI: 10.1021/acsami.6b08024 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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ACS Applied Materials & Interfaces

Figure 1. Atomic models of h-YMnO3, c-Al2O3 and two possible growth modes for the h-YMnO3 film/c-face sapphire substrate system. (a) Hexagonal crystal unit cell of ferroelectric h-YMnO3. (b) Atomic model of the c-Al2O3 substrate viewed along the [110] direction. (c) hYMnO3[100]//c-Al2O3[100] growth mode. (d) h-YMnO3[110̅ ]//c-Al2O3[100] growth mode. In (c) and (d), the h-YMnO3 and Al2O3 unit cells are outlined by blue and black frames, respectively.

found to be distinct from those of single crystal. By using aberration-corrected HAADF (high-angle annular dark field) Zcontrast STEM (scanning transmission electron microscopy) technique, detailed cross-sectional investigations at the atomic level have been conducted, deepening our understanding of the abnormal magnetic properties. Our results suggest the possibility of tailoring the magnetic properties by modulating the in-plane domain structures in the hexagonal manganite systems.

say out-of-plane oxygen vacancies or in-plane vacancies in MnO5 bipyramids.13,14 Γ2 or Γ4 magnetic configurations are stable when out-of-plane oxygen vacancies are stabilized by the compressive in-plane strain, and net magnetic moments will be induced. Whereas, Γ1 or Γ3 magnetic configurations are preferred with the in-plane oxygen vacancies existing.14 Thus, this enlighten us to tune the magnetic properties of h-YMnO3 thin films by using interface strain engineering. For heterojunction films, it is well recognized that multiferroic properties are closely related to the domain configurations, which are invariably modulated by the strain conditions within the film.4,15−17 A large effort has been made, both experimentally and theoretically, to investigate such strain effects on ferroelectricity or (anti)ferromagnetism in multiferroic thin films, highlighting both the opportunities and obstacles in fundamental research and functional applications.15,17−19 Recently, much attention has been focused on strain-induced periodic structures, which invariably contributes to exotic properties.20,21 Examples of such structures include the periodic array of flux-closure quadrants in strained ferroelectric PbTiO3 films,20 and the nanoscale vortex− antivortex pairs in (SrTiO3)n/(PbTiO3)n superlattices.21 Works to verify the nature of the giant magneto-elastic coupling in bulk hexagonal REMnO3, have been reported.7 While, the origins of how strain affects domain configurations and the inherent geometrically frustrated antiferromagnetic ordering in thin films remain ambiguous, which is an important issue in the further development of multiferroic devices.4 In this work, we demonstrate that by finely controlling the growth oxygen partial pressure, a self-assembling in-plane periodic structure can be achieved in h-YMnO3 films on c-face sapphire substrate, in which tensile and compressive domains are distributed alternately. Modulated by such an in-plane periodic domain structure, the correspondingly tailored magnetic states, such as ferromagnetic enhancement and spin glass state, are

2. RESULTS AND DISCUSSION High quality h-YMnO3 films were fabricated on c-face sapphire substrate using pulsed laser deposition (PLD) method. The target was ablated using a KrF excimer laser with a wavelength of 248 nm, at a laser fluence of 1.5 J/cm2 and a repetition rate of 5 Hz. The substrate was maintained at the optimized temperature of 900 °C during deposition. The films were grown to a thickness of ∼30 nm under an oxygen partial pressure of 50 mTorr. The crystalline quality was assessed by high resolution X-ray diffraction (XRD) using a Rigaku Smartlab X-ray diffractometer. Electron diffraction patterns were acquired by a FEI Tecnai G2 20 transmission electron microscope (TEM) operated at 200 kV. HAADF-STEM images were acquired using a double aberration-corrected JEOLARM200CF microscope equipped with a cold-field emission gun operated at 200 kV. The magnetic properties were measured using a Quantum Design MPMS VSM superconducting quantum interference device (SQUID). Figure 1b shows an atomic model of the Al2O3 (sapphire) from [110] direction. c-face sapphire is often chosen as a substrate for hexagonal films.22,23 For our experimental results, there are two possible growth modes for h-YMnO3 films on cface sapphire, namely, (1) the in-plane principal axis directions parallel to each other (Figure 1c) and (2) the film is accommodated by a 30° in-plane rotation (Figure 1d). The B


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Figure 2. XRD characterization of h-YMnO3 film grown on c-face sapphire substrate. (a) θ−2θ scans results of film system (upper channel) and individual substrate (lower channel). c-Al2O3 peaks are marked with asterisks. (b) Φ-scans of the (112) peak for h-YMnO3 (red) and (113) peak for c-Al2O3 (green line). Additional peaks with a 30° offset are each marked with a red rhombus.

Figure 3. SAED patterns and dark field images of the h-YMnO3 film. (a) Experimental SAED pattern for an area with two domain variants in the hYMnO3 film, showing two sets of diffraction patterns. The [100] and [11̅0] zone axis patterns are indicated by white inverted triangles and frames, respectively. (b) Simulated diffraction pattern, which is consistent with the experimental results in (a). (c, d) Dark field images using (030) and (112) diffraction spot, marked with numbers 1 and 2 in (a), respectively.

epitaxial relationship. However, additional peaks with a 30° offset to the c-Al2O3 peaks are also seen (each marked by a rhombus in Figure 2b), which imply the existence of two domain variants.26,27 Thus, it can be deduced that our film was textured with the coexistence of two domain variants, namely hYMnO3[100]//c-Al2O3[100] domain and h-YMnO3[11̅0]//cAl2O3[100] domain. For this system, where large distortions can be expected, microstructural analyses are essential for understanding the strain relaxation behaviors. A selected area electron diffraction (SAED) pattern of h-YMnO3 film, taken with the substrate at the exact [100] zone axis, is shown in Figure 3a. The duplex pattern can be separated into two sets of diffraction spots. One is [100] zone axis pattern (marked by white inverted triangles); the other is [11̅0] zone axis pattern (framed by white rectangles). The SAED observations confirm the presence of two domains, which are in accordance with Φ-scanning results. We selected (030) and (112) diffraction spots for acquiring dark-field images, depicted in Figure 3c and d, respectively. The periodically distributed bright (dark) areas in Figure 3c and dark (bright) areas in Figure 3d are the domains with a [100] ([110̅ ]) zone axis. The alternating patterns and the inverted contrast of Figure 3c and d testify the presence of two domain variants, showing a novel in-plane periodic structure. Further detailed study of the atomic structures across the interface between two domain variants (highlighted by the red rectangle in Figure 3c) was carried out by double aberrationcorrected HAADF-STEM (Figure 4a). Domains with [100]

strain conditions for these two cases are different. For the former case (h-YMnO3[100]//c-Al2O3[100]), the substrate provides a 22.5% in-plane compressive strain by estimation, while for the h-YMnO3 [11̅0]//c-Al2O3[100] mode the film is under a tensile strain of 19.8%. For h-YMnO3 single crystal, the in-plane Mn−O bond (Mn−OP) length is longer than that of out-of-plane Mn−O bond (Mn−OT) in MnO5 bipyramids.24 According to the Harrison’s rule for p−d hybridization (|Vpd|2 ∝ d−7, the hybridization strength is inversely proportional to the seventh power of the bond length), the longer bond length corresponds to a weaker hybridization strength.25 Thus, the inplane oxygen atoms in MnO5 bipyramids are easier to lose in a tensile strain state due to the longer Mn−OP bond length. While out-of-plane oxygen vacancies can be energetically favored, when the Mn−OP bond length is shrunk by a compressive strain. Additionally, the magnetic configurations are closely correlated with the sites of oxygen vacancies.14 Hence, it is highly likely to tune magnetic properties via strain engineering. Figure 2 summarizes the XRD characterization of the sample. Only (00n) reflections of the h-YMnO3 film and the c-Al2O3 substrate (marked by asterisks) can be detected, showing that the film is highly pure phase, with the out-of-plane directions parallel to the c axis of the substrate. We further selected the hYMnO3 (112) peak and the c-Al2O3 (113) peak for Φ-scanning experiments, which show 6-fold symmetry for the c-Al2O3 substrate (Figure 2b). The main peaks of the h-YMnO3 film overlap with those of the substrate, reaffirming a hexagonal C


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Figure 4. Detailed investigations of the periodic structure in the h-YMnO3 film. (a) HAADF-STEM image after Wiener filtering showing domains with [11̅0] zone axis (tensile) and [100] zone axis (compressive) separated by a boundary region (outlined by white dashed lines). (b) Mapping results showing the variation of in-plane lattice parameter a compared to the bulk value for the selected area indicated in (a). (c−e) Fast Fourier transform of different areas in (a); (c) tensile region, (d) boundary region (a square area outlined by red dot lines in (a)), and (e) compressive region.

Figure 5. Magnetic characterization of the h-YMnO3 film. (a) Field-cooling (red line with solid circles) and zero-field-cooling (green line with solid circles) curves. The inset shows the first-order derivative of the M−T curve, determining the Curie temperature as 46 K. (b) M−H loops measured at various temperatures (T = 20, 38, 45, and 77 K) after cooling from room temperature in the field of 2 kOe. The inset enlarges the region near the origin of the loop. (c) M−H loops measured at 10 K after cooling from room temperature in the external fields of +20 kOe (red line with solid spheres) and −20 kOe (black line with solid squares), respectively. The vertical shift of the loop can be seen in the regions indicated by two dashed circles. HEB is plotted as a function of temperature in the inset. The solid blue lines are drawn through the data points for guiding the eye. The error bar is the standard deviation.

and [11̅0] zone axis are separated by a boundary region. The claim that the two domains are under different strain conditions has been tested via a quantitative lattice parameter analysis, based on the Gaussian peak-seek method.28 A subset of the area in Figure 4a, indicated by four red markers, was selected for the analysis. The results (Figure 4b) show the variation of the lattice parameter a compared to the bulk value. It is found that on average the in-plane lattice parameter shrinks 8% for the hYMnO3[100]//c-Al2O3[100] domain and stretches 5.6% for the h-YMnO3[110̅ ]//c-Al2O3[100] domain. The tensile and compressive states are distributed in an alternating fashion for this in-plane periodic structure. This kind of scenario has been theoretically reported to be energetically favorable and conducive to strain relaxation behavior.29 Additionally, compared with bulk value (c = 11.37 Å), the out-of-plane lattice parameters are found to be slightly shrunk for both [100] (c = 10.81 Å) and [11̅0] zone axis domain (c = 11.16 Å). The underlying mechanism for this out-of-plane shrinkage is unclear for us at this stage. Hiratani et al. observed a

considerable decrease in the lattice parameter of SrRuO3 films grown on SrTiO3 substrates as a result of a decrease in oxygen pressure.30 Thus, the formation of oxygen vacancies may be one of the possibilities. The boundary region, between two domain variants with distinct strain states, may relate to this strain relaxation behavior, acting as displacement “absorber” and accommodating the transition from one side to another.29 Though large distortion within the boundary region lays obstacles in the detailed study at this stage, it deserves further investigations due to its special structure and properties. Additionally, the up−down−down configuration of yttrium ions indicates the maintenance of the downward ferroelectric polarization in the strained film.31 By employing a fast Fourier transform (FFT), we obtained the patterns shown in Figure 4c−e, which agree with the SAED observation (Figure 3a). The XRD and (S)TEM investigations confirm the coexistence of two domain variants with opposite strain states periodically distributed in-plane in films grown under D


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Although the features discussed above point to the presence of a spin glass, the formation of frozen state should be further verified. For this we measured M−T curves under various magnetic fields of 1, 2, 5, and 10 kOe (Figure 6). The results

conditions of low oxygen partial pressure. It is well-known that the oxygen vacancy content can affect the electrical and crystal structure in h-YMnO3.32,33 Also, manipulation of the oxygen vacancies can strongly influence the magnetic structure in hYMnO 3 films. For example, the Γ 2 or Γ 4 magnetic configurations, which can induce net magnetic moments, are preferred when an in-plane compressive strain is preserved and the out-of-plane oxygen atoms are easily removed.14 A net magnetic moment can be realized therefore for compressed films. Considering this sensitivity of the magnetic structure to strain state, we expect a diversity of magnetic states in this inplane periodic domain structure film due to its alternating strain states. The magnetic properties of the h-YMnO3 film on c-face sapphire were investigated using a SQUID. For the zero-fieldcooling (ZFC) curve, the sample was cooled down to 5 K without external magnetic field. For the field-cooling (FC) measurement, 2 kOe external magnetic field parallel to the film surface was applied during the cooling-down process. Both the FC and ZFC curves were measured under the magnetic field of 500 Oe in the warming-up cycle, shown in Figure 5a (after subtraction of diamagnetic background). The bifurcation seen in the FC and ZFC curves below the irreversible transition temperature (Tirr), and the peak in the ZFC curve at the lower temperature (TF) provide hints of the formation of a glassy state.34,35 A clear sudden ascent of M−T curve around 50 K corresponds to the magnetic transition. The Curie temperature (TC), which was derived from the first-order derivative of the M−T curve,36 was determined as 46 K (Figure 5a inset). Magnetization loops were also measured at temperatures of 20, 38, 45, and 77 K after field cooling from room temperature (Figure 5b). The shift of the M-H loops along the magnetic field axis can be distinguished in the inset of Figure 5b, where the region near the origin of the loop is blown up to show the extent of exchange bias (EB) clearly. The temperature dependent exchange bias fields (HEB), extracted from the hysteresis loops measured at different temperatures, were plotted in the inset in Figure 5c. The HEB are found to decrease with increasing temperature. Meanwhile, we have measured the sample by cooling from room temperature to 10 K in positive (+20 kOe) and negative magnetic fields (−20 kOe) to further confirm the presence of exchange bias. As shown in Figure 5c, the two hysteresis loops are biased in negative (for +20 kOe cooling field) and positive (for −20 kOe cooling field) directions along the magnetic field axis, respectively. Besides, vertical magnetization shift in the hysteresis loop is also observed, which can be seen in the regions indicated by two dashed circles (Figure 5c). This vertical shift was reported to be related to the uncompensated spins.37,38 Typically, the shift of the hysteresis loops are expected due to the exchange coupling at the ferromagnetic and antiferromagnetic interfaces.35 The frustration states also have relationship with the competition interactions between FM and AFM states.34 Furthermore, the relatively small values of the residual magnetization (Mr) and coercive field (HC) above the freezing temperature, together with the sudden increase of these values with decreasing temperature below TF are in accordance with the previously reported spin glass features.39 Thus, the prominent features of M−T curves and M−H loops, along with the exchange bias effect provide evidence to support the coexistence of the ferromagnetic and antiferromagnetic orderings, with the formation of an associated frustrated state.

Figure 6. M−T curves measured under various magnetic field strengths (H = 1, 2, 5, and 10 kOe). Curves with solid and dash symbols show the FC and ZFC data, respectively. The corresponding plot of irreversible temperature versus H2/3, showing a linear fit to the AT line, is given in the inset. The error bar is the standard deviation.

reveal that both the position and the shape of the peaks in the ZFC curves vary with the external magnetic field in the range from 1 kOe to 10 kOe. In addition, the irreversible temperature (Tirr) decreases monotonously with increasing strength of the applied magnetic field. Such behavior suggests that the spincompeting magnetic interactions in the frozen state are suppressed by a strong external field.35 The case can be different when boundary regions and the corresponding spin glass state are absent in our previous work, resulting in the changeless peak position under different external magnetic fields.14 A detailed analysis of the irreversible temperature (Tirr) shows a clear linear dependence on the two-thirds power of the measurement field (H2/3) (Figure 6). This relationship follows the Almeida−Thouless (AT) line, H(Tirr)/ΔJ ∝ (1 − Tirr/ TF)3/2,34 where ΔJ is the distribution width of the exchange interaction. The fit of the data to the AT line verifies the existence of a spin glass state in the examined film. Our systematic investigation of these films reveals complex magnetic states with clear and obvious differences to single crystal h-YMnO3. These magnetic characteristics all originate from novel in-plane periodicity of the film. Previous study proved that the Γ2 or Γ4 magnetic configuration, where net magnetic moments exist, can be stabilized by the formation of out-of-plane oxygen vacancies.14 According to the Harrison’s rule for p−d hybridization,25 when the Mn−OP bond length is shrunk by a compressive strain, out-of-plane oxygen vacancies can be energetically favored. Thus, the FM enhancement in our film can be reasonably deduced to be related to the compressive strain domains. While, the tensile strain domains remain antiferromagnetic. The exchange coupling therefore induces the EB effect, which can be characterized by the shift of hysteresis loops (Figure 5b and c). Particularly, the unexpected spin glass state, arisen from the competition between the AFM superexchange and the FM double-exchange interactions,34,35 may have relationship with the existence of the boundary region. Thus, the complex magnetic states in our film can therefore be well understood based on its unconventional inE


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(5) Akbashev, A. R.; Kaul, A. R. Structural and Chemical Aspects of the Design of Multiferroic Materials. Russ. Chem. Rev. 2011, 80, 1159− 1177. (6) Wang, J.; Neaton, J. B.; Zheng, H.; Nagarajan, V.; Ogale, S. B.; Liu, B.; Viehland, D.; Vaithyanathan, V.; Schlom, D. G.; Waghmare, U. V.; Spaldin, N. A.; Rabe, K. M.; Wuttig, M.; Ramesh, R. Epitaxial BiFeO3 Multiferroic Thin Film Heterostructures. Science 2003, 299, 1719−1722. (7) Lee, S.; Pirogov, A.; Kang, M.; Jang, K.-H.; Yonemura, M.; Kamiyama, T.; Cheong, S.-W.; Gozzo, F.; Shin, N.; Kimura, H.; Noda, Y.; Park, J.-G. Giant Magneto-Elastic Coupling in Multiferroic Hexagonal Manganites. Nature 2008, 451, 805−808. (8) Choi, T.; Horibe, Y.; Yi, H. T.; Choi, Y. J.; Wu, W. D.; Cheong, S.-W. Insulating Interlocked Ferroelectric and Structural Antiphase Domain Walls in Multiferroic YMnO3. Nat. Mater. 2010, 9, 253−258. (9) Gibbs, A. S.; Knight, K. S.; Lightfoot, P. High-temperature Phase Transitions of Hexagonal YMnO3. Phys. Rev. B: Condens. Matter Mater. Phys. 2011, 83, 094111. (10) Van Aken, B. B.; Palstra, T. T. M.; Filippetti, A.; Spaldin, N. A. The Origin of Ferroelectricity in Magnetoelectric YMnO3. Nat. Mater. 2004, 3, 164−170. (11) Cheng, S. B.; Zhao, Y. G.; Sun, X. F.; Zhu, J. Polarization Structures of Topological Domains in Multiferroic Hexagonal Manganites. J. Am. Ceram. Soc. 2014, 97, 3371−3373. (12) Yu, Y.; Zhang, X. Z.; Zhao, Y. G.; Jiang, N.; Yu, R.; Wang, J. W.; Fan, C.; Sun, X. F.; Zhu, J. Atomic-Scale Study of Topological VortexLike Domain Pattern in Multiferroic Hexagonal Manganites. Appl. Phys. Lett. 2013, 103, 032901. (13) Fabrèges, X.; Petit, S.; Mirebeau, I.; Pailhès, S.; Pinsard, L.; Forget, A.; Fernandez-Diaz, M. T.; Porcher, F. Spin-Lattice Coupling, Frustration, and Magnetic Order in Multiferroic RMnO3. Phys. Rev. Lett. 2009, 103, 067204. (14) Cheng, S. B.; Li, M. L.; Deng, S. Q.; Bao, S. Y.; Tang, P. Z.; Duan, W. H.; Ma, J.; Nan, C. W.; Zhu, J. Manipulation of Magnetic Properties by Oxygen Vacancies in Multiferroic YMnO3. Adv. Funct. Mater. 2016, 26, 3589−3598. (15) Nelson, C. T.; Winchester, B.; Zhang, Y.; Kim, S.-J.; Melville, A.; Adamo, C.; Folkman, C. M.; Baek, S.-H.; Eom, C.-B.; Schlom, D. G.; Chen, L.-Q.; Pan, X. Q. Spontaneous Vortex Nanodomain Arrays at Ferroelectric Heterointerfaces. Nano Lett. 2011, 11, 828−834. (16) Dawber, M.; Rabe, K. M.; Scott, J. F. Physics of Thin-film Ferroelectric Oxides. Rev. Mod. Phys. 2005, 77, 1083−1130. (17) Lu, C. L.; Hu, W. J.; Tian, Y. F.; Wu, T. Multiferroic Oxide Thin Films and Heterostructures. Appl. Phys. Rev. 2015, 2, 021304. (18) Ederer, C.; Spaldin, N. A. Effect of Epitaxial Strain on the Spontaneous Polarization of Thin Film Ferroelectrics. Phys. Rev. Lett. 2005, 95, 257601. (19) Choi, K. J.; Biegalski, M.; Li, Y. L.; Sharan, A.; Schubert, J.; Uecker, R.; Reiche, P.; Chen, Y. B.; Pan, X. Q.; Gopalan, V.; Chen, L.Q.; Schlom, D. G.; Eom, C. B. Enhancement of Ferroelectricity in Strained BaTiO3 Thin Films. Science 2004, 306, 1005−1009. (20) Tang, Y. L.; Zhu, Y. L.; Ma, X. L.; Borisevich, A. Y.; Morozovska, A. N.; Eliseev, E. A.; Wang, W. Y.; Wang, Y. J.; Xu, Y. B.; Zhang, Z. D.; Pennycook, S. J. Observation of a Periodic Array of Flux-closure Quadrants in Strained Ferroelectric PbTiO3 films. Science 2015, 348, 547−551. (21) Yadav, A. K.; Nelson, C. T.; Hsu, S. L.; Hong, Z.; Clarkson, J. D.; Schlepüetz, C. M.; Damodaran, A. R.; Shafer, P.; Arenholz, E.; Dedon, L. R.; Chen, D.; Vishwanath, A.; Minor, A. M.; Chen, L.-Q.; Scott, J. F.; Martin, L. W.; Ramesh, R. Observation of Polar Vortices in Oxide Superlattices. Nature 2016, 530, 198−201. (22) Lee, J. H.; Murugavel, P.; Ryu, H.; Lee, D.; Jo, J. Y.; Kim, J. W.; Kim, H. J.; Kim, K. H.; Jo, Y. H.; Jung, M.-H.; Oh, Y. H.; Kim, Y.-W.; Yoon, J.-G.; Chung, J.-S.; Noh, T. W. Epitaxial Stabilization of a New Multiferroic Hexagonal Phase of TbMnO3 Thin Films. Adv. Mater. 2006, 18, 3125−3129. (23) Murugavel, P.; Lee, J.-H.; Lee, D.; Noh, T. W.; Jo, Y. H.; Jung, M.-H.; Oh, Y. S.; Kim, K. H. Physical Properties of Multiferroic Hexagonal HoMnO3 Thin Films. Appl. Phys. Lett. 2007, 90, 142902.

plane periodic domain structure. Additionally, the local strain conditions and thermal vibration vary depending on the oxygen partial pressure,40 which may account for the formation of the domain structure observed in our film. Our results therefore imply the possibility of modulating the overall magnetic properties in h-YMnO 3 film by tailoring the domain configurations.

3. CONCLUSIONS In brief, we have achieved the formation of a self-assembling inplane periodic structure in h-YMnO3 films on c-face sapphire substrate using pulsed laser deposition method. The domains in this in-plane periodic structure are under alternating compressive and tensile strains, separated by boundary regions, which influences the strain-accommodated properties. The interface strain and finely controlled oxygen partial pressure during deposition may contribute to the formation of such inplane domain structure. The magnetic states are found to be distinct from those of single crystal h-YMnO3, which should have origin in this in-plane domain structure. Magnetic characterization reveals a magnetic transition at ∼46 K, and a frozen state below ∼38 K. The results imply the opportunities for the modulation of magnetic properties through tailored inplane domain structures and provide a strategy for the fabrication of delicate functional interface. The coexistence of ferroelectricity and complex magnetic properties in h-YMnO3 films demonstrates the possibility of achieving coupling of polarization order and spin order in hexagonal multiferroics, which sheds the promising light on potential applications in multiferroic devices.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Author Contributions §

S.D. and S.C. contributed equally to this work.

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was financially supported by the National 973 Project of China (2015CB654902), the Chinese National Natural Science Foundation (Project No.: 11374174, 51390471, and 51527803) and National key research and development program (2016YFB0700402). This work made use of the resources of the National Center for Electron Microscopy in Beijing and the Brookhaven National Laboratory in New York. We appreciate help from Prof. Yimei Zhu for the STEM characterization. We acknowledge helpful discussions with Prof. Guangming Zhang. Prof. Andrew Godfrey is acknowledged for valuable English modifications.

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REFERENCES

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