Domain structure and multiferroic properties of epitaxial hexagonal ErMnO 3 fi lms
Yi Chen
a, Ye Li
a, Dongfeng Zheng
a, Leiyu Li
a, Min Zeng
a,b,*, Minghui Qin
a, Zhipeng Hou
a, Zhen Fan
a, Xingsen Gao
a, Xubing Lu
a, Qiliang Li
b, Jun-Ming Liu
caInstitute for Advanced Materials and Guangdong Provincial Key Laboratory of Quantum Engineering and Quantum Materials, South China Academy of Advanced Optoelectronics, South China Normal University, Guangzhou, 510006, China
bDepartment of Electrical and Computer Engineering, George Mason University, Fairfax, VA, 22033, USA
cLaboratory of Solid State Microstructures and Innovation Center of Advanced Microstructures, Nanjing University, Nanjing, 21009, China
a r t i c l e i n f o
Article history:
Received 12 September 2019 Received in revised form 5 December 2019
Accepted 23 December 2019 Available online 24 December 2019
Keywords:
Multiferroicity Hexagonal ErMnO3film Ferroelectric domains Raman spectrum
a b s t r a c t
Epitaxial ErMnO3thinfilms were grown on Pt-coated Al2O3substrate by pulsed laser deposition. Their structure, multiferroicity, and ferroelectric domain properties have been comprehensively characterized and studied. The XRD measurement indicated an excellent epitaxy of out-of-plane ErMnO3(0001)//
Pt(111)//Al2O3(0001) and in-plane ErMnO3[1000]//Pt[112]//Al2O3[1120] structures. The as-deposited ErMnO3films exhibited spontaneous ferroelectric domains with reversible polarization. A significant remnant polarization of 1.3mC/cm2and an active peak at 662 cm1in Raman spectra were found, further showing a high quality of the ErMnO3thinfilms. Moreover, the magnetic measurements indicated that the thinfilm has an excellent anisotropic magnetic property with a Neel temperature atz53 K.
©2019 Elsevier B.V. All rights reserved.
1. Introduction
Hexagonal rare-earth manganites (h-RMnO3, R ¼ Sc, In, Y, DyeLu) have attracted great attention for both fascinating physical properties and possible applications in spintronic devices after their multiferroic properties were reported [1e4]. They have improper ferroelectricity, which is resulted from a secondary order parameter driven structural transition [5,6]. Below the Curie tem- perature Tc, the ferroelectricity originates from the rotation and tilting of MnO5bipyramids, in which 1/3 ofRions are shifted along the positive direction ofcaxis and 2/3 along the negative direction, resulting in a spontaneous polarizationPs(1e10mC/cm2). On the other hand, the antiferromagnetism is derived from magnetic Mn ions orRions, in which the Neel temperature (TN) related to Mn ion is in the range of 50 Ke130 K, and theTNrelated toRions is below 10 K if theRions have magnetic moments [7,8]. Most interestingly, the improper h-RMnO3 compounds possess six possible
ferroelectric domain states, forming a vortex-like structure owing to topological defects [9e11]. The topologically protected vortices are explicitly robust and exhibit various intriguing phenomena, such as vortex patterns related to microscopy [9,12e14], ferro- electric and magnetic characteristics [15e18], in particular, the conductance in the vortex domain walls [19]. Cheong et al., re- ported the origin and forming mechanism of the topologically protected vortex [1,20,21]. The evolution on vortex domain patterns has been extensively explored at different cooling rates and electric fields [22e24].
Among multiferroic h-RMnO3compounds, the h-ErMnO3, one of the most strongly correlated systems, exhibitsTCatz1400 K, and TN of z78 K depending upon Mn3þ and z2.5 K upon Er3þ [7,22,25,26]. Quite clearly, h-ErMnO3exhibits intriguing topologi- cally protected vortices and a strong magnetoelectric coupling caused by the interplay between Mn3þand Er3þspins. For exam- ples, Chae et al., reported directly observed proliferation of ferro- electric loop domains and vortex-antivortex pairs in h-ErMnO3
single crystal [12]. Geng et al., reported a direct visualization of the coupling between the magnetic and electric vortex domains in h- ErMnO3by a piezo-response and magnetic force microscopy [15]. In addition, a particular interest for h-ErMnO3is the high ferroelectric polarization (Ps¼5.5mC/cm2). As demonstrated by Ruff et al., the
*Corresponding author. Institute for Advanced Materials and Guangdong Pro- vincial Key Laboratory of Quantum Engineering and Quantum Materials, South China Academy of Advanced Optoelectronics, South China Normal University, Guangzhou, 510006, China.
E-mail address:[email protected](M. Zeng).
Contents lists available atScienceDirect
Journal of Alloys and Compounds
j o u rn a l h o m e p a g e :h t t p : / / w w w . e l s e v i e r . c o m / l o c a t e / j a l c o m
https://doi.org/10.1016/j.jallcom.2019.153529 0925-8388/©2019 Elsevier B.V. All rights reserved.
frequency dependence of the saturation polarization in h-ErMnO3
single crystals could be clearly observed in the domain-walls movement measurement [18].
In the reported h-RMnO3 systems, the studies of multiferroic behaviors and vortex domain structures were mainly focused on single crystals [27e31]. The investigations on h-RMnO3films were quite limited due to the difficulty in preparation of perfect struc- tures without any vacancies and grain boundaries. Up to date, the study of epitaxial stabilization technique was focused on fabri- cating artificial h-RMnO3films on some special substrates, such as the Pt-coated Al2O3or yttria-stabilized zirconia (YSZ) with hexag- onal nets on the surface. Among various h-RMnO3films, the most intensively studied compound was YMnO3[32,33]. The reported h- ErMnO3 epitaxial films were mainly focused on their magnetic behaviors [34,35], while the ferroelectric polarization of h-ErMnO3
epitaxialfilms is still quite limited. Especially the study on both domain structure and polarization switching has rarely been re- ported. However, study of domain and domain wall is crucial for the control of many material properties, such as coercivity, resistance, and/or fatigue [24,36]. In this work, we reported the structural characterization of epitaxial h-ErMnO3 films deposited on Pt- coated Al2O3(0001) substrate. The ferroelectric polarization of the h-ErMnO3 films was comprehensively studied by using Raman spectrum, piezoresponse force microscopy (PFM) and transmission electron microscopy (TEM). The h-ErMnO3films clearly exhibited ferroelectric domains, and a remnant polarization of 1.3 mC/cm2 obtained at 150 K. The magnetic properties were characterized with aTNof 53 K. This result represented an important step towards the fabrication and characterization of epitaxial hexagonal manganitefilms.
2. Experimental section
The hexagonal ErMnO3 (h-ErMnO3)films were grown on the platinum (Pt) coated Al2O3(0001) substrates by pulsed laser deposition (PLD) with a KrF excimer laser of 248 nm wavelength).
First, a Pt thin layer was deposited on Al2O3(0001) substrate at a temperature of 450C and a high vacuum of 5104Pa. Next, the ErMnO3 films were grown on the Pt-coated Al2O3 substrates at 850C in an oxygen pressure of about 1.0 Pa. Finally, the ErMnO3/ Pt/Al2O3(0001) heterostructures were in-situ annealed at 600C with an ambient oxygen of 100 Pa for 30 min, and then cooled to room temperature at a rate of 3C/min. The crystal structure and epitaxy of the ErMnO3/Pt/Al2O3(0001) heterostructures were characterized by XRD using the Cu Karadiation (XRD, PANalytical X’Pert PRO). The surface topography and the ferroelectric domain structure were probed by atomic force microscopy/piezoresponse force microscopy (AFM/PFM, Asylum Cypher). The Raman spectra were obtained by Raman microscope (Renishaw). The cross- sectional lattice images were achieved by high resolution trans- mission electron microscopy (HRTEM, JEOL-2011). The electric hysteresis loop of ErMnO3 films were measured by precision ferroelectric tester (Radiant Technologies). The magnetic properties were carried out using a vibrating sample magnetometer incor- porated into a physical properties measurement system (PPMS).
3. Results and discussion
Fig. 1(a) shows the X-ray diffraction (XRD) q-2q scan of the ErMnO3film deposited on Pt-coated Al2O3(0001) substrate. Except the Pt layer and Al2O3(0001) substrate peaks (Fig. S1, Supplemen- tary Material), all the other observed peaks are indexed to the (000l) planes of hexagonal ErMnO3 (h-ErMnO3)film, confirming that a phase-pure h-ErMnO3film with thec-axis normal to thefilm surface is obtained. The in-plane epitaxial orientation was probed
with 4-scans recorded around the ErMnO3(1122), Pt(113), and Al2O3(1123) reflections. As shown inFig. 1(b), the six-fold sym- metry characteristic is clear for the h-ErMnO3(0001), Pt(111)films, and Al2O3(0001) substrate, which confirms the ErMnO3[1000]//Pt [112]//Al2O3[1120] in-plane epitaxial orientation. Theaandclattice constants of the h-ErMnO3film evaluated from these XRD results are 6.08 Å and 11.56 Å, respectively. Compared to h-ErMnO3single crystals (a¼6.12 Å andc¼11.45 Å) [37], the lattice of our h-ErMnO3 films is shrunk along the in-plane direction, but elongated along the out-of-plane one. The reason can be attributed to the epitaxial strain induced by the Pt-coated Al2O3substrate.
The cross-sectional TEM images are displayed inFig. 2(a). It can be seen that the ErMnO3/Pt and Pt/Al2O3interfaces are obvious, as shown in the insert atFig. 2(a), and the thicknesses of Pt layer and ErMnO3 film are z52 nm and z540 nm, respectively. Fig. 2(b) presents the lattice image of ErMnO3/Pt interface at [110] zone axis.
It can be found that the lattice image is clearly identifiable, while some defects, the red dot circles inFig. 2(b), are presented. These defects with nano-scale were originated from the inclusion, e. g., Er2O3due to the missing Mn atom (Fig. S2, Supplementary Mate- rial). Similar result has been reported by Jehanathan et al. [38].
Furthermore, the epitaxial relationship can be further verified by their diffraction patterns obtained through fast Fourier trans- formation. As shown inFig. 2(c) and (d) for Pt layer and ErMnO3 film, respectively, the two diffraction patterns are definitely indexed, where the normal directions of ErMnO3 (0002) and Pt (111) planes are in parallel, evidencing a good epitaxial property.
Finally, the lattice constant cof ErMnO3 film is measured to be 11.4 Å by the HRTEM image, as shown inFig. 2(e), which is in a good agreement with the result evaluated from the XRD data.
In order to explore the ferroelectric property of h-ErMnO3films, the piezoresponse force microscopy (PFM) was first performed Fig. 1.(a) XRD pattern of ErMnO3film deposited on Pt(111)/Al2O3(100) substrate, and (b) XRD4-scans of h-ErMnO3(1122), Pt (113), and h-Al2O3(1123) peaks.
Y. Chen et al. / Journal of Alloys and Compounds 821 (2020) 153529 2
with the dual AC resonance tracking mode.Fig. 3(a) presents a topographic image (55mm2) of the h-ErMnO3film. It can be seen that thefilm surface has grains with size ranging from 0.2mm to 0.3mm. The root mean square surface roughness is evaluated as 5.0 nm which is reasonable for a thickfilm (~540 nm in thickness).
The out-of-plane amplitude and phase images of the as-deposited h-ErMnO3 films are displayed in Fig. 3(b) and (c), respectively.
Note that the vortex-like structure is almost not observed. This means that the defects in ourfilms, see Fig. 2(b) damaged the forming of vortex domain since it is very sensitive to the film quality [34,39]. Although the vortex-like structure has not been formed, the spontaneous ferroelectric domain is clearly seen. As
shown inFig. 3(c), the contrast image indicates that the domain size is identifiable. The local hysteretic phase and butterfly piezores- ponse curves, see Fig. 3(d), also reveal the typical polarization/
ferroelectric behavior. To further investigate the switching behavior of the as-deposited h-ErMnO3films, the poled voltage of±9 V was used by electrically writing two adjacent areas. As shownFig. 3(e), it is clearly observed that the left regions (9 V) is 180 phase different with the right region (þ9 V) in the image. Although the grain has somehow effect on the phase contrast, while the obser- vation of electrically switchable bi-stable states can be considered as an evidence of ferroelectric property in the h-ErMnO3 film.
Moreover, the contrast image stills remain after 1 h, seeFig. 3(f), Fig. 2.TEM characterization of the ErMnO3(0001)/Pt(111)/Al2O3(0001) heterostructure. (a) Low magnification TEM image. The insert in (a) shows the Pt layer with a thickness of 52 nm and the clear Pt/Al2O3and ErMnO3/Pt interfaces. (b) Cross-sectional TEM image of ErMnO3/Pt interface at (110) zone axis. The diffraction patterns for (c) Pt layer and (d) ErMnO3film by fast Fourier transformation. (e) High resolution TEM of ErMnO3film.
Fig. 3.Piezoelectric force microscopy (PFM) image based on the in situ epitaxial ErMnO3film in a 55mm2area. The out-of-plane (a) topography, (b) amplitude, and (c) phase images. (d) The local voltage butterfly loop (red line) and phase voltage hysteresis loop (blue line). Out-of-plane phase images by using poling bias of±9 V after (e) 0, and (f) 1 h. (For interpretation of the references to colour in thisfigure legend, the reader is referred to the Web version of this article.)
indicating the good polarization stability.
Next, to perform macroscopic ferroelectric polarization mea- surement of the h-ErMnO3film, the Pt top electrodes with 180mm in diameter were sputtered onto thefilm surface by using PLD. The measurement was performed at low temperature due to the large electrical leakage of h-ErMnO3 at room temperature, which is difficult to perform a reliable measurement at high electricfield [9,18]. Here, a pulsed positive-up-negative-down (PUND) polari- zation measurement was used at 150 K, and the result is displayed inFig. 4. A remnant polarizationProf 1.3mC/cm2and the two typical polarization current peaks are achieved, implying the ferroelectric behavior. Note that thePrvalue is smaller than that of reported h- ErMnO3single crystal (z5.5mC/cm2at 1 kHz and 120 K [18]). The reason can be attributed to the defects and epitaxial strain. The former induces that the domains of h-ErMnO3films are difficult to achieve saturation [39], and the latter generates a shrinkage lattice c,i.e., reduced polarization displacement.
It is well recognized that the polarization of h-ErMnO3originates from the rotation and tilting of the MnO5polyhedra [9,40]. The lo- cations and variations of Raman peaks, in response to phonon modes of MnO5bipyramids, can reflect the special symmetry of crystals and ferroelectric vibrating modes [37,41]. Therefore, Raman spectros- copy was used to study lattice structure of our h-ErMnO3films, and results are shown inFig. 5for Pt(111)/Al2O3(0001) substrate (black line), and ErMnO3(0001)/Pt(111)/Al2O3(0001) heterostructure (blue line). Here, an excitation laser with the wavelength of 532 nm was used vertical toabplane. According to the group theory, Raman- active modes are predicted for theG-point zone-center irreducible representations in ferroelectric h-ErMnO3(space group:P63cm) as:
G¼10A1þ15E1þ5A2þ10B1þ5B2þ15E2 [40]. Among, Raman-active modes (15E2, 9A1) can be estimated when the incident light is par- allel to thecaxis. In ErMnO3single crystal (see Refs, [37,41], or our single crystal), the 683 cm1(A1) corresponding to the O1and O2
apical oxygen stretching vibrations along thecaxis, indicated the tilting and trimerization of MnO5bipyramids and the ferroelectric polarization. In our epitaxial h-ErMnO3film, as depicted inFig. 5, the strong peak at 662 cm1 can be clearly observed. A compared analysis of Raman-active modes between our ErMnO3 film, our single crystal, and other reported ErMnO3films was summarized in Table 1. Ourfilm peaks are quite consistent with previous reports, implying the existence of ferroelectricity. Note that the Raman modes of 390 cm1(E1), 238 cm1(E1) and 107 cm1(A1) are shifted towards lower wavenumber, which can be ascribed to the smaller lattice constant with the higher bonding dissociation energy compared to the single crystal [41].
Finally, the magnetic properties of the epitaxial h-ErMnO3film were explored. Fig. 6(a) and (b) display the temperature (T) dependence of the zero-field cooled (ZFC) and field-cooled (FC) magnetization (M) curves at 2 kOe along theabplane and thecaxis, respectively (The magnetic behavior with weak diamagnetism of Pt(111)/Al2O3(0001) substrate can be found in theFig. S3, Supple- mentary Material). It can be found that the values of Mat any temperature are clearly different along two crystallographic di- rections, reflecting the anisotropic nature. Interestingly, all theM-T curves exhibits a clear change atTN¼53 K depended on Mn3þions, whileTNdepended on Er3þions is not observed. Note that the value is significantly smaller than that reported for single crystal (~78 K) [7,8,42]. The reason can be attributed to three aspects. First is the small grain size as shown inFig. 2. Fedorova et al. [25], reported a decreasingTNfrom 75 K to 62 K with grain size decreasing from 300 nm to 50 nm in polycrystalline h-ErMnO3ceramics. Second is the epitaxial strain. As demonstrated by Cheng et al. [43], the epitaxial h-YMnO3 films exhibited large in-plane compressive strain and a reducedTN(z36 K), much lower than that of YMnO3
single crystal (z70 K). Therefore, we believed that the in-plane compressive strain in the EMO film has a significant impact on the spin interaction, leading to a reducedTN. Last is the disorders or other competing interactions causing frustration of spins [34,44,45]. For example, it was reported that the excess Mn atoms in hexagonalRMnO3 system shifted the TNtowards lower tem- perature [34]. In general, modulation of the AFM spin ordering of Mn ions forms a triangular network within theabplane for the hexagonalRMnO3system. To avoid geometric magnetic frustration, the Mn spins are AFM ordering with a rotation of 120belowTN. It Fig. 4.P-Ehysteresis loop (red line) and polarization current (blue line) by using PUND
measurement with frequency of 1 kHz and temperature of 150 K for ErMnO3(0001)/
Pt(111)/Al2O3(0001) heterostructure. (For interpretation of the references to colour in thisfigure legend, the reader is referred to the Web version of this article.)
Fig. 5.Raman spectra of Pt(111)/Al2O3(0001) substrate (black line), single crystal ErMnO3(red line), and ErMnO3(0001)/Pt(111)/Al2O3(0001) heterostructure (blue line).
(For interpretation of the references to colour in thisfigure legend, the reader is referred to the Web version of this article.)
Table 1
A comparison in Raman-active modes between h-ErMnO3single crystals andfilms.
Modes our single crystal Reported single crystal [37]
Ourfilms Reported ErMnO3films [41]
A1 116 123 107
E2 218 234
E1 252 266 238
E2 295 297 300
E1 363 343
E1 417 444 390 420
A1 468 464 500
E1 639 592 640
A1 683 684 662 685
Y. Chen et al. / Journal of Alloys and Compounds 821 (2020) 153529 4
is quite clear that the loss Mn atoms [seeFig. S2, Supplementary Material] in ourfilm plays a major role for the inhibition of spin- ordering, reducing theTN. Moreover, the FC curves under various magneticfield (see the insets inFig. 6(a) and 6(b)), theTNpeaks at 53 K are still unchanged.
TheMcurves at differentT with magneticfield along theab plane and thecaxis are displayed inFig. 6(c) and (d), respectively. It can be found that the shapes ofM(H) curves present an excellent linearity above TN ¼ 53 K, indicating the paramagnetic (PM) property. BelowTN, the curves present a slightly saturated ten- dency, indicating an AFM property. TheM(H) curves reveal a clear AFM-PM phase transition, which is consistent with the above- mentionedM-Tresults.
4. Summary
Epitaxial hexagonal ErMnO3 films were grown on Pt(111)/
Al2O3(0001) substrates by pulsed laser deposition. The XRD and TEM indicated the h-ErMnO3films had a (0001)-orientation texture equipped with an in-plane compressive strain. A strong Raman- active peak at 662 cm1indicated the distorted MnO5polyhedra.
The full polarization reversal upon the electrical poling process, piezoresponse hysteresis loops and the excellent polarization retention performance were obtained by using PFM. A remnant polarization Pr ¼1.3 mC/cm2 at 150 K was measured by PUND.
Unlike the h-ErMnO3single crystal, Neel temperature for ErMnO3
films was 53 K due to the epitaxial compressive strain. This study provides an attractive approach to deeply understand the multi- ferroic hexagonal manganitefilms.
Author contribution statement
Yi Chen:Methodology, Writing- original draft preparation.Ye Li:Methodology.Dongfeng Zheng:Data curation.Leiyu Li:Data curation. Min Zeng: Methodology，Supervision,
Conceptualization.Minghui Qin:Formal analysis.Zhipeng Hou:
Visualization.Zhen Fan:Editing.Xingsen Gao:Resources.Xubing Lu: Validation. Qiliang Li: Writing- reviewing. Jun-Ming Liu:
Writing- reviewing, Conceptualization.
Declaration of competing interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgements
This work was supported by the National Key Research Program of China (No. 2016YFA0201004), the National Science Foundation of China (Grant Nos.: 11574091), the National Science Foundation of Guangdong Province (Grant No.: 2016A030308019), and X. Lu thanks for the support from the Project for Guangdong Province Universities and Colleges Pearl River Scholar Funded Scheme (2016).
Appendix A. Supplementary data
Supplementary data to this article can be found online at https://doi.org/10.1016/j.jallcom.2019.153529.
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