Fabrication of epitaxial ferroelectric BiFeO

(1)

Contents lists available atScienceDirect

Ceramics International

journal homepage:www.elsevier.com/locate/ceramint

Fabrication of epitaxial ferroelectric BiFeO

₃

nanoring structures by a two- step nano-patterning method

Qiuyuan Luo

^a

, Deyang Chen

^a,⁎

, Wenda Yang

^a

, Chao Chen

^a

, Peilian Li

^a

, Zhifeng Huang

^a

, Guo Tian

^a

, Zhen Fan

^a

, Xingsen Gao

^a,⁎

, Jun-Ming Liu

^b

aInstitute 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

bLaboratory of Solid State Microstructures and Innovation Center of Advanced Microstructures, Nanjing University, Nanjing 210093, China

A R T I C L E I N F O

Keywords:

BiFeO3

Ferroelectric Nanodot Nanoring Ion-beam etching

A B S T R A C T

A novel two-step nano-patterning method is proposed to fabricate epitaxial ferroelectric BiFeO₃(BFO) nanoring array, which maintains well-epitaxial structure and possesses strong ferroelectricity demonstrated by X-ray diffraction (XRD) and piezoresponse force microscopy (PFM). The ferroelectric polarizations were examined by PFM, revealing the reversible switching behavior under an electricfield. This novel method could also be ex- tended to other oxide material systems. The fabrication of high quality ferroelectric nanoring structure provides the possibility to explore novel functionalities (e.g., ferroelectric vortices) and offers application potentials for the high-density non-volatile memory devices.

1. Introduction

In 1994, Gorbatsevich[1]and Grimmer[2]reported the possible existence of a unique domain structure called vortices or toroidal or- dering in ferroelectrics. In recent years, ferroelectric vortices are at- tracting much attention both for the study of the emerging fundamental physical phenomena as a new state of matter and due to potential applications in ultra-high density memory“bits”applications[3–8]. So far, there is tantalizing experimental evidence for vortices (or closure structure) in ferroelectric nanodots [9–11], paraelectric/ferroelectric superlattices[6]or as a transient state during switching in thinﬁlms [4,9,12,13]. However, it is diﬃcult to observe static ferroelectric vortices due to the tremendous disclination strain in the core[5,14]. The theory study[3]predicted that the ferroelectric vortices could be sta- bilized in the nanoring structure as the release of the strain energy in the core, which sheds light on the path to explore the ferroelectric vortices. The ring structured multiferroics are also promising for a range of fascinating phenomena such as multiferroic toroidal orders and magnetoelectric couplings.

Nevertheless, unlike the great progress in the fabrication methods of ferroelectric nanodots, such as focused ion beam (FIB) milling, electron beam direct writing (EBDW), anodized alumina (AAO) template-assisted ion beam etching and self-assembly, etc[11,15–19], the fabrication approaches of ferroelectric nanorings are less successful. Com- pared to the well-studied metallic nanorings [20–23], ferroelectric

oxides are very diﬃcult to be patterned into the ring structure by conventional lithography method. Zhu et al.[24]have attempted to fabricate lead zirconium titanate (PZT) nanorings by template assisted wet chemical solution deposition method and Byrne et al.[25]prepared PZT nanorings using a self-assembly technique, while these nanorings are polycrystalline and the ferroelectricity was not reported.

Han et al.[26]prepared epitaxial PZT ring-like structure using the laser interference lithography process combined with pulsed laser deposition (PLD), nevertheless the rings exhibits very poor epitaxial quality with some impurities. Therefore, the diﬃculties in fabrication of high quality epitaxial ferroelectric nanorings have hindered the study of the unique exotic domains and the realization of high-density memory device applications.

In this study, we report an effective two-step nano-patterning method to fabricate well-epitaxial ferroelectric nanorings by patterning high quality ferroelectricfilms with top-down technique, which is dif- ferent from the AAO template-assisted pulsed laser deposition and ion beam etching method we proposed very recently[27].. We choose the multiferroic BiFeO3 (BFO) epitaxial thin film grown by pulsed laser deposition (PLD) as the starting material for its strong ferroelectricity [28–30], rich domain structures [31–33], various phase transitions [34–36]and superior magnetoelectric couplings[37–40]. Ordered BFO nanoring array with well-epitaxial structure and good ferroelectric properties is fabricated using this method, which makes it possible for investigation and manipulation of the refined polar domain structures.

http://dx.doi.org/10.1016/j.ceramint.2017.08.188

Received 26 August 2017; Received in revised form 28 August 2017; Accepted 28 August 2017

⁎Corresponding authors.

E-mail addresses:dychen1987@gmail.com(D. Chen),xingsengao@scnu.edu.cn(X. Gao).

Available online 30 August 2017

MARK

(2)

This method also paves a path for exploring the exotic domains structures (e.g. vortex topological domain) and promotes the realization of nanoscale electronic devices.

2. Experimental details

BFO epitaxial thin ﬁlms (thickness ~ 40 nm) with 71° stripe domains grown by PLD on single crystal STO (001) substrate with SrRuO3

(SRO, thickness ~ 50 nm) as the bottom electrode are used to fabricate nanorings in this work. The preparation procedures of BFO nanoring structure by a two-step nano-patterning method, including the fabrication of nanodots (step I) and the fabrication of nanoring array (step II), are illustrated inFig. 1.

To fabricate BFO nanorings, nanodots are firstly produced by polystyrene spheres (PS)-assisted Ar ion-beam etching in Step I, as shown in Fig. 1a–d. First, the monolayer PS (diameter of the nano- spheres ~ 500 nm)floating on the mixture of ethanol and water are transferred onto the epitaxial BFO thinfilm (Fig. 1a). Then, O2plasma etching is applied to obtain the isolated monolayer PS array and the size of PS are reduced to around 450 nm, as presented in Fig. 1b. After- wards, the PS covered BFO thinfilm is etched by Ar ion-beam to pro- duce the BFO nanodots, as shown inFig. 1c. Next, O2plasma etching is used again to further reduce the size of PS around 250 nm (Fig. 1d) in order to be prepared for the fabrication of BFO nanorings in Step II (Fig. 1e–h).

To synthesize BFO nanoring array, a 12 nm-thick aluminum (Al) thinﬁlm is deposited as a mask layer by thermal evaporation, as shown inFig. 1e. By cleaning the sample with chloroform and acetone, the PS are removed and Al nanoring array are produced (Fig. 1f). Then, Ar ion- beam is applied to etch the sample with appropriate etching time. The Al nanoring array is taken as a scariﬁed mask layer during this process, so that the regions blocked by Al nanorings are protected while the rest areas are etched away. Thereby, the BFO nanoring structure is fabricated as presented inFig. 1g. Following by cleaning the rest Al mask

with NaOH solution, we successfully obtain the ordered BFO nanoring array (Fig. 1h).

3. Results and discussions

After the synthesis of BFO nanoring structure, atomic force microscopy (AFM) and scanning electron microscopy (SEM) are used to measure the topographies. To demonstrate the epitaxy and crystallinity of the nanoring structure, X-ray diﬀraction (XRD)θ–2θ scan and reciprocal space mapping (RSM) are performed. The ferroelectric properties and polarization reversal behaviors are also characterized by piezoresponse force microscopy (PFM).

To better understand the morphology evolutions in the fabrication process of BFO nanoring array, topography images in some key steps are captured by AFM and SEM, as shown inFig. 2. The AFM image in Fig. 2a shows the morphology of the sample after O2plasma etching (corresponding toFig. 1b), demonstrating the formation of isolated monolayer PS nanodot array on the BFO thinﬁlm. Following the Ar ion- beam etching to synthesize BFO nanodots (Fig. 1c), second time O2

plasma etching is applied and thus the PS nanodot (diameter ~ 250 nm) covered BFO nanodots (diameter ~ 450 nm) are fabricated, as presented inFig. 2b which is in accordance withFig. 1d. After the deposition of the Al thinﬁlm, the PS nandots are removed, leading to the production of Al nanorings (Figs. 1f and2c). Al nanorings are taken as the mask during the following Ar ion-beam etching process (Fig. 1g).

Ordered BFO nanoring array (with outer diameter around 400 nm, the width of ring wall around 70 nm, and the height of 15 nm) is obtained after the Al cleaning, as shown inFig. 2d which is corresponding to Fig. 1h. These AFM images not only demonstrate the morphology evolutions during the synthesis process, but also prove that the two-step nano-patterning approach, proposed in this study, is eﬀective to fabricate BFO nanoring structure. The corresponding SEM images are displayed inFig. 2e and f, which are consistent with the morphologies in AFM images, further revealing the successful fabrication of ordered Fig. 1.Schematic diagrams of fabrication process of BFO nanoring structures: Step I, fabrication of nanodots (a–d): (a) monolayer PS transferring, (b) etching PS by O2plasma, (c) Ar ion-beam etching of BFO, (d) second time O2plasma milling to obtain smaller size PS; Step II, fabrication of BFO nanoring array (e–h): (e) Al deposition, (f) PS removing, (g) using Ar ion-beam etching to form nanoring structures, and (h) Al cleaning.

(3)

BFO nanoring array using this novel two-step nano-patterning method.

In addition, size-tunable nanorings could be produced by easily changing the size of PS. The BFO nanoring array with diameter of 250 nm has been fabricated in our work (not shown here) and further study for the synthesis of smaller nanorings are in process.

Next, we turn to study the structure of the BFO nanoring array. As presented inFig. 3a, XRDθ–2θ scan shows the (001) and (002) diffraction peaks of BFO nanorings, SRO bottom electrode and STO substrate, revealing the (001) epitaxy of the obtained BFO nanoring structure. The (103) RSM data shown inFig. 3b further conﬁrms the epitaxy of the nanorings. According to the XRDθ–2θscan and RSM data inFig. 3, we can obtain the in-plane and out-of-plane lattice constants of BFO nanodots, a ~ 3.905 Å and c ~ 4.04 Å, indicating the in-plane compressive strain imposed by the STO substrate, which is in agree- ment with the previous study[40].

The ferroelectric properties of the BFO thinﬁlm and nanoring array were measured by PFM. The topography, out of plane (OOP) and in plane (IP) phases of the as grown BFO thin ﬁlm are displayed in Fig. 4a–c, revealing the typical 71° stripe domain structure. The

topography presented inFig. 4d shows the well-ordered BFO nanorings, consistent with the AFM and SEM data inFig. 2. As shown inFig. 4e and f, the in-plane (IP) PFM phase images were captured at 0° and 90° orientation (the details of the scanning directions are illustrated in the top sketches), respectively. According to the in-plane PFM images (Fig. 4e and f) and the corresponding out of plane (OOP) PFM image which shows a uniform bright contrast (not shown here), it is found that the BFO nanorings mainly consist of the typical 71° domain structure with single orientated vertical polarization and stripe-like lateral domain patterns. To study the domain switching behaviors of the BFO nanoring structure, the out-of-plane (OOP) PFM phase images of a random selected nanoring were captured at the as prepared state (Fig. 4g), after sequentially applying electric pulses of−5 V and +5 V within the blue rectangle dashed line region (Fig. 4h and i). It is observed that the OOP PFM contrast of the region applied with electric pulses changes from bright (Fig. 4g) to dark (Fig. 4h) after theﬁrst pulse and then return to bright again after the second pulse (Fig. 4i), which indicates the OOP polarization can be reversibly switched back and forth. Moreover, the piezoresponse phase-voltage hysteresis loops (Fig. 4k) and amplitude- Fig. 2.AFM images showing the morphology evolution during the fabrication process. (a) The morphology of the PS array after theﬁrst time O2plasma etching, (b) Small PS nanodots on the BFO nanodots after the second time O2plasma milling, (c) Al nanoring array mask, (d) the obtained BFO nanoring structures, (e–f) the corresponding SEM images of (a–d). Scale bar, 1 µm.

Fig. 3.(a) X-ray diﬀractionθ-2θscan, and (b) (103) Reciprocal space mapping (RSM) data of the obtained BFO nanoring array.

(4)

voltage loops (Fig. 4l) of three randomly selected points in a random BFO nanoring shown inFig. 4j were conducted. All the piezoresponse phase loops show 180° sharp ferroelectric switching along with the butterﬂy-like amplitude-voltage loops. Therefore, PFM data shown in Fig. 4demonstrate the strong ferroelectricity and reversible polarization switching behaviors of the BFO nanorings, which is promising for high-density non-volatile memory devices. Further reduction of the nanoring size (which is under process in our lab) may induce the ferroelectric vortex state.

4. Conclusion

In summary, the BFO nanoring structure, with the outer diameter of

~ 400 nm, wall thickness of 70 nm and the height of 15 nm, has been successfully fabricated by the proposed novel two-step nano-patterning route in this study. The obtained BFO nanoring array synthesized using this method maintains the epitaxial structure, strong ferroelectricity and reversible polarization switching, which is promising for high- density non-volatile memory devices and provides a possibility for Fig. 4.(a) Topography, (b) and (c) In-plane(IP) PFM phase images of the nanoring array captured at 0° and 90° orientation, respectively (the details of the scanning directions are illustrated in the top sketches), Scale bar, 500 nm. Out-of-plane (OOP) PFM phase images of a nanoring: (d) as prepared state, (e) after applying electric pulses of−5 V (e) and (f) +5 V (f) in the blue rectangle dashed line region. (g) AFM topography image of a random BFO nanoring; (h) the phase-voltage loops, and (i) butterﬂy-like amplitude-voltage loops of three randomly selected points of the BFO nanoring in (g). Scale bar, 200 nm. (For interpretation of the references to color in thisﬁgure legend, the reader is referred to the web version of this article.)

(5)

exploring their novel functionalities, e.g. ferroelectric vortices in nanorings. The size of the nanorings could be tunable by easily changing the size of PS and this nanoring-fabrication method could also be ex- tended to a variety of functional oxide material systems (such as BaTiO3

and PZT).

Acknowledgments

This work was supported by the National Key Research Program of China (No. 2016YFA0201002), the State Key Program for Basic Researches of China (No. 2015CB921202), the National Natural Science Foundation of China (Nos. 11674108, 51272078), the Project for Guangdong Province Universities and Colleges Pearl River Scholar Funded Scheme (2014), the Natural Science Foundation of Guangdong Province (No. 2016A030308019), and the Science and Technology Planning Project of Guangdong Province (No. 2015B090927006).

References

[1] A.A. Gorbatsevich, Y.V. Kopaev, Toroidal order in crystals, Ferroelectrics 161 (1994) 321–334.

[2] H. Grimmer, The forms of tensors describing magnetic, electric and toroidal properties, Ferroelectrics 161 (1994) 181–189.

[3] I.I. Naumov, L. Bellaiche, H. Fu, Unusual phase transitions in ferroelectric nanodisks and nanorods, Nature 432 (2004) 737–740.

[4] I.I. Naumov, H. Fu, Vortex-to-polarization phase transformation path in ferroelectric Pb(ZrTi)O3nanoparticles, Phys. Rev. Lett. 98 (2007) 077603.

[5] G. Catalan, J. Seidel, R. Ramesh, J.F. Scott, Domain wall nanoelectronics, Rev. Mod.

Phys. 84 (2012) 119–156.

[6] A.K. Yadav, C.T. Nelson, S.L. Hsu, Z. Hong, J.D. Clarkson, C.M. Schleputz, A.R. Damodaran, P. Shafer, E. Arenholz, L.R. Dedon, D. Chen, A. Vishwanath, A.M. Minor, L.Q. Chen, J.F. Scott, L.W. Martin, R. Ramesh, Observation of polar vortices in oxide superlattices, Nature 530 (2016) 198–201.

[7] Z. Li, Y. Wang, G. Tian, P. Li, L. Zhao, F. Zhang, J. Yao, H. Fan, X. Song, D. Chen, Z. Fan, M. Qin, M. Zeng, Z. Zhang, X. Lu, S. Hu, C. Lei, Q. Zhu, J. Li, X. Gao, J.M. Liu, High-density array of ferroelectric nanodots with robust and reversibly switchable topological domain states, Sci. Adv. 3 (2017) e1700919.

[8] Y. Li, Y. Jin, X. Lu, J.C. Yang, Y.H. Chu, F. Huang, J. Zhu, S.W. Cheong, Rewritable ferroelectric vortex pairs in BiFeO3, NPJ Quantum Mater. 2 (2017) 43.

[9] A. Gruverman, D. Wu, H.J. Fan, I. Vrejoiu, M. Alexe, R.J. Harrison, J.F. Scott, Vortex ferroelectric domains, J. Phys.: Condens. Matter 20 (2008) 342201.

[10] B.J. Rodriguez, X.S. Gao, L.F. Liu, W. Lee, I.I. Naumov, A.M. Bratkovsky, D. Hesse, M. Alexe, Vortex polarization states in nanoscale ferroelectric arrays, Nano Lett. 9 (2009) 1127–1131.

[11] A. Schilling, D. Byrne, G. Catalan, K.G. Webber, Y.A. Genenko, G.S. Wu, J.F. Scott, J.M. Gregg, Domains in ferroelectric nanodots, Nano Lett. 9 (2009) 3359–3364.

[12] N. Balke, B. Winchester, W. Ren, Y.H. Chu, A.N. Morozovska, E.A. Eliseev, M. Huijben, R.K. Vasudevan, P. Maksymovych, J. Britson, S. Jesse, I. Kornev, R. Ramesh, L. Bellaiche, L.Q. Chen, S.V. Kalinin, Enhanced electric conductivity at ferroelectric vortex cores in BiFeO3, Nat. Phys. 8 (2011) 81–88.

[13] R.K. Vasudevan, Y.C. Chen, H.H. Tai, N. Balke, P. Wu, S. Bhattacharya, L.Q. Chen, Y.H. Chu, I.N. Lin, S.V. Kalinin, V. Nagarajan, Exploring topological defects in epitaxial BiFeO3thinﬁlms, ACS Nano 5 (2011) 879–887.

[14] G. Arlt, P. Sasko, Domain conﬁguration and equilibrium size of domains in BaTiO3

ceramics, J. Appl. Phys. 51 (1980) 4956–4960.

[15] H. Han, Y. Kim, M. Alexe, D. Hesse, W. Lee, Nanostructured ferroelectrics: fabrication and structure-property relations, Adv. Mater. 23 (2011) 4599–4613.

[16] S.R. Burns, J.M. Gregg, V. Nagarajan, Nanostructuring ferroelectrics via focused ion beam methodologies, Adv. Funct. Mater. 26 (2016) 8367–8381.

[17] Y. Kim, Y. Kim, H. Han, S. Jesse, S. Hyun, W. Lee, S.V. Kalinin, J.K. Kim, Towards the limit of ferroelectric nanostructures: switchable sub-10 nm nanoisland arrays, J.

Mater. Chem. C 1 (2013) 5299.

[18] G. Tian, L. Zhao, Z. Lu, J. Yao, H. Fan, Z. Fan, Z. Li, P. Li, D. Chen, X. Zhang, M. Qin, M. Zeng, Z. Zhang, J. Dai, X. Gao, J.M. Liu, Fabrication of high-density BiFeO3

nanodot and anti-nanodot arrays by anodic alumina template-assisted ion beam etching, Nanotechnology 27 (2016) 485302.

[19] H. Zheng, J. Wang, S.E. Loﬂand, Z. Ma, L.M. Ardabili, T. Zhao, L.S. Riba, S.R. Shinde, S.B. Ogale, F. Bai, D. Viehland, Y. Jia, D.G. Schlom, M. Wuttig,

A. Roytburd, R. Ramesh, Multiferroic BaTiO3-CoFe2O4nanostructures, Science 303 (2004) 661–663.

[20] A.R. Halpern, R.M. Corn, Lithographically patterned electrodeposition of gold, silver, and nickel nanoring arrays with widely tunable near-infrared plasmonic resonances, ACS Nano 7 (2013) 1755–1762.

[21] R. Near, C. Tabor, J. Duan, R. Pachter, M. El-Sayed, Pronounced eﬀects of aniso- tropy on plasmonic properties of nanorings fabricated by electron beam lithography, Nano Lett. 12 (2012) 2158–2164.

[22] J. Li, S. Zhang, C. Grigas, R. Misra, J. Bartell, V.H. Crespi, P. Schiﬀer, Magnetization states and switching in narrow-gapped ferromagnetic nanorings, AIP Adv. 2 (2012) 012136.

[23] T. Ozel, M.J. Ashley, G.R. Bourret, M.B. Ross, G.C. Schatz, C.A. Mirkin, Solution- dispersible metal nanorings with deliberately controllable compositions and ar- chitectural parameters for tunable plasmonic response, Nano Lett. 15 (2015) 5273–5278.

[24] X.H. Zhu, P.R. Evans, D. Byrne, A. Schilling, C. Douglas, R.J. Pollard, R.M. Bowman, J.M. Gregg, F.D. Morrison, J.F. Scott, Perovskite lead zirconium titanate nanorings:

towards nanoscale ferroelectric“solenoids, Appl. Phys. Lett. 89 (2006) 122913.

[25] H. Han, R. Ji, Y.J. Park, S.K. Lee, G. Le Rhun, M. Alexe, K. Nielsch, D. Hesse, U. Gosele, S. Baik, Wafer-scale arrays of epitaxial ferroelectric nanodiscs and nanorings, Nanotechnology 20 (2009) 015301.

[26] J. Wang, J.B. Neaton, H. Zheng, V. Nagarajan, S.B. Ogale, B. Liu, D. Viehland, V. Vaithyanathan, D.G. Schlom, U.V. Waghmare, N.A. Spaldin, K.M. Rabe, M. Wuttig, R. Ramesh, Epitaxial BiFeO3multiferroic thinﬁlm heterostructures, Science 299 (2003) 1719–1722.

[27] G. Tian, D. Chen, J. Yao, Q. Luo, Z. Fan, M. Zeng, Z. Zhang, J. Dai, X. Gao, J.-M. Liu, BiFeO3 nanorings synthesized via AAO template-assisted pulsed laser deposition and ion beam etching, RSC Adv. 7 (2017) 41210–41216.

[28] D. Chen, X. Gao, J.-M. Liu, Domain structures and magnetoelectric eﬀects in multiferroic nanostructures, MRS Commun. 6 (2016) 330–340.

[29] Y.H. Chu, Q. He, C.H. Yang, P. Yu, L.W. Martin, P. Shafer, R. Ramesh, Nanoscale control of domain architectures in BiFeO3thinﬁlms, Nano Lett. 9 (2009) 1726–1730.

[30] J. Wu, J. Wang, BiFeO3 thinﬁlms of (111)-orientation deposited on SrRuO3 buf- fered Pt/TiO2/SiO2/Si(100) substrates, Acta Mater. 58 (2010) 1688–1697.

[31] Z. Chen, J. Liu, Y. Qi, D. Chen, S.L. Hsu, A.R. Damodaran, X. He, A.T. N'Diaye, A. Rockett, L.W. Martin, 180 degrees ferroelectric stripe nanodomains in BiFeO3

thinﬁlms, Nano Lett. 15 (2015) 6506–6513.

[32] D. Chen, Z. Chen, Q. He, J.D. Clarkson, C.R. Serrao, A.K. Yadav, M.E. Nowakowski, Z. Fan, L. You, X. Gao, D. Zeng, L. Chen, A.Y. Borisevich, S. Salahuddin, J.M. Liu, J. Bokor, Interface engineering of domain structures in BiFeO3thinﬁlms, Nano Lett.

17 (2017) 486–493.

[33] R.J. Zeches, M.D. Rossell, J.X. Zhang, A.J. Hatt, Q. He, C.H. Yang, A. Kumar, C.H. Wang, A. Melville, C. Adamo, G. Sheng, Y.H. Chu, J.F. Ihlefeld, R. Erni, C. Ederer, V. Gopalan, L.Q. Chen, D.G. Schlom, N.A. Spaldin, L.W. Martin, R. Ramesh, A strain-driven morphotropic phase boundary in BiFeO3, Science 326 (2009) 977–980.

[34] D. Chen, C.T. Nelson, X. Zhu, C.R. Serrao, J.D. Clarkson, Z. Wang, Y. Gao, S.L. Hsu, L.R. Dedon, Z. Chen, D. Yi, H.J. Liu, D. Zeng, Y.H. Chu, J. Liu, D. Schlom, R. Ramesh,, A strain-driven, Antiferroelectric-to-Ferroelectric Phase Transition in La-doped BiFeO3thinﬁlms on Si, Nano Lett. (2017),http://dx.doi.org/10.1021/

acs.nanolett.7b03030.

[35] G. Tian, F. Zhang, J. Yao, H. Fan, P. Li, Z. Li, X. Song, X. Zhang, M. Qin, M. Zeng, Z. Zhang, J. Yao, X. Gao, J. Liu, Magnetoelectric coupling in well-ordered epitaxial BiFeO3/CoFe2O4/SrRuO3heterostructured nanodot array, ACS Nano 10 (2016) 1025–1032.

[36] J.G. Wu, Z. Fan, D.Q. Xiao, J.G. Zhu, J. Wang, Multiferroic bismuth ferrite-based materials for multifunctional applications: ceramic bulks, thinﬁlms and nanostructures, Prog. Mater. Sci. 84 (2016) 335–402.

[37] S. Dong, J.M. Liu, S.W. Cheong, Z. Ren, Multiferroic materials and magnetoelectric physics: symmetry, entanglement, excitation, and topology, Adv. Phys. 64 (2015) 519–626.

[38] Y.H. Chu, L.W. Martin, M.B. Holcomb, M. Gajek, S.J. Han, Q. He, N. Balke, C.H. Yang, D. Lee, W. Hu, Q. Zhan, P.L. Yang, A.F. Rodriguez, A. Scholl, S.X. Wang, R. Ramesh, Electric-ﬁeld control of local ferromagnetism using a magnetoelectric multiferroic, Nat. Mater. 7 (2008) 478–482.

[39] J.T. Heron, J.L. Bosse, Q. He, Y. Gao, M. Trassin, L. Ye, J.D. Clarkson, C. Wang, J. Liu, S. Salahuddin, D.C. Ralph, D.G. Schlom, J. Iniguez, B.D. Huey, R. Ramesh, Deterministic switching of ferromagnetism at room temperature using an electric ﬁeld, Nature 516 (2014) 370–373.

[40] Y. Feng, C. Wang, S. Tian, Y. Zhou, C. Ge, H. Guo, M. He, K. Jin, G. Yang, Controllable growth of ultrathin BiFeO3fromfinger-like nanostripes to atomically flatfilms, Nanotechnology 27 (2016) 355604.