Applied Physics Letters

(1)

Magnetoelectric couplings in high-density array of nanoscale Co/BiFeO ₃ multiferroic heterostructures

Cite as: Appl. Phys. Lett.114, 012901 (2019);doi: 10.1063/1.5066997

Submitted: 16 October 2018

.

Accepted: 15 December 2018

.

Published Online:

04 January 2019

XinZhong,¹GuoTian,¹XiaoSong,¹YadongWang,¹WendaYang,¹PeilianLi,¹QiuyuanLuo,¹ZhipengHou,^1,a) ZhenFan,¹ DeyangChen,¹MinghuiQin,¹MinZeng,¹ XingsenGao,^1,a) and Jun-MingLiu²

AFFILIATIONS

1Guangdong Provincial Key Laboratory of Quantum Engineering and Quantum Materials and Institute for Advanced Materials, South China Academy of Advanced Optoelectronics, South China Normal University, Guangzhou 510006, China

2Laboratory of Solid State Microstructures and Innovation Center of Advanced Microstructures, Nanjing University, Nanjing 21009, China

a)Authors to whom correspondence should be addressed:houzp@m.scnu.edu.cnandxingsengao@scnu.edu.cn

ABSTRACT

We have systematically explored the magnetoelectric (ME) coupling effect of Co/BiFeO₃multiferroic heterostructured nanodot arrays, fabricated by the anodic aluminum oxide template method. Piezoresponse hysteresis loops of these nanodots demonstrate a significant enhancement of the ME coupling effect. More intriguingly, we have realized a magnetic domain transformation from an initial single-domain state to a vortex state by applying a regional or local voltage, and the single-domain state can be recovered by using an external in-plane magnetic field. Our results will guide the invention of high-density, energy- efficient, non-volatile multifunctional ME microdevices.

Published under license by AIP Publishing.https://doi.org/10.1063/1.5066997

The family of multiferroic magnetoelectric (ME) materials, which accommodates both electric and magnetic orders, has been receiving increasing interest from the research community due to their wide range of applications in the micro-electronic industry, including ME sensors,¹logical devices,²and nonvolatile random access memories (RAMs).³Recently, numerous material systems have been experimentally proved to show the ME coupling effect, including vertical structures (denoted 1–3 structures),⁴ multilayered films (denoted 2–2),⁵ multiferroic tunnel junctions (denoted 0–0),⁶and others.^7–11Among them, the multilayered heterostructures are of particular appeal because they possess a relatively high ME performance, small substrate clamping effect, and high operational temperatures.^12–14Here, we design and fabricate a vertical Co/BiFeO₃(Co/BFO) heterostructure to rationally achieve a good ME performance. In this structure, BFO is chosen to be the ferroelectric layer because it is a well-known room-temperature single-phase multiferroic material with large ferroelectric polarization that brings a large strain.^15–17Meanwhile, Co is selected as the ferromagnetic layer to obtain a sensitive strain-magnetism response due to its large magnetostriction.¹⁸Furthermore, we fabricate the heterostructure into nanodot arrays to meet the high-density storage

requirements. More intriguingly, the nanodot arrays result in a further enhancement of the ME coupling effect because in such a highly geometrically confined structure, the atomic-level mechanical coupling is significantly improved and the substrate clamping, which hampers the ME coupling in multiferroic composite films, is suppressed.¹⁹As a result, a notable enhancement of ME coupling is observed in the nanodot arrays. Based on the significant ME coupling, we further realize a transformation from single-domain to the vortex in the nanodots by imple- menting either a regional or local voltage. These results provide an opportunity to realize the high-density, non-volatile ME devices.

The ordered Co/BFO epitaxial nanodot array is fabricated by pulsed laser deposition (PLD) through an ultrathin nanoporous anodic aluminum oxide (AAO) membrane template.²⁰ In Fig. 1(a), we present a schematic diagram to illustrate the details of the fabrication process. First, the (Ca,Ce)MnO3(CCMO) bot- tom electrode layer and BFO films were grown on a LaAlO3

(LAO) substrate using PLD at an ambient temperature of 640C and an oxygen pressure of 15 Pa. Then, an ultrathin AAO membrane with a pore size of 300 nm is transferred onto the BFO

Appl. Phys. Lett.114, 012901 (2019); doi: 10.1063/1.5066997 114, 012901-1

Published under license by AIP Publishing

Applied Physics Letters

ARTICLE scitation.org/journal/apl

(2)

film [see the left panel inFig. 1(a)]. Subsequently, the Co layer is deposited by thermal evaporation at a low oxygen pressure ranging from 110³to 210³Pa at room temperature [see the middle panel ofFig. 1(a)]. In this process, a thin Au layer is deposited on top of the Co nanomagnets to protect the Co nanodots from oxidation under the fabrication conditions. Finally, by mechanically or chemically lifting off the AAO mask, we can obtain a well-ordered Co nanodot array [see the right panel of Fig. 1(a)].Figure 1(b)shows the surface topography of the heterostructure visualized by atomic force microscopy (AFM). One can observe that the Co pillars are arranged periodically on the BFO, and their corresponding sizes range from 300 to 330 nm. These features suggest that the well-ordered epitaxial Co-pillars are in high quality. To establish the orientation of the heterostructured nanodots, we have measured the XRD diffractogram along the out-of-plane direction of the sample. The diffraction peaks from LAO (002), CCMO (002), BFO (002), and Co (111) clearly demonstrate that the normal direction of LAO, CCMO, and BFO is along the [001]-axis and the normal direction of the Co layer is along the [111]-axis. We also measure the reciprocal space map (RSM) to determine the lattice parameters [seeFig. 1(d)]. The corresponding lattice constants of CCMO and BFO are established to be 0.369 nm and 0.465 nm, respectively, agreeing well with previous reports.²¹We have further measured the ferroelectricity of the heterostructure (seesupplementary material Fig. S1). The well-defined saturated phase-voltage hysteresis loop and butterfly-like amplitude-voltage hysteresis loop suggest that its ferroelectricity is good, laying a good material platform for further investigations.

Hereafter, we investigate the ME coupling effect of the heterostructure by using piezoresponse force microscopy (PFM) to

measure the piezoresponse characteristics before and after applying an in-plane magnetic field of 3000 Oe (seeFig. 2). By comparing piezoresponse hysteresis loops with and without an external in-plane magnetic field of 3000 Oe, one can observe that the average phase-contrast curves of switching are close to 180(see the upper panel ofFig. 2), which suggests that a polarization reversal occurs in the heterostructure. Moreover, under a zero magnetic field, we find that the maximum PFM amplitude is 125 pm with a coercive voltage of 5 V (see the lower panel of Fig. 2). After the application of an in-plane magnetic field of 3000 Oe, the maximum PFM amplitude increases to 175 pm, and meanwhile, the corresponding coercive voltage increases to 6 V.

These properties clearly show that the magnetic field affects the electrical properties of the heterostructure and hence imply a strong ME coupling in the heterostructure.

To analyze the ME coupling quantitatively, we next calcu- lated the ME coupling coefficienta31of the heterostructure by using Xie’s method based on the physical parameters obtained above.²²The ME coupling coefficient a31 can be expressed by the equation: a₃₁¼VDu=uDDH1, where V is the driving AC voltage that is used to induce piezoelectric vibration, and here, V is fixed to be 1 V;uis the measured piezoresponse displacement, and here, u is 125 pm, obtained from the amplitude- voltage loop in the remnant state when the voltage is equal to zero;Duis the displacement change with and without the magnetic field and possesses a fixed value of 50 pm;Dis the measured nanodot thickness and is equal to 20 nm; DH1 is the FIG. 1.Co/BFO heterostructured nanodot arrays. (a) Schematic diagrams of the

fabrication process. An ultrathin AAO membrane with a pore size of300 nm is adopted. (b) AFM image of Co nanomagnets, which suggests that the well-ordered Co nanodots are of high quality. The scale bar in panel (b) is 1lm. (c) XRDh-2h scan diffraction pattern. (d) Reciprocal space mapping of heterostructures. The lattice constants of CCMO and BFO are determined to be 0.369 nm and 0.465 nm, respectively.

FIG. 2.Controlling electric properties via a magnetic field. The piezoresponse hysteresis exhibits phase hysteresis loops (upper panel) and butterfly-shaped amplitude loops (lower panel) in the absence (black line) or the presence (red line) of an in-plane magnetic field.

Applied Physics Letters

(3)

variation of the magnetic field, and here,DH1is 3000 Oe. Based on the established physical parameters, the valuea31can be cal- culated to be 6.710⁴mV/cm Oe. It should be noted here that we obtain a similar value ofa31in other regions of the heterostructure, which suggests that our results are reasonable (see supplementary materialFig. S2). The ME coefficient of the Co/

BFO epitaxial nanodot array is two orders of magnitude higher than the quasi (0–3) type composite films²²and nearly two times higher than that of the CoFe₂O₄(CFO)–Pb(Zr_0.52Ti_0.48)O₃(PZT) nanofibers.²³We propose that the enhancement of the ME coefficient is closely related to the structure of the heterostructure.

As is known, an in-plane magnetic field can make the Co layer generate a remarkable in-plane compressive strain and out-of- plane tensile strain due to its large magnetostriction effect.

When the in-plane compressive stain is transferred to the BFO layer, the out-of-plane lattice constant of the BFO may be increased.^22,24This feature can increase the polarization charac- teristic along thecaxis,^25–27which primarily contributes to the increase in the piezoresponse of BFO.²²On the other hand, the out-of-plane tensile strain may also directly stretch the BFO layer along the lateral surface of the Co layer. Although this process may also account for the enhancement of piezoresponse, the magnitude of enhancement is much smaller due to a smaller contact surface area.²²

To confirm the significant ME coupling in the heterostructure, we have further studied the converse ME effect via the magnetization changes induced by the ferroelectric phase transition. The magnetization states of nanomagnets can be charac- terized by magnetic force microscopy (MFM) before and after the application of voltage. Note that we use low-moment tips for the sake of avoiding the interference of the magnetic tip with the magnetization states of the nanomagnets. The MFM tip is magnetized along the vertical direction of the substrate, which is sensitive to changes in the vertical components of magnetic fields. A bright contrast indicates that the tip is repelled, while a dark contrast shows that the tip is attracted to the sample. The control of magnetization in our nanodot arrays via the electric field can be clearly confirmed by the MFM switching characteristics before and after electrically writing the region (seeFig. 3).

The MFM images of initial states show that most of the nanomagnets are magnetized in one direction (rightward) and exhibit a single domain, similar to the domain states observed insupple- mentary materialFig. S1. After aþ7 V voltage is applied to the region, most of the single domains are switched into the vortex states. Subsequently, when we increase the voltage toþ8 V, the vortexes are unaffected. It should be noted that this transformation is not a special case and can be widely observed in the nanodot arrays. A small region is highlighted in detail to present the magnetization switching more clearly (seesupplementary materialFig. S3). The intriguing phenomenon can also be confirmed by previous phase field simulations.²⁸It is demonstrated that both the single domain state and the vortex state show a minimum energy in the initial state of the Co thin film, but there exists an energy barrier between them.²⁸When an external out- of-plane voltage is applied, a transient T/R-phase may occur in the BFO layer.²⁹ Since the in-plane lattice constant of the R-phase (a 0.40 nm) is larger than that of the T-phase

(a 0.3665 nm), such a phase transition brings about an in- plane tensile stress. When the tensile stress is transferred to the Co nanodots, the easy magnetization axis of Co tends to move along the in-plane direction,³⁰which drives the switching from the single domain state to the vortex state.

Our above results have demonstrated that the magnetization state of the Co nanodots can be significantly changed by applying a regional writing model of the voltage electric field through converse ME coupling, which demonstrates a great potential utility in the application of ME memory devices.

However, in the field of practical applications, a controllable nanodot writing mode is preferred. Therefore, we have further investigated whether a similar domain transformation in the nanodots can be realized by a single-nanodot writing mode.

Figures 4(b)and4(c)illustrate the MFM images taken before and after electrically writing a single nanodot by þ7 V. The initial magnetization orientation of the nanomagnet is found to be a single domain. An obvious change in the MFM contrast is observed after applying an electric bias, which strongly suggests that the single domains transform into the vortex states. This feature verifies that domain transformation can also be realized by a single-nanodot writing model, which is highly appreciated for practical applications. It should be noted here that the mag- netizations of some nanodots cannot switch under the application of the vertical electric bias. This may be attributed to the inhomogeneity of the BFO, such as the surface roughness, resulting in an applied tensile strain that is not uniformly strong enough to overcome the ME energy of the nanomagnets to switch the magnetization state.^30–32

FIG. 3.Controlling the magnetization of the nanodot arrays by electrically writing a region. (a) AFM topological image. (b)–(d) The corresponding MFM micrographs by varying the external voltage: (b) 0 V, (c)þ7 V, and (d) þ8 V. The domains enclosed by colored circles in (b)–(d) represent one-to-one correspondence to the domain evolution process. The left panel of (e) shows an enlarged MFM image of the single domain enclosed by the blue circle in (b), and the right panel of (e) shows an enlarged MFM image of the vortex domain enclosed by the blue circle in (d).

FIG. 4.Electrically induced domain switching of individual nanodots. (a) AFM topological image. (b) and (c) show the corresponding MFM micrographs under an external voltage of 0 V andþ7 V, respectively. (d) The domain states under an external in-plane magnetic ﬁeld of 1200 Oe. Almost all single domains move along the direction of the magnetic ﬁeld (blue line). The scale bar is 500 nm.

Applied Physics Letters

(4)

However, we find that the vortex state cannot be switched back to the single-domain magnetization state in our experi- ments even if we apply a voltage of9 V (seesupplementary material Fig. S3). Meanwhile, when the applied voltage is decreased to zero, the vortex domains remain unaffected.

However, the robust vortex domains can recover back to the single domains by applying a large external in-plane magnetic field. The MFM images inFig. 4(d)clearly demonstrate that the vortex states transform back into the uniform single-domain initial states when we apply a magnetic field of 1200 Oe. However, we find that some of the single domains (nanodots marked as

“1”) do not exactly orientate along the direction of the applied in-plane magnetic field. This may be attributed to the inhomogeneity of the Co film, which results in a higher saturation magnetic field.

In conclusion, we have systematically explored the fabrication techniques and the ME coupling effect of nanoscale ferromagnetic Co arrays on the ferroelectric BiFeO3(BFO) substrate.

Periodic arrays of Co/BFO heterostructured nanodots with diameters ranging from 300 nm to 350 nm are fabricated by using thermal evaporation and the ultrathin nanoporous anodic alumina (AAO) membrane. By measuring the piezoresponse hysteresis loops of these nanodots, we find an enhancement of ME coupling at room temperature and that the corresponding ME coefficient is two orders of magnitude higher than quasi (0–3) type thin films. More intriguingly, based on the strong ME coupling, we can realize a magnetic domain transformation from the initial single-domain state to a vortex state by applying a regional or a local voltage. After applying an in-plane magnetic field, the vortex can be switched back to the original state.

These results provide us an opportunity to realize high-density, nonvolatile ME devices.

See supplementary material for the saturated phase- voltage hysteresis loop and the butterfly-like amplitude-voltage hysteresis loop (S1), the piezoresponse hysteresis for different regions of the heterostructure (S2), and magnetic domain evolution with the variation of the electric field and the magnetic field (S3).

The authors thank the financial support from the National Key Research and Development Program of China (Nos.

2016YFA0201002, 2016YFA0300101, and 2016YFA0201004), the State Key Program for Basic Researches of China (No.

2015CB921202), the Natural Science Foundation of China (Nos.

11674108 and 51272078), the Project for Guangdong Province Universities and Colleges Pearl River Scholar Funded Scheme (2014), Science and Technology Planning Project of Guangdong Province (No. 2015B090927006), and the Natural Science Foundation of Guangdong Province (No. 2016A030308019).

REFERENCES

1P. Zhao, Z. Zhao, D. Hunter, R. Suchoski, C. Gao, S. Mathews, M. Wuttig, and I. Takeuchi,Appl. Phys. Lett.94, 243507 (2009).

2D. E. Nikonov and I. A. Young,J. Mater. Res.29, 2109 (2014).

3J.-M. Hu, Z. Li, L.-Q. Chen, and C.-W. Nan,Nat. Commun.2, 553 (2011).

4J.-H. Li, I. Levin, J. Slutsker, V. Provenzano, P. K. Schenck, R. Ramesh, J.

Ouyang, and A. L. Roytburd,Appl. Phys. Lett.87, 072909 (2005).

5M. Ghidini, F. Maccherozzi, X. Moya, L. C. Phillips, W. Yan, J. Soussi, N.

Metallier, M. E. Vickers, N. J. Steinke, R. Mansellet al.,Adv. Mater.27, 1460 (2015).

6N. A. Pertsev and H. Kohlstedt,Appl. Phys. Lett.95, 163503 (2009).

7J. G. Wan, X. W. Wang, Y. J. Wu, M. Zeng, Y. Wang, H. Jiang, W. Q. Zhou, G.

H. Wang, and J. M. Liu,Appl. Phys. Lett.86, 122501 (2005).

8F. Zavaliche, T. Zhao, H. Zheng, F. Straub, M. P. Cruz, P. L. Yang, D. Hao, and R. Ramesh,Nano Lett.7, 1586 (2007).

9W. Eerenstein, M. Wiora, J. L. Prieto, J. F. Scott, and N. D. Mathur,Nat.

Mater.6, 348 (2007).

10C. W. Nan, M. I. Bichurin, S. Dong, D. Viehland, and G. Srinivasan,J. Appl.

Phys.103, 031101 (2008).

11Y. Wang, J. Hu, Y. Lin, and C.-W. Nan,NPG Asia Mater.2, 61 (2010).

12H. Zheng, J. Wang, S. E. Lofland, Z. Ma, L. Mohaddes-Ardabili, T. Zhao, L.

Salamanca-Riba, S. R. Shinde, S. B. Ogale, F. Baiet al.,Science303, 661 (2004).

13H. J. Liu, W.-I. Liang, Y.-H. Chu, H. Zheng, and R. Ramesh,MRS Commun.

4(2), 31 (2014).

14N. M. Aimon, D. H. Kim, X. Sun, and C. A. Ross,ACS Appl. Mater. Interfaces 7, 2263 (2015).

15J. Allibe, S. Fusil, K. Bouzehouane, C. Daumont, D. Sando, E. Jacquet, C.

Deranlot, M. Bibes, and A. Barthelemy,Nano Lett.12, 1141 (2012).

16S. H. Baek, H. W. Jang, C. M. Folkman, Y. L. Li, B. Winchester, J. X. Zhang, Q.

He, Y. H. Chu, C. T. Nelson, M. S. Rzchowskiet al.,Nat. Mater.9(4), 309 (2010).

17J. M. Hu, L. Q. Chen, and C. W. Nan,Adv. Mater.28, 15 (2016).

18J. R. Choi, S. J. Oh, H. Ju, and J. Cheon,Nano Lett.5, 2179 (2005).

19S. M. Stratulat, X. Lu, A. Morelli, D. Hesse, W. Erfurth, and M. Alexe,Nano Lett.13(8), 3884 (2013).

20J. B. Yi, H. Pan, J. Y. Lin, J. Ding, Y. P. Feng, S. Thongmee, T. Liu, H. Gong, and L. Wang,Adv. Mater.20, 1170 (2008).

21P. Li, Z. Huang, Z. Fan, H. Fan, Q. Luo, C. Chen, D. Chen, M. Zeng, M. Qin, Z.

Zhanget al.,ACS Appl. Mater. Interfaces9, 27120 (2017).

22Y. Li, Z. Wang, J. Yao, T. Yang, Z. Wang, J. M. Hu, C. Chen, R. Sun, Z. Tian, J.

Liet al.,Nat. Commun.6, 6680 (2015).

23S. Xie, F. Ma, Y. Liu, and J. Li,Nanoscale3, 3152 (2011).

24T. N. Yang, J. M. Hu, C. W. Nan, and L. Q. Chen,Appl. Phys. Lett.104, 052904 (2014).

25H. Bea, B. Dupe, S. Fusil, R. Mattana, E. Jacquet, B. Warot-Fonrose, F.

Wilhelm, A. Rogalev, S. Petit, V. Croset al.,Phys. Rev. Lett.102, 217603 (2009).

26C. Daumont, W. Ren, I. C. Infante, S. Lisenkov, J. Allibe, C. Carretero, S.

Fusil, E. Jacquet, T. Bouvet, F. Bouamraneet al.,J. Phys. Condens. Matter 24, 162202 (2012).

27Y. Uratani, T. Shishidou, F. Ishii, and T. Oguchi,Jpn. J. Appl. Phys., Part 144, 7130 (2005).

28M. Yi, B. Xu, R. Muller, and D. Gross,€ Acta Mech.2017, 1–10.

29J. Yao, X. Song, X. Gao, G. Tian, P. Li, H. Fan, Z. Huang, W. Yang, D. Chen, Z.

Fanet al.,ACS Nano12, 6767 (2018).

30V. Sampath, N. D’Souza, D. Bhattacharya, G. M. Atkinson, S.

Bandyopadhyay, and J. Atulasimha,Nano Lett.16, 5681 (2016).

31G. Tian, F. Zhang, J. Yao, H. Fan, P. Li, Z. Li, X. Song, X. Zhang, M. Qin, M.

Zenget al.,ACS Nano10, 1025 (2016).

32A. K. Biswas, H. Ahmad, J. Atulasimha, and S. Bandyopadhyay,Nano Lett.

17, 3478 (2017).

Applied Physics Letters

Magnetoelectric couplings in high-density array of nanoscale Co/BiFeO 3 multiferroic heterostructures

.

.

Applied Physics Letters

Applied Physics Letters

Applied Physics Letters

Applied Physics Letters

Magnetoelectric couplings in high-density array of nanoscale Co/BiFeO ₃ multiferroic heterostructures