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Self-poling induced magnetoelectric effect in highly strained epitaxial BiFeO3/La0.67Sr0.33MnO3-# multiferroic heterostructures Dong Li, Dongxing Zheng, Junlu Gong, Wanchao Zheng, Chao Jin, and Haili Bai ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b05803 • Publication Date (Web): 26 Jun 2017 Downloaded from http://pubs.acs.org on June 27, 2017

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ACS Applied Materials & Interfaces

MS to ACS Applied Materials & Interfaces

Self-Poling Induced Magnetoelectric Effect in Highly Strained Epitaxial BiFeO3/La0.67Sr0.33MnO3−δ Multiferroic Heterostructures

Dong Li, Dongxing Zheng, Junlu Gong, Wanchao Zheng, Chao Jin, and Haili Bai∗

Tianjin Key Laboratory of Low Dimensional Materials Physics and Preparing Technology, Institute of Advanced Materials Physics, Faculty of Science, Tianjin University, Tianjin 300350, PRC

*

Author to whom all correspondence should be addressed. E-mail: [email protected]

1

ACS Paragon Plus Environment


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ABSTRACT

Highly strained epitaxial BiFeO3/La0.67Sr0.33MnO3−δ heterostructures were fabricated on LaAlO3 substrates by magnetron sputtering. The as-grown downward self-polarization of BiFeO3 (BFO) capping layers was confirmed by piezoelectric force microscope. Through the electrostatic field induced charge screening, a hole depletion state was induced in ultrathin (8 nm) La0.67Sr0.33MnO3−δ (LSMO) films. As a result of the interfacial charge coupling, appreciable saturated magnetization (MS) increase of about 500% and 100% can be observed in LSMO with BFO capping at 5 K and 300 K, respectively. Besides, LSMO phase translations can be revealed by the BFO thickness related exchange bias field (HE) and MS of the BFO/LSMO heterostructures. The results established a new approach in achieving interfacial magnetoelectric couplings with thin self-polarized multiferroic layers.

Keywords:

bismuth

ferrite,

polarization,

multiferroic

magnetoelectric coupling effect, phase transitions

2


heterosutructures,

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INTRODUCTION

Multiferroic (MF) materials offer a fascinating perspective of new possibilities in next-generation information technologies and systems, which possess higher memory speed and density or lower energy assumptions.1-4 They also stand for important scientific challenges in multiple correlations between different degrees of freedom in various systems.5 The magnetoelectric (ME) effect of multiferroic materials provides the possibility for achieving electric writing and magnetic reading. However, the ME couplings observed in single phase multiferroics are often too weak, thus limiting their practical applications.6 By now, many attempts have been made to seek the new potential in electrical tunable magnetization of thin film systems. For instance, the ME effects in the exchange bias (EB) systems, and the interfacial strain or charge mediated systems are studied the most.7-10 At the same time, plenty of recent works have focused on new multifunctional devices such as multiferroic tunnel junctions (MFTJ) with the promising photovoltaic tenability and magnetoelectric four states.11-13 In ferroelectric tunnel junctions (FTJ) devices, the most popular electrodes are doped lanthanum manganites, where the coexistence of strong interplay between electron transport, magnetism, and lattice distortions can be observed as a result of the various electronic behaviors.14 The Mn moments in La0.67Sr0.33MnO3−δ (LSMO) could be altered by the polarization induced accumulation or depletion of different charges, as predicted by first-principles calculations and observed in experiments.15, 16 BiFeO3

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(BFO) is the most studied multiferroic material due to the unique room temperature multiferroic properties.17 Bulk BFO possesses the rhombohedral structure, while a highly compressive strained BFO can be epitaxially stabilized with misfit strain over 4.5%.18, 19 This new pseudo-tetragonal with extremely large polarization of about 150 µC/cm2 is of great scientific interest, especially in T-BFO based tunnel junctions.20-22 Uniform direction of the ferroelectric polarization in as-grown samples is called self-poling. Polarizing some ferroelectrics by applying electric fields is sometimes impossible due to the high leakage current, the large coercive field or the difficulty in making electrodes. Hence, the existence of self-poling can offer a new freedom to develop novel functional devices. However, despite its importance for applications, how self-poling could affect the interfacial magnetoelectric effect has been rarely reported. In this study, we report the multiferroic capping layer induced interfacial magnetoelectric effect in highly strained epitaxial BFO/LSMO heterostructures. Our work should be helpful in understanding the interfacial couplings of MF/FM heterostructures and the designing for advanced multifunctional devices.

EXPERIMENTAL DETAILS

8-nm thick LSMO bottom ferromagnetic electrodes were firstly deposited epitaxially on (001) oriented LaAlO3 (LAO) single crystal substrates using facing-target magnetron sputtering. Then the BFO layer with various thickness was

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deposited on the LSMO layer by radio-frequency magnetron sputtering. The temperature, pressure and Ar/O2 ratio during the growth for LSMO and BFO were 670 °C, 0.5 Pa, 50/4.0 and 650 °C, 0.4 Pa, 50/0.1, respectively. After the deposition, the substrate was maintained at 650 °C for half an hour in pure oxygen atmosphere of 100 Pa and then cooled down to room temperature at a rate of 5 K/min. The microstructure was analyzed by X-ray diffraction (XRD, Rigaku D/max−2500) and FEI Tecnai G2 F20 transmission electron microscopy (TEM). The thicknesses of the BFO and LSMO were confirmed by a Dektak 6M surface profiler and TEM cross section imaging. The stoichiometry of the LSMO films were checked by X-ray photoelectron spectroscopy (XPS). The morphology, kelvin probe force microscopy (KPFM) and nanoscale ferroelectric properties of the BFO layers were examined with Bruker Multimode 8 atomic force microscope (AFM) equipped with Nanoscope V controller. Magnetic properties were measured with a Quantum Design magnetic property measurement system (SQUID-VSM).

RESULTS AND DISCUSSION

Figure 1(a) shows the XRD θ−2θ patterns of the highly strained BFO/LSMO heterostructures on (001) LAO substrates. For comparison, the XRD θ−2θ patterns of 30 nm T-like BFO (001) and 40 nm LSMO grown on LAO (001) substrates are also presented. The out-of-plane lattice parameter c of the T-BFO calculated from the (00l) peaks is ~4.64 Å, revealing that the T-BFO is stabilized by the large compressive

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Figure 1. (a) X−ray diffraction θ−2θ scan of the deposited BFO (37 nm)/LSMO (8 nm)/LAO (001) heterostructure, 30 nm BFO on LAO, and 40 nm LSMO on LAO. (b) AFM morphology of the 8 nm LSMO layer on LAO. (c) Morphology of the BFO (7 nm)/LSMO (8 nm)/LAO (001) heterostructure.

strain. The LSMO films on LAO show lower 2θ angles of the (00l) reflections than the bulk, which indicates an out-of-plane expansion as a result of the in-plane compressive strain. Note that the growth of high quality, atomically flat heterostructures is a critical step for the required ferroelectric properties in highly strained BFO and the interfacial magnetoelectric couplings.23 Figures 1(b) and 1(c) show the AFM surface morphology of the single LSMO layer on LAO and the BFO/LSMO/LAO heterostructure with 7 nm BFO capping layer. The calculated root-mean-square roughness of the LSMO single layer and the BFO capping layer are

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Figure 2. (a) Low-magnification of the BFO (37 nm)/LSMO (8 nm)/LAO (001) heterostructure. Inset is atomic resolution TEM image of the BFO/LSMO interface on the zoomed area. (b) Cross-sectional HRTEM images of the BFO/LSMO and LSMO/LAO interfaces. (c) Cross-sectional HRTEM image of the BFO layer in the same heterostructure and inset is the corresponding SAED pattern of BFO layer (d) SAED of the BFO/LSMO/LAO heterostructure captured near the interfaces. Diffraction spots of BFO, LSMO, and LAO are marked with different colors, namely green, cyan, and yellow, respectively. The inset of (d) is a zoomed area for the [001] diffraction spots of BFO, LSMO and LAO.

0.12 and 0.18 nm, respectively. Atomically flat terraces separated by ~4 Å steps (a single unit cell) can be clearly observed. The low-magnification TEM image of the entire heterostructure in Figure 2(a) 7



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shows an overview of the BFO/LSMO/LAO (001) heterostructure. The zoomed area shows a high-resolution TEM (HRTEM) cross section image of the highly strained BFO/LSMO interface in the [010] orientation. It demonstrates atomically sharp interfaces between BFO and LSMO. The HRTEM result presented in Figure 2(b) shows the abrupt atomic interfaces and well-defined perovskite crystal structure and hence indicates good coherent growth of the heterostructures. A high-resolution scanning transmission electron microscope (STEM) image of the BFO thin film and its corresponding selected area electron diffraction (SAED) is shown in Figure 2(c). The LAO substrate causes a highly compressive strain in the BFO thin films because the bulk a-axis lattice constant of BFO (a=3.965 Å) is 4.5% larger than that of LAO (a=3.792 Å). The measured c and a are ~4.64 and ~3.78 Å with an axial ratio c/a=1.23, which is consistent with the T-like phase of BFO.24 Figure 2(d) presents the SAED patterns of the BFO/LSMO/LAO heterostructure taken from the interfaces along the [100] zone axis. Well defined epitaxial growth of BFO/LSMO heterostructure is confirmed. Contrary to the ferroelectrics grown in paraelectric phase such as BaTiO3, BFO is typically grown in ferroelectric phase because the TC (~1100 K) is higher than deposit temperature (~930 K). In this case, an intricate interplay between polarization and surface could result in the accumulation of charges at the interface.25-27 Thus, it is likely that an interfacial charge coupling will persist and favors a specific polarization direction. To explore the ferroelectric properties of highly strained BFO in our heterostructures, we performed ferroelectric domain imaging and piezoresponse

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Figure 3. (a) and (b) Morphology and out-of-plane PFM phase images in 5×5 µm2 area after a negative (−5 V) and positive (+5 V) tip bias switching of the 33 nm thick BFO layer with 8 nm LSMO bottom electrode on LAO. The relative bright and dark contrasts indicate the polarization states point upward and downward, respectively. (c) PFM ramp curves (phase and amplitude) for the BFO layer. (d) Corresponding KPFM surface potential at the same place.

measurements by piezoelectric force microscope (PFM). In general, the naturally formed MPBs are slightly different in morphology with the electric field induced ordered MPBs. In order to observe pure electric field induced ordered MPBs, the 33 nm thick BFO layer with a small amount of naturally formed MPBs is adopted in the PFM measurement. As can be seen in Figure 3(a), an electric field induced squared pattern composed of morphotropic phase boundaries (MPBs) can be found in corresponding morphology image after the domain writing process. Nanoscale 9



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grooves in the MPBs and stripe like contrast in the in-plane PFM images can be clearly observed (Figure S1), which reveals the electric field induced phase transitions in highly strained BFO films during the process of polarization switching. A typical out-of-plane PFM phase image by ±5 V is presented in Figure 3(b). The as-grown BFO exhibits the single domain architecture with a downward polarization and clear ferroelectric switching property. It is consistent with the shift of local piezoelectric hysteresis and butterfly loops. Note that the out-of-plane monodomain configuration has been found in typical highly strained BFO films with the polarization points toward the substrate.28,

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Figure 3(c) shows standard ferroelectric switching,

indicating the reliable ferroelectricity in the BFO films. The PFM phase changes by 180° at the coercive voltages (VC) of −0.96 and 3.55 V, which displays a significant asymmetry reversal behavior. The horizontal shift in VC means that it takes a higher voltage to switch the polarization away from the as-grown direction than switching it back. This off-set switching behavior can be understood as a consequence of pinning effects, where the self-poling of BFO could play a dominant role. Other mechanisms such as work functions discrepancy between top and bottom electrodes might be involved as well. To study the ferroelectric polarization related surface potential variation in the BFO/LSMO heterostructures, KPFM measurement was performed. The electrical field generated by contact potential difference (CPD) between the scanning tip and sample can be nullified by adjusting an external DC bias as a compensatory potential. When the electrostatic force between tip and sample is minimized, the applied

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Figure 4. (a) and (b) Schematic band diagrams of KPFM measurement for the BFO/LSMO heterostructures with downward and upward ferroelectric polarization, respectively. Φtip and ΦBFO are the work function of scanning tip and BFO; EC, EV, and EF denote the conduction band, valence band, and Fermi level. VDC and VCPD denote the applied dc bias for KPFM measurement and the contact potential difference, respectively. (c) BFO thickness dependent average surface potential measured by KPFM and surface potential distribution obtained from KPFM scanning at each point illustrated as the background depth histogram.

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VDC = VCPD . All KPFM measurements were performed in the dark to avoid the photovoltaic interference, which is not neglectable in BFO heterostructures.30, 31 As can be seen in Figure 3(d), obvious surface potential difference reveals the polarization induced surface band bending after domain writing process. Having the photovoltaic effect ruled out, the band bending induced surface potential variation could be mainly due to the BFO ferroelectric polarization. Thus the CPD can be written as VCPD = ΦBFO + ΦFE − Φtip , where Φtip, ΦBFO are the work functions of scanning tip and BFO, ΦFE is the band bending contribution induced by the ferroelectric polarization. Schematic interfacial band diagrams are sketched in Figures 4(a) and 4(b) with the polarization reversal. The downward polarization with much uncompensated negative bound charges will highten the BFO surface potential and cause the upward band bending. Conversely, the upward polarization would lower the surface potential with uncompensated positive bound charges. Furthermore, the CPD of the BFO/LSMO heterostructures with increasing BFO thickness were analysised and the result has been summarized in Figure 4(c). At first sight, it is confusing that the surface potential does not change monotonously with increasing BFO thickness, but reaches its maximum at about 25–30 nm. It seems inconsistent with the monotonic trend stemed from the depolarization theory, which tells that the thin ferroelectric film would show reduced values of polarization and transition temperature.32 To understand this scenario, we performed KPFM measurement on 45 nm BFO for a zoomed area with distinct MPBs in the center (Figure S2). Although approaching the KPFM resolution limit of instrument, it is still found that the surface potential of

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MPBs region is lower than the surrounding T-phase area, which is due to the strain relaxation happened to BFO in MPBs. In this strain relaxation process, BFO will change from T-phase to mixed phase with a reduced value of ferroelectric polarization. Therefore, the trend observed in surface potential can be explained by the thickness related evolution of self-polarization value in highly strained BFO with a maximum at about 25–30 nm. To understand the magnetic properties of the highly strained BFO/LSMO heterostructures, we first measured the magnetic hysteresis loops in the presence of +2 T cooling field from 300 to 5 K. From Figure 5(a), we found that the hysteresis loops display a negative horizontal shift, which is the typical behavior of EB effect. In addition, on cooling with a −2 T field, the loop is biased in the positive direction, which confirms the existence of EB effect. The exchange field (HE) is defined as

HE = H + H

2 C

1 C

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/ 2 , where HC1 and HC2 are the coercive fields of the hysteresis loop.

According to the reports by Tebano and Sun et al., the strain induced intrinsic HE appears in the LSMO films grown on LAO substrates as a result of strain induced phase separation.33,

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The temperature dependent HE of the BFO/LSMO

heterostructure and different LSMO single layers are presented in Figure 5(b). It is found that in all samples, HE decreases with increasing temperature and finally vanishes above 30 K, which is consistent with the determined blocking temperature (TB) in BFO/LSMO system and phase separated LSMO.8, 35 Compared with the highly strained single LSMO layers, HE is reduced in BFO/LSMO heterostructures. Figure 5(c) summarizes the magnetic properties of the BFO/LSMO heterostructures

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Figure 5. (a) M−H loops of the highly strained epitaxial BFO/LSMO heterostructures at 5 K after the field cooling from 300 K. (b) Temperature-dependent EB field in the LSMO single layers and BFO/LSMO heterostructures on LAO. (c) and (d) BFO thickness dependent MS and HE of highly strained epitaxial BFO/LSMO/LAO heterostructures with the LSMO layers fixed at 8 nm.

as a function of the BFO thickness. With the increasing BFO thickness, the average saturation magnetization (MS) first increases and then decreases. Similar trend was observed both at 5 and 300 K. It is suggested that the BFO capping layer can induce distinct

MS

increase

in

BFO/LSMO

heterostructures.

Considering

the

antiferromagnetic (AF) ground state of BFO layers, the magnetic variation in the BFO/LSMO heterostructures with the same 8 nm LSMO layers should be mainly affected by interfacial charge couplings.36 In general, the origin of EB effect usually lies at the interface shared by FM and AF magnetically ordered systems.37 As 14


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summarized in Figure 5(d), the trend of HE is similar to the variation of MS, which shows a maximum at the intermediate thickness of BFO. The occurrence of HE reflects the coexistence of FM and AF phases and the distribution and population of FM and AF domains can determine the amplitude of HE. As a symbol of frustrated magnetic systems, spin glass has been widely reported in oxide heterostructures with intricate magnetic interfacial interactions, where FM enhancement is often observed.38 In Figure S4，the occurrence of clear bifurcation between the ZFC and FC M-T curves and the peak of ZFC M-T curve at low temperature confirms the existence of spin glass behavior in highly strained BFO/LSMO heterostructure. In brief, the observed enhancement of MS and suppression of HE reveal that the BFO capping layers can effectively enhance the ferromagnetic interactions in highly strained LSMO layers. To illustrate the observed magnetic variations, we sketch the mechanism diagram of interfacial charge couplings in highly strained BFO/LSMO heterostructures in Figure 6. The double exchange (DE) mechanism has successfully explained the observed ferromagnetic signal in LSMO and the magnetic phase transitions between the FM and AF states. It originates from exchange couplings between Mn3+ and Mn4+ via oxygen ions, thus could be quite sensitive to Mn valence states and carrier concentration as shown in the zoomed area of Figure 6(a). The observed magnetic variation can be understood in the framework of polarization charge screening induced magnetic phase transitions. In addition, the occupation of the d 3 z 2 -r 2 orbitals is much preferential in thin LSMO films grown on the LAO substrates due to the compressive epitaxial strain induced tetragonal distortion.39 As a result, the strained

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LSMO will show a suppressed ferromagnetism because of the transition from FM ground state to C-type AF phase.40 While it is clear that the transition is incomplete with some ferromagnetic signal persists in the films, which fits well with the strain induced phase separation scenario, where multiple magnetic phases coexist.41 The screen length of bulk like half metallic LSMO is only several unit cells deep, while in phase separated semiconductor like LSMO, the depletion region can be much thicker, thus improving the interfacial charge couplings. To get more specific information on the Mn valence in the LSMO films, XPS measurements were performed on the 8 nm LSMO bottom electrode (Figure S3). The fitted curves of XPS show that Mn4+/Mn3+ in our LSMO films is about 48%. Note that the as-grown BFO layers with downward polarization can lead to hole depletion state in thin LSMO films. Based on the phase diagram of strained manganites, the Mn4+ depletion can strengthen the double exchange interaction in LSMO on LAO.42 Analogous results have been observed by Jiang and Barrionuevo et al. in the systems where ultrathin LSMO films capped with ferroelectric layers.43, 44 In our previous report on interfacial charge couplings, we have observed electric field induced magnetization swithings in Mn doped ZnO/BFO epitaxial

heterostructure

and

CoFe2O4/Pb(Mg1/3Nb2/3)0.7Ti0.3O3

multiferroic

heterostructures.45, 46 The electric bias induced polarization reversal combined with the redistribution of oxygen vacancies could be responsible for the observed interfacial magnetoelectric coupling. Similarly, the hole depletion in LSMO induced by BFO self-poling can give rise to the transition from AF to FM phase and the enhancement of magnetization.

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Figure 6. Illustration of the interfacial charge couplings in highly strained BFO/LSMO heterostructures with (a) thin and (b) thick BFO layers. The arrows marked with M, P and D represent the saturated magnetization, polarization and depolarization field, respectively. The size dependent depolarization fields opposite to the BFO downward self-polarization could affect the amplitude of hole depletion in highly strained LSMO layers, thus inducing the observed magnetic transitions.

To further understand the BFO thickness dependence of MS and HE, we should take the above hole depletion induced transition from AF to FM phase into consideration. Based on the analyses of KPFM surface potential results and depolarization field theory, the magnitude of the polarization in ferroelectric films is size dependent as illustrated in Figure 6. With the increase of BFO thickness, the polarization induced charge screening effect at the BFO/LSMO interface becomes more prominent. However, in highly strained BFO, the polarization will not always increase with thickness, T-R phase transition of BFO accompanying with reduced polarization would weaken the interfacial screening effect. We found that the 17



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magnetic properties of the BFO/LSMO heterostructures share the same trend with the BFO thickness dependence of hole depletion, which can increase the amount of FM/AF interfaces by transforming some AF phase into FM phase. Therefore, it reveals that the enhanced population of FM phase in LSMO induced by BFO polarization screening should be responsible for the variation of MS and HE. We have noticed that a HE depression occurs in the BFO/LSMO heterostructures compared with the single LSMO layers. Considering the amount of AF phase in ultrathin EB systems, HE might deminish with the decreased ratio of AF phase in LSMO.47 For reference, as can be seen in Figure 5(b), the HE of 20 nm LSMO single layer is smaller than that of 8 nm film. It stands for the deduction that the decrease of HE should be ascribed to the reduced proportion of AF phase by strain relaxation. In such MF/FM heterostructures, the complexity of short-range interface magnetic coupling comes from the competing phases in LSMO and the interfacial pinning between AF ordered BFO and FM phase of LSMO. The key factor of the observed magnetic properties changes should be the hole depletion induced transition from AF to FM phase in LSMO. Hence, the MS and HE keeps increasing until the critical thickness of fully strained T-like BFO layer, which is about 25–30 nm.48 The occurrence of T-R phase transition in thicker BFO layers will lead to the formation of MPBs with the release of epitaxial strain.49 In this condition, the electrostatic field induced FM domains will shrink with the reduced hole depletion, resulting in the decrease of MS and HE. In addition to the previous statements, other possible factors for the BFO

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thickness dependent magnetic variation are also worth considering. Such as strain relaxation effect, interfacial orbital reconstruction and the formation of MPBs et al.. However, their contribution won’t be dominant for some reasons. Firstly, the relaxation of strain should increase monotonously with the increasing BFO thickness, which would result in a monotonously variation but inconsistent with the observed results. Secondly, based on our previous first principle calculation, the interfacial orbital reconstruction at highly strained BFO/LSMO interface indeed can induce an significantly enhanced magnetization with the polarization pointing at the interface through the modulation of orbital occupation.50 Nevertheless, this kind of effects should be only limited to interfacial atomic layers and independent to the BFO thickness. The formation of as-grown MPBs which might bring about the ferromagnetic enhancement of BFO can only found above the critical thickness of ~30 nm, which is opposite to the observed MS decrease in that range.51

CONCLUSION

In summary, highly strained epitaxial heterostructures consisted of multiferroic BFO and ferromagnetic LSMO layers were fabricated using magnetron sputtering technique on LAO substrates. Ferroelectric self-poling in the highly strained epitaxial BFO layers was confirmed by PFM measurements. The MS and HE of the BFO/LSMO heterostructures reach their maximums with 25–30 nm BFO capping layers, as a result of the BFO thickness dependent interfacial charge screening. Thus we have

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demonstrated that the magnetic phase of LSMO is not only controlable with chemically doping and misfit strain, but also through electrostatic field. Note that the modulation by ultra-thin T-BFO films is quite impressive, providing valuable information for FTJ and MFTJ designing with thickness limitation. Although more investigations are needed to definitely settle the microscopic nature of spin and orbital ordering in these films, the findings reported here should be helpful in designing the multifunctional devices.

ASSOCIATED CONTENT

*Supporting Information AFM morphology of the highly strained epitaxial 45 nm BFO with 8 nm LSMO bottom electrode on LAO before and after electric writing. The out-of-plane and in-plane phase images of the corresponding area. AFM morphology of distinct MPBs and corresponding KPFM potential images. Mn 3d XPS of the 8 nm epitaxial LSMO film on LAO. ZFC and FC curves of the BFO (37 nm)/LSMO (8 nm)/LAO (001) heterostructure.

AUTHOR INFORMATION

Corresponding Author *E-mail: [email protected]

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Author Contributions D.L. and H.B. designed the outline of the manuscript and wrote the main manuscript text. D.Z., J.G., W.Z. and C.J. contributed detailed discussions and revisions. All authors reviewed the manuscript.

Notes The authors declare no competing ﬁnancial interest.

ACKNOWLEDGEMENTS

This work was supported by the National Natural Science Foundation of China (51272174&11434006).

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