Effects of Interface Layers and Domain Walls on the Ferroelectric

Subscriber access provided by The Chinese University of Hong Kong

Article

Effects of interface layers and domain walls on the ferroelectric resistive switching behavior of Au/BiFeO3/La0.6Sr0.4MnO3 heterostructures Lei Feng, Shengwei Yang, Yue Lin, Dalong Zhang, Weichuan Huang, Wenbo Zhao, Yuewei Yin, Sining Dong, and Xiao-Guang Li ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.5b10210 • Publication Date (Web): 10 Nov 2015 Downloaded from http://pubs.acs.org on November 14, 2015

Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a free service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are accessible to all readers and citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.

ACS Applied Materials & Interfaces is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

Page 1 of 34

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ACS Applied Materials & Interfaces

Effects of Interface Layers and Domain Walls on the Ferroelectric Resistive Switching Behavior of Au/BiFeO3/La0.6Sr0.4MnO3 Heterostructures Lei Feng,1 Shengwei Yang,1 Yue Lin,1 Dalong Zhang,1 Weichuan Huang,1 Wenbo Zhao,1 Yuewei Yin,1 Sining Dong1, Xiaoguang Li1, 2,

1

*

Hefei National Laboratory for Physical Sciences at Microscale, Department of Physics, University of Science and Technology of China, Hefei 230026, P. R. China.

2

Collaborative Innovation Center of Advanced Microstructures, Nanjing 210093, P. R. China.

KEYWORDS: ferroelectric heterostructures, resistive switching, interface layer, domain walls, ion interdiffusion

ABSTRACT: The electric field effects on the electric and magnetic properties in multiferroic heterostructures are important for not only understanding the mechanisms of certain novel physical phenomena occurring at heterointerfaces but also offering a route for promising spintronic applications. Using the Au/BiFeO3/La0.6Sr0.4MnO3 (Au/BFO/LSMO) multiferroic heterostructure as a model system, we investigated the ferroelectric resistive switching (RS) behaviors of the heterostructure. Via the manipulation of the BFO ferroelectric polarizations, the non-volatile tri-state of RS is observed, which is closely related to the Au/BFO and BFO/LSMO interface 1

ACS Paragon Plus Environment


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 2 of 34

layers and the highly conducting of BFO domain walls (DWs). More interestingly, according to the magnetic field dependence of the RS behavior, the negative magnetoresistance effect of the third resistance state, corresponding to the abnormal current peak in current-pulse voltage hysteresis near the electric coercive field, is also observed at room temperature, which mainly arises from the possible oxygen vacancy accumulation and Fe ion valence variation in the DWs.

2


Page 3 of 34

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


1. INTRODUCTION Abundant investigations into various ferroelectric materials have revealed remarkable polarization dependent electric transport properties, including resistive switching (RS) effects in ferroelectric diodes,1-4 ferroelectric tunnel junctions (FTJs)5-10 and conduction change at ferroelectric domain walls (DWs).11-15 RS effects in epitaxial ferroelectric heterostructures, which arise from the manipulation of band alignment and contact resistance at the ferroelectric/electrode interfaces by switchable ferroelectric polarization, have captured immense interest due to their potential applications in nondestructive readout memory devices.16 The RS behaviors of multiferroic BiFeO3 (BFO) based heterostructures have been widely studied due to the relative narrow band gap (∼2.8 eV) and large remnant polarization of BFO,1, 17-19 which can be categorized into two types: i) diode-like rectification effect based on two interfaces,3-4, 20-21 and ii) bi-stable currents relied on only one interface.18-19,

22

In a switchable ferroelectric diode, such as Ag/BiFeO3/Ag,23 two

Schottky barriers (SCBs) formed at two metallic/semiconducting Ag/BiFeO3 interfaces, can be equivalently treated as two diodes connected back-to-back. When ferroelectric layer is polarized, the existence of remnant polarization charges and the corresponding screening charges at interfaces will modify the SCB height and width, driving one interface forward-biased and the other reverse-biased. Then the rectification direction and the overall resistance of heterostructures can be switched by polarization switching, resulting in an RS behavior.2-4 However, A. Sawa19, 24, J. L. Wang25 and D. Wu18 et al. have reported that in Pt/BiFeO3/SrRuO3 and Pt/BiFeO3/La0.7Sr0.3MnO3 the ferroelectric RS effects, showing bi-stable currents relying on only one interface, cannot be 3



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 4 of 34

simply explained by the “diode” mechanism. It should be noted that the metallic La0.7Sr0.3MnO3 can be regarded as a special electrode because of the similar work functions (∼4.8 eV for La0.7Sr0.3MnO3, and ∼4.7 eV for BiFeO3) as well as matched lattice parameters. In this case, the SCB between BFO and La0.7Sr0.3MnO3 diminishes or even often disappears.18, 25-26 Although the SCB between La0.7Sr0.3MnO3 and BFO is negligible, further studies show that there exists some kind of interface layer between them, which has been proved to produce novel electric and magnetic properties.27-28 Thus, it should be considered whether the electronic properties at this interface and RS behaviors can be changed by polarization charges. Besides the interfaces mentioned above, a certain type of ferroelectric DWs shows a much higher conductivity than domain in BFO, ferromagnetic behavior, as well as a negative magnetoresistance (MR) effect below 200K.11, resistance

states

with

different

29-31

polarization

It has been found that, besides the two directions,

the

resistance

of

the

Ag/La0.1Bi0.9FeO3/La0.7Sr0.3MnO3 shows a third state with the lowest resistance near the ferroelectric coercive voltage of La0.1Bi0.9FeO3 due to the high DW density at zero-polarization state, which proves the possibility of ferroelectric DWs participating macroscopic conduction.32 Recently, J. H. Lee et al. also reported that the conducting BFO DWs are ferromagnetic and have anisotropic MR effect below 100 K via macroscopic measurements.31 Although the ferromagnetic property of BFO DWs with much higher conductivity is demonstrated, whether the negative MR effect of these DWs can be observed at room temperature or not, when RS effects occur, is very important for practical applications of ferroelectric-resistive memories. 4


Page 5 of 34

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


The above discussions reveal that the ferroelectric RS effects as well as the related mechanisms are very complex, which should be deeply investigated. In the present work, we report the RS behavior of Au/BiFeO3/La0.6Sr0.4MnO3 (Au/BFO/LSMO) heterostructures. A ferroelectric related tri-state has been observed in the current versus pulse voltages (I-Vpulse) curves without any electroforming process. Based on the effects of interface layers and conducting domain walls, the non-volatile RS characteristics are analyzed. Furthermore, a negative MR effect near electric coercive field owning to the magnetic interactions of ferroelectric DWs of BiFeO3 is observed at room temperature. 2. EXPERIMENTAL DETAILS The Au/BFO/LSMO multiferroic heterostructure was grown by a magnetron sputtering technique. An LSMO layer with a thickness of 80 nm was first deposited on (001) oriented SrTiO3 (STO) single crystal substrate at 750 °C, followed by a 2 h annealing process in a flowing O2 at 800 °C. Subsequently, a BFO layer with 150 nm thickness was deposited on the LSMO film at 670 °C. The Bi/Fe ratio of BFO is 1.02 (±0.01) estimated by inductively coupled plasma atomic emission spectrometry. Together with the oxygen deficiencies induced during the sample preparation, the BFO is of n-type. The thicknesses of the heterostructure were determined by cross-sectional scanning electron microscope, shown in supporting information Figure S1. The structure of the films was characterized by X-ray diffraction (XRD) using Cu Kα1 radiation (λ=1.540598Å) (Panalytical X’pert) and scanning transmission electron microscopy (STEM). The high-angle annular dark field (HAADF) STEM was carried out at 200 kV on a JEOL JEM-ARM200F 5



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 6 of 34

equipped with a spherical aberration corrector on the condenser lens system. For the transport measurements, the configuration with current perpendicular to the film plane was employed and the forward bias was defined as a positive voltage applied to the Au top electrodes with 0.3 mm in diameter sputtered on the BFO film surface, as shown in Figure 1(a). The morphology and piezoelectric force microscopy (PFM) phase images of the BFO/LSMO film were measured by atomic force microscope (AFM, Bruker Cor.), which are shown in supporting information Figure S3. The ferroelectric properties were measured by Radiant Technologies Precision Premier II (Radiant Tech., USA). The non-volatility of the tri-state was demonstrated by resistance measurements after writing pulses which were also applied via Radiant Technologies Precision Premier II. The current-voltages (I-V) characteristics at different polarization states were measured by Agilent 5270A semiconductor parameter analyzer and during the measurement we set a current limitation of 0.1 µA to avoid the breakdown of the sample. The magnetoresistances were measured by using a physical property measurement system (PPMS, Quantum Design). All the measurements were carried out at room temperature. 3. RESULTS AND DISCUSSIONS Figure 1(b) shows the XRD patterns of the BFO/LSMO heterostructure grown on STO. Only the (00l) peaks of the films and substrate appear, confirming that the BFO/LSMO heterostructure is epitaxially grown on the STO substrate. The cross section of BFO/LSMO heterostructure is illustrated in Figure 2(a) by atomically resolved images obtained via STEM using an HAADF detector, and the position of the BFO/LSMO interface is indicated by a green line. Within the 6


Page 7 of 34

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


BFO, the Bi atoms appear as the brighter spots forming a rectangular lattice with the Fe located at the center of these rectangles. In LSMO, La (Sr) columns are the brightest with Mn located at the center of their rectangles. The interfacial sharpness is investigated using electron energy loss spectroscopy (EELS). EELS maps taken from the red box of Figure 2(a), with atomic-scale spatial resolution, have been used to determine the chemical species in the BFO/LSMO interface as shown in Figure 2(b). Figure 2(c) shows the integrated intensities corresponding to the Fe-L2,3 (red), Mn-L2,3 (green), and La-M4,5 (yellow) edges. Interestingly, we detect a certain amount of Fe beyond the BFO and Mn beyond the LSMO. This suggests a certain degree of the interdiffusion between Fe and Mn near the interface, which is similar to that reported by Y. M. Kim et al.33 The ferroelectric hysteresis loop (P-V) at 5 kHz and room temperature is shown in Figure 3(a). Note that the P-V hysteresis loop is nearly rectangular with remnant polarization of ∼45 µC/cm2. The remnant polarization is slightly smaller than typical values reported for [001]-oriented BFO films (∼60 µC/cm2), which may result from the leakage charges caused by oxygen vacancies and the existence of pinned domains due to epitaxial strain from LSMO.19 To study the RS behaviors of the heterostructure, the I-Vpulse was measured with the testing pulses composed of a series of write-pulses (w, 0 V→20 V→−20 V→0 V) with different amplitudes and a read-pulse (r, 0.3 V) following each write-pulse as schematically shown in Figure 3(b). The step of the writing pulses is 0.05 V, the time width of the write-and read-pulse are 100 µs and 5 s, respectively. The time width of the read-pulse is long enough to exclude the influence of the ferroelectric reversal current. With the heterostructure pre-polarized upward by a −20 V pulse, the non-volatile RS behavior of the 7



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 8 of 34

heterostructure is demonstrated by the clear hysteretic variation of I-Vpulse curve, as shown in Figure 3(c). The I-Vpulse variation tendency is similar to that of the P-V curve, indicating that the hysteretic loop of the steady-state current is manipulated by the ferroelectric polarization.34 Obviously, the positive pulses set the heterostructure to the low resistance state (LRS) defined as “ON” state, whereas negative pulses switch the heterostructure to the high resistance state (HRS) defined as “OFF” state by the polarization reversal, and the ON/OFF current ratio is about 400%. Besides the two stable resistance states corresponding to the upward and downward polarizations, the most interesting result is that a third resistance state corresponding to the highest current around electric coercive voltage Vc appears. According to the I-Vpulse curve, two important pending issues should be addressed unambiguously. One is to figure out the origins of the ON and OFF states, the other is what happens around the peak near Vc for the heterostructure. To investigate the origins of the ON and OFF states, we measured the current versus voltage (I-V) at the fully downward and upward polarization states, as shown in Figure 3(d). It is notable that the I-V curves do not show obvious rectifying behaviors between −2 V and +2 V in either polarization state. The current in the downward polarization state is always higher than that in the upward polarization state. Similar behaviors have also been reported in other ferroelectric thin-film capacitors.35-36 Obviously, the RS behavior of our sample is different from that of a ferroelectric diode for which the RS effect depends on the polarity of the readout voltage.1, 4, 17, 20-21

8


Page 9 of 34

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Before addressing the detailed physical mechanism of the RS behaviors, it is necessary to investigate the dominant transport mechanism for carriers inside the heterostructure. The possible conduction mechanisms for metal/insulator/metal heterostructures include interface-limited Schottky emission, bulk-limited Poole-Frenkel emission, Fowler-Nordheim tunneling, and space-charge-limited bulk conduction.20,

37-38

The detailed fitting results presented in the

supporting information Figure S4 suggest that all the models for the conduction mechanisms mentioned above seem to be too simple to fully explain the I-V behavior of our heterostructures. Therefore, based on the RS behaviors observed in our sample, the polarization-direction dependences of the asymmetrical electrical characteristics at the Au/BFO and BFO/LSMO interfaces should be considered: one is the Schottky-like barrier manipulated by polarization charges at the top Au/BFO interface, and the other is the ferroelectric field-effect induced interface resistance variations closely related to the interdiffusion of Mn and Fe ions at the bottom BFO/LSMO interface.39 We now discuss the Au/BFO interface first. It is known that the work function of Au is about 5.1 eV, and for BFO the band gap and electron affinity are ~2.8 eV and ~3.3 eV, respectively. The work function of this n-type BFO is about (4.7−δ) eV, where δ is a small value.17, 22 It has been widely reported that an interface layer (IL) often forms at the metal/BiFeO3 interface,19, 36 which is suggested to cover the SCB of Au/BFO and the polarization bound charge as well as corresponding screening charge. The existence of the IL at the Au/BFO interface is proved by the STEM result, as shown in supporting information Figure S2. Then the variations of SCB-field (ESc 9



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 10 of 34

in IL should point to IL from BFO) at the top interface upon polarization reversal should occur in the IL. This IL will separate the screening charges from the metal electrode with the polarization bound charges from the ferroelectric layer, and then a finite potential distribution will form here.36 Figure 4 depicts the schematic charge distributions at the interface layers in different polarization directions. We suppose that the oxygen vacancies ( VO ) and extra Bi will release electrons to compensate the positive boundary charge, while the left positively charged oxygen vacancies ( VO•• or VO• ) will compensate the negative boundary charge at the opposite end.2 Both the released electrons and oxygen vacancies can act as compensation charges just like the electrode charges. For the downward polarization case, as shown in Figure 4(a), the negative bound charges aggregate at the IL on the BFO side and VO•• / VO• are attracted by these negative bound charges at the IL close to Au side, which leads to an electric field EIn pointing from the IL to BFO, resulting in a decrease of ESc,36 in accordance with the I-V at LRS. The enhancement of ESc due to the reversal of EIn together with polarization will result in HRS, as shown in Figure 4(b). It can be seen that at HRS the resistance read by a positive voltage is apparently larger than that read by a negative voltage as shown in Figure 3(d), which is opposite with that of an enhanced pure SCB. This means that just only the manipulated SCB cannot explain the details of the observed RS behaviors. Hence, it is necessary to take the other interface, namely the BFO/LSMO interface, into consideration. Let’s consider the contribution of the bottom interface between BFO and LSMO. The work function of LSMO is about 4.7 eV, similar to that of BFO,25 which could make the BFO/LSMO 10


Page 11 of 34

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


interface show nearly flat-band condition and an quasi-Ohmic contact in the bottom interface. However, a transition region between BFO and LSMO with barrier inevitably occurs due to the obvious ionic interdiffusion occurring at the bottom BFO/LSMO interface as shown in the STEM and EELS results of Figure 2. This barrier can also be manipulated by the ferroelectric polarization reversal, which leads to the resistance change in the vicinity of the bottom interface and cannot be ignored as compared with the resistance of the whole heterostructure.5 As for the ion interdiffusion, we can separate the interdiffusion region between BFO and LSMO into two regions affected by polarization reversal. Considering the Mn beyond the LSMO, which has also been reported by Y. M. Kim et al., it is proved that Mn on the BFO side of the interface for the upward polarization stays in a lower valence state, i.e., the coexistence of Mn2+ and Mn3+, whereas it maintains Mn3+ for the downward polarization.33 While Fe3+ on the LSMO side of the interface will substitute for Mn3+,40-42 which will decrease the ratio Mn3+/Mn4+ and break the Mn-O-Mn network, so that the double exchange coupling between Mn3+ and Mn4+ on the surface of LSMO will be obviously weakened. Then the resistivity of the LSMO part adjacent to the bottom interface will be greatly increased. Therefore, for the upward polarization, on the one hand the interaction between Mn2+ and Fe3+ shows antiferromagnetic and no electron hopping occurs because of their both half full 3d shell,43 on the other hand besides Fe3+ substitution for Mn3+, the ratio Mn3+/Mn4+ will further decrease due to the hole accumulation by ferroelectric screening making a considerable amount of Mn3+ oxidized into Mn4+, corresponding to a higher interface resistance state.44 Combining the 11



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 12 of 34

contribution of Au/BFO, then the HRS appears for the upward polarized state as a result. While for the downward polarization, the positive carriers are repelled away by the positive bound charges at the bottom interface in which Mn4+ can be changed to Mn3+, and thus the ratio Mn3+/Mn4+ increases, resulting the decrease of the resistance at the interface.44 In addition, there is no Mn2+ appearing in BFO near the bottom interface in the downward polarized state.33 It implies that the resistances in LSMO and BFO near the bottom interface in the downward polarization state are both less than those in the upward polarization state. Therefore, considering the contributions of interface of BFO/LSMO together with that of Au/BFO in the downward polarized state, LRS will appear. To summarize, the deviation of fitting parameters of I-V for the all conduction mechanisms imply that besides SCB and IL of Au/BFO, the interface of BFO/LSMO also plays an important role in the RS behavior. We now turn towards discussing the behaviors of the conductivity peaks around Vc, which have rarely been reported in previous results related to the RS of BFO.32 Compared with a fully switched state, there is a great increase of the current when the polarization approaches to zero. In addition, the value of the negative pulse voltage (about −8.6 V) corresponding to the peak is smaller than that of the positive pulse voltage (about +10 V), as shown in Figure 3(c). This can be explained in terms of the RS behavior and the different nucleation dynamics of domains with opposite polarization reversal: when the polarization state of the heterostructure changes from the upward to downward, the resistances of the top and bottom interfaces should both alter from the higher state to the lower state. That is to say, before the polarization reversal from the upward, the 12


Page 13 of 34

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


two interfaces will share more partial voltage from the applied voltage, then when applying voltage to reverse polarization state, larger voltage should be needed, resulting in the large value of Vc in this process and vice versa.45 From the PFM data as illustrated in S3 of supporting information, we can justify that the as-grown polarization prefers upward. This makes the process of reversing polarization from upward to downward harder too, leading to a larger Vc and wider current peak. As for the higher current peak around the negative Vc, because the applied read-pulse is invariable, thus near the positive peak the bulk BFO will obtain less partial voltage than that near the negative one, resulting the lower peak in the positive process. In fact, the conductivity peaks should be closely related to the high population and maximal effective length of the high conductive ferroelectric DWs near Vc.31 This enhanced conductivity of DWs has been explained by the following combined three factors related to the structural changes at DWs:11, 29, 46 (1) An increased carrier density (the accumulation of oxygen vacancies is most possible)29, 46 as a consequence of the electrostatic potential step at DWs; (2) A decrease in the band gap within DWs;47 (3) The possible presence of accumulated oxygen vacancies in DWs should also produce electron doping and some Fe3+ ions changing into Fe2+. The double-exchange interaction between the nearest-neighbor Fe3+ and Fe2+ ions through oxygen bridges can cause a high electrical conductivity in DWs.48-50 In this case, the applied magnetic fields will result in the alignment of both the localized and conduction electron spins, decrease the scattering probability of spins and thus give rise to an increase of the conductivity, i.e., the appearance of negative MR,49-51 which has been realized in BFO//DyScO3 (110) heterostructures below 200K.30 13



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 14 of 34

For the purpose of further verifying this high conductance arising from the oxygen vacancies and double-exchange interaction between Fe3+ and Fe2+ in the DWs, we measured the I-Vpulse in different magnetic fields. It can be seen from Figure 5(a) that the peak height increases with increasing magnetic fields. The variations of the resistance ∆R=R(H)−R(0) as function of writing pulse voltages in different magnetic fields show a negative MR effect as shown in Figure 5(b), which further demonstrates the origin of the peak. It should be pointed out that this negative MR effect is mainly due to the response of ferroelectric DWs of BFO rather than LSMO film. The contribution from BFO or LSMO to the negative MR is discussed in S5 of supporting information. From the viewpoint of application, the peak current arising from DWs is so distinguishable from the ON and OFF currents that we can combine it with the other two stable current states to realize tri-state-like RS device with appropriate programming pulse voltages, where the peak current can be denoted as a third state. With the purpose of confirming the stability of the tri-state resistive currents for non-volatile random access memories, the steady currents read at ±0.3V in three different polarization states for 20 cycles are summarized in Figures 6(a) and (b), respectively, demonstrating the good reproductivity of the three non-volatile resistance states. 4. CONCLUSION In summary, we report a tri-state RS behaviors manipulated by electric and magnetic fields for the Au/BFO/LSMO heterostructure. By changing the direction of the ferroelectric polarization, the resistances of the heterostructure can be switched among three stable states without an electroforming process. It is found that the non-volatile RS behaviors in our sample are attributed 14


Page 15 of 34

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


to co-contributions of Schottky-like barrier in the Au/BFO interface, the resistance changes of BFO/LSMO interface, and the domain wall density manipulated by polarizations. Most interestingly, the room-temperature current peaks near Vc owning to the high conductivity of ferroelectric DWs prove the negative MR properties of ferroelectric DWs macroscopically. The above findings are helpful for us to understand the effects of interfaces and the ferromagnetism of ferroelectric DWs in multiferroic heterostructures on the non-volatile RS behaviors, which may give a possibility for combining RS device with spintronics.

ASSOCIATED CONTENT Supporting Information Details regarding the cross-sectional SEM of the Au/BFO/LSMO heterostructure, STEM result at the Au/BFO interface, AFM and PFM images of the BFO/LSMO film, additional I-V data fittings by possible models for conduction mechanisms, and the analysis of negative MR effects of BFO and LSMO. This material is available free of charge via the Internet at http://pubs.acs.org.

AUTHOR INFORMATION Corresponding Author *E-mail: [email protected].

ACKNOWLEDGEMENTS

15



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 16 of 34

This work is supported by NSFC (51332007, 21521001), NBRPC (2012CB922003, 2015CB921201), FRFCU (WK2030020026) and CPSF (2014T70590).

16


Page 17 of 34

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


REFERENCES

(1) Choi, T.; Lee, S.; Choi, Y. J.; Kiryukhin, V.; Cheong, S. W., Switchable Ferroelectric Diode and Photovoltaic Effect in BiFeO3. Science 2009, 324 (5923), 63-66. (2) Yuan, G.; Wang, J., Evidences for the Depletion Region Induced by the Polarization of Ferroelectric Semiconductors. Appl. Phys. Lett. 2009, 95 (25), 252904. (3) Yi, H. T.; Choi, T.; Choi, S. G.; Oh, Y. S.; Cheong, S. W., Mechanism of the Switchable Photovoltaic Effect in Ferroelectric BiFeO3. Adv. Mater. 2011, 23 (30), 3403-3407. (4) Jiang, A. Q.; Wang, C.; Jin, K. J.; Liu, X. B.; Scott, J. F.; Hwang, C. S.; Tang, T. A.; Lu, H. B.; Yang, G. Z., A Resistive Memory in Semiconducting BiFeO3 Thin-Film Capacitors. Adv. Mater. 2011, 23 (10), 1277-1281. (5) Zhuravlev, M.; Sabirianov, R.; Jaswal, S.; Tsymbal, E., Giant Electroresistance in Ferroelectric Tunnel Junctions. Phys. Rev. Lett. 2005, 94 (24), 246802. (6) Garcia, V.; Fusil, S.; Bouzehouane, K.; Enouz-Vedrenne, S.; Mathur, N. D.; Barthelemy, A.; Bibes, M., Giant Tunnel Electroresistance for non-Destructive Readout of Ferroelectric States. Nature 2009, 460 (7251), 81-84. (7) Pantel, D.; Goetze, S.; Hesse, D.; Alexe, M., Reversible Electrical Switching of Spin Polarization in Multiferroic Tunnel Junctions. Nat. Mater. 2012, 11 (4), 289-293. (8) Maksymovych, P.; Jesse, S.; Yu, P.; Ramesh, R.; Baddorf, A. P.; Kalinin, S. V., Polarization Control of Electron Tunneling into Ferroelectric Surfaces. Science 2009, 324 (5933), 1421-1425.

17



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 18 of 34

(9) Chanthbouala, A.; Crassous, A.; Garcia, V.; Bouzehouane, K.; Fusil, S.; Moya, X.; Allibe, J.; Dlubak, B.; Grollier, J.; Xavier, S.; Deranlot, C.; Moshar, A.; Proksch, R.; Mathur, N. D.; Bibes, M.; Barthelemy, A., Solid-State Memories Based on Ferroelectric Tunnel Junctions. Nat. Nanotechnol. 2012, 7 (2), 101-104. (10) Yin, Y. W.; Burton, J. D.; Kim, Y. M.; Borisevich, A. Y.; Pennycook, S. J.; Yang, S. M.; Noh, T. W.; Gruverman, A.; Li, X. G.; Tsymbal, E. Y.; Li, Q., Enhanced Tunnelling Electroresistance Effect due to a Ferroelectrically Induced Phase Transition at a Magnetic Complex Oxide Interface. Nat. Mater. 2013, 12 (5), 397-402. (11) Seidel, J.; Martin, L. W.; He, Q.; Zhan, Q.; Chu, Y. H.; Rother, A.; Hawkridge, M. E.; Maksymovych, P.; Yu, P.; Gajek, M.; Balke, N.; Kalinin, S. V.; Gemming, S.; Wang, F.; Catalan, G.; Scott, J. F.; Spaldin, N. A.; Orenstein, J.; Ramesh, R., Conduction at Domain Walls in Oxide Multiferroics. Nat. Mater. 2009, 8 (3), 229-234. (12) Choi, T.; Horibe, Y.; Yi, H. T.; Choi, Y. J.; Wu, W.; Cheong, S. W., Insulating Interlocked Ferroelectric and Structural Antiphase Domain Walls in Multiferroic YMnO3. Nat. Mater. 2010, 9 (3), 253-258. (13) Maksymovych, P.; Seidel, J.; Chu, Y. H.; Wu, P.; Baddorf, A. P.; Chen, L.; Kalinin, S. V.; Ramesh, R., Dynamic Conductivity of Ferroelectric Domain Walls in BiFeO3. Nano Lett. 2011, 11 (5), 1906-1912.

18


Page 19 of 34

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(14) Vasudevan, R. K.; Morozovska, A. N.; Eliseev, E. A.; Britson, J.; Yang, J. C.; Chu, Y. H.; Maksymovych, P.; Chen, L. Q.; Nagarajan, V.; Kalinin, S. V., Domain Wall Geometry Controls Conduction in Ferroelectrics. Nano Lett. 2012, 12 (11), 5524-5531. (15) Vasudevan, R. K.; Wu, W.; Guest, J. R.; Baddorf, A. P.; Morozovska, A. N.; Eliseev, E. A.; Balke, N.; Nagarajan, V.; Maksymovych, P.; Kalinin, S. V., Domain Wall Conduction and Polarization-Mediated Transport in Ferroelectrics. Adv. Funct. Mater. 2013, 23 (20), 2592-2616. (16) Pintilie, I.; Teodorescu, C. M.; Ghica, C.; Chirila, C.; Boni, A. G.; Hrib, L.; Pasuk, I.; Negrea, R.; Apostol, N.; Pintilie, L., Polarization-Control of the Potential Barrier at the Electrode Interfaces in Epitaxial Ferroelectric Thin Films. ACS Appl. Mater. Interfaces 2014, 6 (4), 2929-2939. (17) Hong, S.; Choi, T.; Jeon, J. H.; Kim, Y.; Lee, H.; Joo, H. Y.; Hwang, I.; Kim, J. S.; Kang, S. O.; Kalinin, S. V.; Park, B. H., Large Resistive Switching in Ferroelectric BiFeO3 Nano-Island Based Switchable Diodes. Adv. Mater. 2013, 25 (16), 2339-2343. (18) Chen, D. X.; Li, A. D.; Wu, D., Resistive Switching in BiFeO3-based Heterostructures due to Ferroelectric Modulation on Interface Schottky Barriers. J. Mater. Sci.: Mater. Electron. 2014, 25 (8), 3251-3256. (19) Tsurumaki, A.; Yamada, H.; Sawa, A., Impact of Bi Deficiencies on Ferroelectric Resistive Switching Characteristics Observed at p-Type Schottky-Like Pt/Bi1-δFeO3 Interfaces. Adv. Funct. Mater. 2012, 22 (5), 1040-1047.

19



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 20 of 34

(20) Lee, D.; Baek, S. H.; Kim, T. H.; Yoon, J. G.; Folkman, C. M.; Eom, C. B.; Noh, T. W., Polarity Control of Carrier Injection at Ferroelectric/Metal Interfaces for Electrically Switchable Diode and Photovoltaic Effects. Phys. Rev. B 2011, 84 (12), 125305. (21) Wang, C.; Jin, K.; Xu, Z.; Wang, L.; Ge, C.; Lu, H.; Guo, H.; He, M.; Yang, G., Switchable Diode Effect and Ferroelectric Resistive Switching in Epitaxial BiFeO3 Thin Films. Appl. Phys. Lett. 2011, 98 (19), 192901. (22) Yang, H.; Luo, H. M.; Wang, H.; Usov, I. O.; Suvorova, N. A.; Jain, M.; Feldmann, D. M.; Dowden, P. C.; DePaula, R. F.; Jia, Q. X., Rectifying Current-Voltage Characteristics of BiFeO3/Nb-doped SrTiO3 Heterojunction. Appl. Phys. Lett. 2008, 92 (10), 102113. (23) Choi, T.; Lee, S.; Choi, Y. J.; Kiryukhin, V.; Cheong, S. W., Switchable Ferroelectric Diode and Photovoltaic Effect in BiFeO3. Science 2009, 324 (5923), 63-66. (24) Jiménez, D.; Miranda, E.; Tsurumaki-Fukuchi, A.; Yamada, H.; Suñé, J.; Sawa, A., Multilevel Recording in Bi-Deficient Pt/BFO/SRO Heterostructures Based on Ferroelectric Resistive Switching Targeting High-Density Information Storage in Nonvolatile Memories. Appl. Phys. Lett. 2013, 103 (26), 263502. (25) Fang, L.; You, L.; Zhou, Y.; Ren, P.; Shiuh Lim, Z.; Wang, J., Switchable Photovoltaic Response from Polarization Modulated Interfaces in BiFeO3 Thin Films. Appl. Phys. Lett. 2014, 104 (14), 142903.

20


Page 21 of 34

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(26) Zhou, Y.; Zou, X.; You, L.; Guo, R.; Shiuh Lim, Z.; Chen, L.; Yuan, G.; Wang, J., Mechanism of Polarization Fatigue in BiFeO3: The Role of Schottky Barrier. Appl. Phys. Lett. 2014, 104 (1), 012903. (27) Yu, P.; Lee, J. S.; Okamoto, S.; Rossell, M. D.; Huijben, M.; Yang, C. H.; He, Q.; Zhang, J. X.; Yang, S. Y.; Lee, M. J.; Ramasse, Q. M.; Erni, R.; Chu, Y. H.; Arena, D. A.; Kao, C. C.; Martin, L. W.; Ramesh, R., Interface Ferromagnetism and Orbital Reconstruction in BiFeO3/La0.7Sr0.3MnO3 Heterostructures. Phys. Rev. Lett. 2010, 105 (2), 027201. (28) Ma, J.; Hu, J. M.; Li, Z.; Nan, C. W., Recent Progress in Multiferroic Magnetoelectric Composites: from Bulk to Thin Films. Adv. Mater. 2011, 23 (9), 1062-1087. (29) Farokhipoor, S.; Noheda, B., Conduction through 71° Domain Walls in BiFeO3 Thin Films. Phys. Rev. Lett. 2011, 107 (12), 127601. (30) He, Q.; Yeh, C. H.; Yang, J. C.; Singh-Bhalla, G.; Liang, C. W.; Chiu, P. W.; Catalan, G.; Martin, L. W.; Chu, Y. H.; Scott, J. F.; Ramesh, R., Magnetotransport at Domain Walls in BiFeO3. Phys. Rev. Lett. 2012, 108 (6), 067203. (31) Lee, J. H.; Fina, I.; Marti, X.; Kim, Y. H.; Hesse, D.; Alexe, M., Spintronic Functionality of BiFeO3 Domain Walls. Adv. Mater. 2014, 26 (41), 7078-7082. (32) Gao, R. L.; Chen, Y. S.; Sun, J. R.; Zhao, Y. G.; Li, J. B.; Shen, B. G., Complext Tansport Behavior Accompanying Domain Switching in La0.1Bi0.9FeO3 Sandwiched Capacitors. Appl. Phys. Lett. 2012, 101 (15), 152901.

21



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 22 of 34

(33) Kim, Y. M.; Morozovska, A.; Eliseev, E.; Oxley, M. P.; Mishra, R.; Selbach, S. M.; Grande, T.; Pantelides, S. T.; Kalinin, S. V.; Borisevich, A. Y., Direct Observation of Ferroelectric Field Effect and Vacancy-Controlled Screening at the BiFeO3/LaxSr1−xMnO3 Interface. Nat. Mater. 2014, 13 (11), 1019-1025. (34) Yao, Y. P.; Liu, Y. K.; Dong, S. N.; Yin, Y. W.; Yang, S. W.; Li, X. G., Multi-state Resistive

Switching

Memory

with

Secure

Information

Storage

in

Au/BiFe0.95Mn0.05O3/La5/8Ca3/8MnO3 Heterostructure. Appl. Phys. Lett. 2012, 100 (19), 193504. (35) Chen, X.; Jia, C. H.; Chen, Y. H.; Yang, G.; Zhang, W. F., Ferroelectric Memristive Effect in BaTiO3 Epitaxial Thin Films. J. Phys. D: Appl. Phys. 2014, 47 (36), 365102. (36) Meyer, R.; Contreras, J. R.; Petraru, A.; Kohlstedt, H., On a Novel Ferro Resistive Random Access Memory (FRRAM): Basic Model and First Experiments. Integr. Ferroelectr. 2004, 64 (1), 77-88. (37) Rana, A.; Lu, H.; Bogle, K.; Zhang, Q.; Vasudevan, R.; Thakare, V.; Gruverman, A.; Ogale, S.; Valanoor, N., Scaling Behavior of Resistive Switching in Epitaxial Bismuth Ferrite Heterostructures. Adv. Funct. Mater. 2014, 24 (25), 3962-3969. (38) Chen, Z. H.; He, L.; Zhang, F.; Jiang, J.; Meng, J. W.; Zhao, B. Y.; Jiang, A. Q., The Conduction Mechanism of Large on/off Ferroelectric Diode Currents in Epitaxial (111) BiFeO3 Thin Film. J. Appl. Phys. 2013, 113 (18), 184106. (39) You, T.; Du, N.; Slesazeck, S.; Mikolajick, T.; Li, G.; Bürger, D.; Skorupa, I.; Stöcker, H.; Abendroth, B.; Beyer, A.; Volz, K.; Schmidt, O. G.; Schmidt, H., Bipolar Electric-Field Enhanced 22


Page 23 of 34

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Trapping and Detrapping of Mobile Donors in BiFeO3 Memristors. ACS Appl. Mater. Interfaces 2014, 6 (22), 19758-19765. (40) Souza Filho, A. G.; Faria, J. L. B.; Guedes, I.; Sasaki, J. M.; Freire, P. T. C.; Freire, V. N.; Mendes Filho, J.; Xavier, M. M.; Cabral, F. A. O.; de Araújo, J. H.; da Costa, J. A. P., Evidence of Magnetic Polaronic States in La0.7Sr0.3Mn1-xFexO3 Manganites. Phys. Rev. B 2003, 67 (5), 052405. (41) Ahn, K. H.; Wu, X. W.; Liu, K.; Chien, C. L., Magnetic Properties and Colossal Magnetoresistance of La(Ca)MnO3 Materials Doped with Fe. Phys. Rev. B 1996, 54 (21), 15299-15302. (42) Xavier, M. M.; Cabral, F. A. O.; de Araújo, J. H.; Chesman, C.; Dumelow, T., Magnetic and Transport Properties of Polycrystalline La0.7Sr0.3Mn1-xFexO3. Phys. Rev. B 2000, 63 (1), 012408. (43) Coey, J. M. D.; Venkatesan, M.; Fitzgerald, C. B., Donor Impurity Band Exchange in Dilute Ferromagnetic Oxides. Nat. Mater. 2005, 4 (2), 173-179. (44) Jiang, L.; Choi, W. S.; Jeen, H.; Dong, S.; Kim, Y.; Han, M. G.; Zhu, Y. M.; Kalinin, S. V.; Dagotto, E.; Egami, T.; Lee, H. N., Tunneling Electroresistance Induced by Interfacial Phase Transitions in Ultrathin Oxide Heterostructures. Nano Lett. 2013, 13 (12), 5837-5843. (45) Liu, Y. K.; Yao, Y. P.; Dong, S. N.; Yang, S. W.; Li, X. G., Effect of Magnetic Field on Ferroelectric Properties of BiFeO3/La5/8Ca3/8MnO3 Epitaxial Heterostructures. Phys. Rev. B 2012, 86 (7), 075113. (46) Seidel, J.; Maksymovych, P.; Batra, Y.; Katan, A.; Yang, S. Y.; He, Q.; Baddorf, A. P.; Kalinin, S. V.; Yang, C. H.; Yang, J. C.; Chu, Y. H.; Salje, E. K. H.; Wormeester, H.; Salmeron, 23



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 24 of 34

M.; Ramesh, R., Domain Wall Conductivity in La-Doped BiFeO3. Phys. Rev. Lett. 2010, 105 (19), 197603. (47) Catalan, G.; Scott, J. F., Physics and Applications of Bismuth Ferrite. Adv. Mater. 2009, 21 (24), 2463-2485. (48) Rosencwa, A., Double Exchange and Electron Hopping in Magnetite. Can. J. Phys. 1969, 47 (21), 2309-2317. (49) Kostopou, D., Magnetoresistance of Magnetite. Phys. Status Solidi A 1972, 9 (2), 523-527. (50) Zhou, Y.; McEvoy, C.; Ramos, R.; Shvets, I. V., The Magnetic and Magnetoresistance Properties of Ultrathin Magnetite Films Grown on MgO Substrate. J. Appl. Phys. 2006, 99 (8), 08J111. (51) Toyozawa, Y., Theory of Localized Spins and Negative Magnetoresistance in the Metallic Impurity Conduction. J. Phys. Soc. Jpn. 1962, 17 (6), 986-1004.

24


Page 25 of 34

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Figure captions Figure 1. (a) Schematic illustration for the measuring configuration of the sample. (b) XRD pattern of the BFO/LSMO heterostructure grown on STO substrate. Figure 2. (a) A typical STEM-HAADF image of BFO/LSMO interface, showing the good epitaxial properties of the heterostructure. (b) HAADF images and EELS maps of the BFO/LSMO interface taken from the red box of (a), including the Fe-L2,3, Mn-L2,3 and La-M4,5 edges. (c) Elemental profiles obtained from the EELS line scan across the BFO/LSMO interface. Figure 3. (a) Remanent P-V hysteresis loop of the heterostructure with f = 5 kHz at room temperature. (b) Schematical pulse train for the I-Vpulse loop. (c) Steady-state current measured as a function of 100 µs writing pulses using the pulse train shown schematically in (b). The corresponding domain structures measured by the out of plane piezoelectric atomic force microscope are shown in the middle-right, bottom-left and top-left insets for the ON, OFF and zero polarized states, respectively. (d) I-V curves measured after applying Vp = ±20V. Figure 4. Schematic charge distributions in the interface layers of the heterostructures: (a) at LRS for the downward polarization, and (b) at HRS for the upward polarization. Figure 5. (a) Steady-state current reading with +0.3 V as a function of 100 µs writing pulse voltages in different magnetic fields, the inset shows the enlarged picture around the peaks. (b) Magnetoresistance-pulse voltage curves at different magnetic fields

25



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 26 of 34

Figure 6. (a) and (b) Tri-state currents read at +0.3 V and −0.3 V for 20 cycles, respectively. The insets schematically show the corresponding domain structures of downward, upward and zero polarized states, respectively.

26


Page 27 of 34

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


For Table of Contents only

Abstract Graphic: Up: Steady-state current measured as a function writing pulses. The corresponding domain evolutions are schematically shown for the three different states. The inset is the enlarged picture around the left peak in different magnetic fields, showing a negative magnetoresistance. Down: HAADF images and electron energy loss spectroscopy maps of the BFO/LSMO interface, suggesting a certain degree of interdiffusion between Fe and Mn around of the interface.

27



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Figure 1. (a)Schematic illustration for the measuring configuration of the sample. (b) XRD pattern of the BFO/LSMO heterostructure grown on STO substrate. 315x189mm (300 x 300 DPI)


Page 28 of 34

Page 29 of 34

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Figure 2. (a) A typical STEM-HAADF image of BFO/LSMO interface, showing the good epitaxial properties of the heterostructure. (b) HAADF images and EELS maps of the BFO/LSMO interface taken from the red box of (a), including the Fe-L2,3, Mn-L2,3 and La-M4,5 edges. (c) Elemental profiles obtained from the EELS line scan across the BFO/LSMO interface. 363x165mm (300 x 300 DPI)



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Figure 3. (a) Remanent P-V hysteresis loop of the heterostructure with f = 5 kHz at room temperature. (b) Schematical pulse train for the I-Vpulse loop. (c) Steady-state current measured as a function of 100 µs writing pulses using the pulse train shown schematically in (b). The corresponding domain structures measured by the out of plane piezoelectric atomic force microscope are shown in the middle-right, bottomleft and top-left insets for the ON, OFF and zero polarized states, respectively. (d) I-V curves measured after applying Vp = ±20V. 406x406mm (300 x 300 DPI)


Page 30 of 34

Page 31 of 34

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Figure 4. Schematic charge distributions in the interface layers of the heterostructures: (a) at LRS for the downward polarization, and (b) at HRS for the upward polarization. 144x87mm (300 x 300 DPI)



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Figure 5. (a) Steady-state current reading with +0.3 V as a function of 100 µs writing pulse voltages in different magnetic fields, the inset shows the enlarged picture around the peaks. (b) Magnetoresistancepulse voltage curves at different magnetic fields 178x327mm (300 x 300 DPI)


Page 32 of 34

Page 33 of 34

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Figure 6. (a) and (b) Tri-state currents read at +0.3 V and −0.3 V for 20 cycles, respectively. The insets schematically show the corresponding domain structures of downward, upward and zero polarized states, respectively. 318x152mm (300 x 300 DPI)



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Abstract Graphic: Up: Steady-state current measured as a function writing pulses. The corresponding domain evolutions are schematically shown for the three different states. The inset is the enlarged picture around the left peak in different magnetic fields, showing a negative magnetoresistance. Down: HAADF images and electron energy loss spectroscopy maps of the BFO/LSMO interface, suggesting a certain degree of interdiffusion between Fe and Mn around of the interface. 48x86mm (300 x 300 DPI)


Page 34 of 34

Effects of Interface Layers and Domain Walls on the Ferroelectric

Recommend Documents