Unusual Behaviors of Electric-Field Control of Magnetism in

Jun 20, 2019 - Electric-field control of magnetism (EFCM) is very important for the exploration of high-density, fast, and nonvolatile random-access m...
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Research Article Cite This: ACS Appl. Mater. Interfaces 2019, 11, 25569−25577

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Unusual Behaviors of Electric-Field Control of Magnetism in Multiferroic Heterostructures via Multifactor Cooperation Ce Feng,†,‡ Yan Liu,†,‡,⊥ Haoliang Huang,§ Zhaozhao Zhu,∥ Yuanjun Yang,# You Ba,†,‡ Shuying Yan,¶ Jianwang Cai,∥ Yalin Lu,§ Jinxing Zhang,¶ Sen Zhang,*,∇ and Yonggang Zhao*,†,‡

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Department of Physics and State Key Laboratory of Low-Dimensional Quantum Physics, Tsinghua University, Beijing 100084, China ‡ Collaborative Innovation Center of Quantum Matter, Beijing 100084, China § National Synchrotron Radiation Laboratory, University of Science and Technology of China, Hefei 230026, China ∥ Beijing National Laboratory for Condensed Matter Physics, Institute of Physics, Chinese Academy of Sciences, Beijing 100190, China ⊥ Key Laboratory of Space Utilization, Technology and Engineering Center for Space Utilization, Chinese Academy of Sciences, Beijing 100094, China # School of Electronic Science and Applied Physics, Hefei University of Technology, Hefei 230009, China ¶ Department of Physics, Beijing Normal University, Beijing 100875, China ∇ College of Science, National University of Defense Technology, Changsha 410073, China S Supporting Information *

ABSTRACT: Electric-field control of magnetism (EFCM) is very important for the exploration of high-density, fast, and nonvolatile random-access memory with ultralow energy consumption. Here, we report the electric-field-induced ferroelectric phase transitions in Pb(Mg1/3Nb2/3)0.82Ti0.18O3 (PMN-0.18PT) and symmetry breaking of EFCM behaviors for corresponding directions in multiferroic heterostructures composed of amorphous ferromagnetic Co40Fe40B20 (CoFeB) and PMN-0.18PT. We uncover a new mechanism behind the unusual phenomena, involving coupling between CoFeB and PMN-0.18PT via complex cooperation of electric-field-induced ferroelectric phase transitions, competition of different ferroelectric domains, and internal electric field in PMN-0.18PT. The deterministic EFCM with reversible and nonvolatile nature opens up a new avenue for exploring EFCM in multiferroic heterostructures and is also significant for applications. KEYWORDS: electric-field control of magnetism, multiferroic heterostructures, electric-field-induced ferroelectric phase transition, ferroelectric domain ratio, internal field

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density modulation,8 and strain-mediated effect.9−15 Among them, the strain-mediated EFCM has attracted much attention because of its remarkable ME coupling which is of vital importance for applications.9 PMN-xPT around the morphotropic phase boundary (MPB) with 0.31 ≤ x ≤ 0.3716−18 have played important roles in the study of FM/FE multiferroic heterostructures9−15 because of their excellent piezoelectric properties.19 Wu et al. reported a reversible and nonvolatile EFCM in the Ni/PMN-0.30PT(011) heterostructure via remanent strain originating from a non-180° FE polarization reorientation when operating an FE substrate in a specific electric field sweeping with one end larger than the electric coercive field (full FE domain switching) and the other just

INTRODUCTION Electric-field control of magnetism (EFCM) is very important for the exploration of high-density, fast, and nonvolatile random-access memory with reduced energy consumption, which is a long sought-after goal in the Information Age. Multiferroic materials, exhibiting both ferroelectricity and magnetism, are promising for achieving this target.1,2 Nevertheless, single-phase multiferroic materials with a large magnetoelectric (ME) coupling effect at room temperature (RT) are rare,3 so multiferroic heterostructures composed of ferromagnetic (FM) and ferroelectric (FE) materials have been widely investigated owing to their various choices of materials with RT performance.4 Even heterostructures combining single-phase multiferroic materials with FE, FM, and superconducting materials, respectively, have also attracted much attention.5 EFCM in multiferroic heterostructures can be realized via exchange coupling,6,7 interface charge carrier © 2019 American Chemical Society

Received: April 14, 2019 Accepted: June 20, 2019 Published: June 20, 2019 25569

DOI: 10.1021/acsami.9b06532 ACS Appl. Mater. Interfaces 2019, 11, 25569−25577

Research Article

ACS Applied Materials & Interfaces

Figure 1. Schematic of the sample configuration and electric-field-controlled magnetizations. (a) Schematic of the sample structure and experimental configuration. (b) Magnetic hysteresis loops under electric fields of −0 kV/cm (blue dot) and +8 kV/cm (red square) along the [110] direction. The inset is the loops with a larger scale. (c) M−E curves for the [010], [−110], [100], and [110] directions. (d) Curve of polarization current vs electric field.

below the electric coercive field (half-way FE domain switching),12 which is disadvantageous for applications. In our previous work, we realized another type of reversible and nonvolatile EFCM with loop-like magnetization−electric-field (M−E) curves in the CoFeB/PMN-0.30PT(001) heterostructure under bipolar electric fields. In this case, the 90° rotation of in-plane-strain induced by the net 109° rhombohedral (R) phase FE domain switching in PMN0.30PT leads to the rotation of magnetic easy axis of CoFeB via the change of strain-generated magnetoelastic energy. Therefore, the complementary strains along the [110] and [−110] directions result in the complementary converse M−E curves, namely antisymmetric behavior, along these two directions as shown by experiments.13 Afterwards, the looplike and butterfly-like M−E behaviors corresponding to the net 109° and 71/180° R phase FE domain switching, respectively, were demonstrated,14,15 suggesting EFCM is not deterministic in this case, which is disadvantageous for applications. Hence, deterministic EFCM with reversible and nonvolatile nature is highly desired. So far, for EFCM in FM/PMN-xPT, all the previous reports focused on PMN-xPT with x around 0.30. Recently, we studied EFCM in CoFeB/PMN-xPT heterostructures with x = 0.18 (outside the MPB region) and revealed a remarkable loop-like nonvolatile M−E behavior along the [1−10] direction in addition to a giant increase of magnetization at the macro−micro domain transition temperature (this transition is unique for the PMN-xPT with a smaller values of x).20 This work demonstrated that CoFeB/PMN-xPT with x outside the MPB region can also show a remarkable loop-like nonvolatile M−E behavior. However, the mechanism is still elusive. Very recently, theoretical calculations by Wu et al. suggested that the electric-field-induced rhombohedral− monoclinic−tetragonal (R−MA−T) FE phase transitions can be realized at RT in PMN-0.18PT with attainable electric fields whereas FE phase transition for PMN-xPT near the MPB can only occur above RT.21 So far, there has been no experiment demonstration for this electric-field-induced R−MA−T FE

phase transition in PMN-0.18PT, which may provide a good chance to explore unusual phenomena in EFCM in FM/PMN0.18PT heterostructures. In this paper, we report the electric-field-induced FE phase transition in PMN-0.18PT by using X-ray diffraction reciprocal space mapping (XRD-RSM) technique. Moreover, the breaking of complementary symmetry on the EFCM behaviors for corresponding directions, which has never been reported for FM/FE heterostructures, is observed for CoFeB/PMN0.18PT(001) because of cooperation of different contributions. The magnetizations show remarkable changes with the electric field, revealing loop-like and butterfly-like M−E curves along the [110] and [−110] directions, respectively. The absence of complementary M−E curves or symmetry breaking along these two directions is observed in contrast with that of CoFeB/ PMN-0.30PT(001), which showed complementary converse (antisymmetric) M−E curves.13 For the [100] and [010] directions, the magnetizations also show remarkable changes with the electric field, revealing similar loop-like M−E curves in contrast to the butterfly-like M−E curves along these two directions in CoFeB/PMN-0.30PT(001).14 These unusual behaviors of EFCM strongly suggest a new mechanism. Through systematic investigations of magnetic anisotropy, FE domains, and crystal structures under in situ electric fields, we demonstrate that the unusual phenomena are related to the cooperation of electric-field-induced R−MA−T FE phase transitions, internal electric field in PMN-0.18PT, and the competition of different FE domains. This work shows deterministic EFCM with reversible and nonvolatile nature in CoFeB/PMN-0.18PT(001) and is significant for exploring EFCM in FM/FE multiferroic heterostructures and applications.

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RESULTS AND DISCUSSION FM/FE heterostructures composed of amorphous CoFeB films and (001)-cut PMN-0.18PT substrates were prepared by magnetron sputtering and various techniques were utilized to 25570

DOI: 10.1021/acsami.9b06532 ACS Appl. Mater. Interfaces 2019, 11, 25569−25577

Research Article

ACS Applied Materials & Interfaces

Figure 2. ESR measurements of the in-plane magnetic anisotropies under in situ electric fields. (a) Configuration of the measurement directions and the definition of in-plane rotation angles φ for the case of −0 kV/cm, showing two kinds of FM domains with perpendicular in-plane magnetic easy axes (FE-EA) induced by strains of FE. (b) ESR signal dP/dH(φ,H) mapping for the case of −0 kV/cm. The resonance magnetic fields (Hr) are outlined by the black line for the main peaks and the red line for the weak peaks. (c) In-plane Hr obtained for the case of −0 kV/cm. The black squares are for the Hr of the main peaks and the red dots for the Hr of the weak peaks, guided by the smooth lines fitted with the uniaxial magnetic anisotropy model. (d) Configuration of the sample for the case of +8 kV/cm with the absence of magnetic anisotropy because of the in-plane isotropic strain of the T phase. (e) ESR signal dP/dH(φ,H) mapping for the case of +8 kV/cm with Hr outlined by the blue line. (f) In-plane Hr obtained for the case of +8 kV/cm at various angles, marked with the blue triangles connected with a smooth line.

measure the properties of the samples (see details in the Methods section). Figure 1a shows the schematic configuration of the sample. In all measurements, the positive electric field is downward. The magnetic hysteresis (M−H) loops were measured along the [110] direction under −0 kV/cm (after being poled by −8 kV/cm and the same in the following text) and +8 kV/cm (Figure 1b). It indicates that a positive electric field increases the magnetization in a certain range of magnetic field although the saturation magnetization and coercive field remain the same, suggesting that a positive electric field makes the FM film more easily magnetized along this direction. In contrast, the M−H loops are almost unchanged along the [−110] direction under −0 and +8 kV/cm (Figure S1, Supporting Information). The reason for choosing these electric fields to measure the M−H loops will be mentioned later. To explore the correlation between the change of magnetization and electric field directly, we measured the M− E curves along different directions (Figure 1c) with a 5 Oe magnetic field applied along the corresponding direction. As shown in Figure 1c, the M−E curves exhibit a loop-like behavior along the [110] direction and a butterfly-like behavior along the [−110] direction in contrast to the complementary converse loop-like M−E curves along these two directions in the CoFeB/PMN-0.30PT heterostructure.13 Moreover, for the [100] and [010] directions, the magnetizations show remarkable changes with the electric field, exhibiting a similar loop-like M−E curve, in sharp contrast to the butterfly-like M− E curves along these two directions in CoFeB/PMN0.30PT(001).14 This abnormal behavior is quite surprising as the variation of magnetization projection along these two directions with the electric field should be butterfly-like if PMN-0.18PT stays in the R phase distortion, for which the magnetization orientation rotates between the [110] and [−110] directions because of the in-plane strain-generated easy axes on different FE domains. Therefore, this suggests that

PMN-0.18PT should not always stay in the R phase under electric fields. Instead, electric-field-induced FE phase transition should be involved. Therefore, the unusual behaviors of EFCM shown in Figure 1c strongly suggest that a new mechanism is involved in CoFeB/PMN-0.18PT. Besides the M−E curves, the polarization current, whose peaks are related to the FE polarization switching processes, was also measured simultaneously (Figure 1d). Comparing Figure 1c with 1d, it can be deduced that the sharp magnetization changes are correlated with FE polarization switching processes. As demonstrated later, electric-field-induced R−MA−T FE phase transition, internal electric field, and FE domain ratio play important roles in EFCM in CoFeB/PMN-0.18PT. In Figure 1d, it can be noted that the two polarization current peaks have different peak heights with the higher peak at −0.8 kV/cm and the much lower peak at +1.6 kV/cm. The asymmetry of the negative and positive electric fields also appears in the polarization versus electric field (P−E) hysteresis loops of PMN-0.18PT (Supporting Information S2), indicating the existence of a negative internal electric field22 in PMN-0.18PT whose origin will be discussed later. It is worth to mention that the ME coupling coefficient (α = μ0 dM/dE) accompanying the polarization switching process can reach around 10−6 s/m (Figure S3, Supporting Information). Thus, the α and the magnetization modulation by electric fields for CoFeB/PMN0.18PT are comparable or a little bit smaller than those for CoFeB/PMN-xPT with x equal to or near 0.30, reported in the previous literature.13,20 Moreover, the α in our work is pretty high in the research on FM/FE multiferroic heterostructures.23 To understand the unusual phenomena of EFCM in CoFeB/PMN-0.18PT, the in-plane magnetic anisotropy of CoFeB was investigated by electron spin resonance (ESR) under in situ electric fields of −0 and +8 kV/cm (Figure 2a). For −0 kV/cm, the raw in-plane ESR data gathered at different angles exhibit two kinds of resonance magnetic field (Hr), 25571

DOI: 10.1021/acsami.9b06532 ACS Appl. Mater. Interfaces 2019, 11, 25569−25577

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Figure 3. RSM measurements to investigate the crystal structural distortions with the diffraction patterns around (113) and (103) for different electric fields. (a) Configuration of the (113) and (103) crystal faces (translucent blue plane and green plane). The R phase polarizations r1−, r2−, r3−, and r4− are shown by the arrows for the case of −0 kV/cm. (b,c) Diffraction pattern around (113) and (103), respectively, for the case of −0 kV/cm. (d) MA phase polarizations of a1+, a2+, a3+, and a4+ shown by the arrows for the case of +3 kV/cm. (e,f) Diffraction pattern around (113) and (103), respectively, for the case of +3 kV/cm. (g) T phase polarization shown by the arrow for the case of +8 kV/cm. (h,i) Diffraction pattern around (113) and (103), respectively, for the case of +8 kV/cm. (j) Change of the in-plane lattice under the sequence of electric fields. There is hysteresis on the transition between the R phase and the MA phase for increasing and decreasing fields and the angle α inside the diamond structure indicates the change of in-plane lattice obviously. (k) Change of the angle α under the sequence of electric fields. The green, purple, and dark blue regions stand for the R phase, MA phase, and T phase, respectively.

which have maximum separation in the ⟨110⟩ directions of PMN-0.18PT with a peak-intensity ratio of nearly 1:2 (data for some typical angles are shown in Figure S4a, Supporting Information). On the basis of all data at different angles, we can plot the dP/dH(φ,H) mapping (Figure 2b) with the weaker-peak intensity outlined by the red line besides the main peak by the black line. Figure 2c shows the angular dependences of Hr with the black squares for the main peak and the red dots for the weaker peak, guided by the smooth lines fitted by the Kittel formula based on the uniaxial magnetic anisotropy model (Supporting Information S5). The fitting lines for the two kinds of Hr give the same uniaxial magnetic anisotropy constant (K1) of 7.5 × 103 J/m3 (Supporting Information S5), with the magnetic easy axes on the [110] and [−110], respectively. Therefore, the ESR results for −0 kV/cm reveal the coexistence of two kinds of FM domains with perpendicular easy axes and the same K1 induced by FE domains (FE-EA, Figure 2a) with two perpendicular in-plane

distortions. Moreover, the peak-intensity ratio of the two kinds of Hr also implies that the ratio of the two kinds of FM domains is around 1:2 in this case. Nevertheless, the raw ESR data for +8 kV/cm gathered at different angles only show a single peak, in contrast to the two peaks for −0 kV/cm, even along the ⟨110⟩ directions, which have maximum separation of two peaks for −0 kV/cm (data for some typical angles are shown in Figure S4a,b, Supporting Information). On the basis of all data at different angles, we then plot the dP/dH(φ,H) mapping (Figure 2e). The angular dependence of Hr obtained from the blue line (Figure 2e) is shown in Figure 2f, which reflects that the in-plane magnetic anisotropy of the FM film is nearly absent with ΔHr/Hr < 10%. The weak easy axis on the ⟨110⟩ directions shown in Figure 2f will be explained later. The near absence of uniaxial magnetic anisotropy implies the inplane isotropic strain in FE under +8 kV/cm, which cannot be explained by the R phase, suggesting that another kind of FE phase without an in-plane strain anisotropy should play a role 25572

DOI: 10.1021/acsami.9b06532 ACS Appl. Mater. Interfaces 2019, 11, 25569−25577

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ACS Applied Materials & Interfaces

length of strain.23 The value of K1 is comparable to that obtained by ESR results (7.5 × 103 J/m3). By comparing the positions of the (113) and (103) spots under +3 kV/cm with those related to the MA and MC phase distortions,25 especially the values along the Q[00l], the MA phase distortions can be deduced in our case as shown in Figure 3d, and the marks for the four distortions corresponding to the diffraction spots are shown in Figure 3e,f. Similarly, the T phase distortion as shown in Figure 3g can be deduced under +8 kV/cm, with the diffraction spots shown in Figure 3h,i, considering the highly enhanced c/a ratio with c about 0.547% longer than a in this case (defined in Supporting Information S10). For the T phase, the in-plane strain is isotropic, consistent with the aforementioned ESR results. Therefore, we demonstrate the R−MA−T phase transitions at RT, predicted by theoretical calculations.21 It should be pointed out that, under the positive electric fields around +8 kV/cm, there are weak diffraction spots in Figure 3h,i as pointed by the arrows in addition to the diffraction spots of the T phase. By comparing the positions of these weak spots with those in Figure 3b,c, it can be deduced that these weak spots are related to some tiny regions with the R phase similar to that under negative electric fields or −0 kV/ cm, suggesting that the tiny regions of the R phase with a negative polarization embed in the main T phase with positive polarization in PMN-0.18PT under positive electric fields around +8 kV/cm. The existence of the tiny regions of the R phase with negative polarization is consistent with the asymmetry of the P−E loop (Supporting Information S2), implying an internal electric field along the negative direction ([001]), which is also the reason for choosing the +8 kV/cm electric field in the M−H and ESR measurements, leading to the symmetry breaking along the out-of-plane direction. The origin of this internal field will be discussed later. The existence of the tiny regions of the R phase under positive electric fields around +8 kV/cm can also account for the weak easy axis on the ⟨110⟩ direction (Figure 2f). For the phase transitions of R−MA−T, we can utilize the same method that was used to calculate the strain under −0 kV/cm as mentioned above to analyze the strains under the sequence of electric fields (Supporting Information S9). The strains (Figure S10b, Supporting Information) and the spot broadening (Figure S5, Supporting Information) also exhibit hysteresis with the transition between the R phase and the M A phase accompanying the polarization switching in FE for increasing and decreasing fields (see the schematic shown in Figure 3j), which can account for the hysteresis in the aforementioned M−E curves (Figure 1c). To get the details of phase distortions, we propose a model of distortion for the MA phase (with the R and T phases as the special cases of MA) for quantitative calculations so that we can distinguish distortions for the three phases (Supporting Information S10) and analyze the polarization history in PMN-0.18PT under the sequence of electric fields (Supporting Information S6). Moreover, the angle α of the in-plane diamond structure (Figures 3j and S11a, Supporting Information) changes with the electric field, indicating electric-field-induced phase transition and hysteresis, as shown in Figure 3k. In addition, piezo-response force microscopy (PFM) was used to investigate the FE domain state and switching (Figure S12, Supporting Information). It was observed that the complex structure consisting of micronsized domains embedded in the FE domains with opposite orientation appears in the as-grown PMN-0.18PT, similar to the results reported by Shvartsman et al.26 The asymmetry of

in this case, that is, an FE phase transition occurs under large positive electric fields as demonstrated later by the XRD-RSM study. Therefore, ESR results of CoFeB/PMN-0.18PT reveal two kinds of FM domains with perpendicular magnetic easy axes for −0 kV/cm and near absence of magnetic anisotropy for +8 kV/cm. It should be pointed out that these phenomena cannot be explained by the scenario related to the electric-fieldinduced charge accumulation/depletion at the interface of CoFeB and PMN-0.18PT or effect due to the polarization switching in the FE considering the small screening length (1− 2 unit cells) in FM metal,23 which is too small to account for the observed large change in magnetization. To understand the unusual phenomena of EFCM in CoFeB/PMN-0.18PT, it is also essential to explore the change of PMN-0.18PT under different electric fields, especially the possible electric-field-induced R−MA−T FE phase transition predicted by theory.21 So far, there have been no reports on the experimental demonstration of this phase transition in PMN-0.18PT. We carried out a systematic study of the PMN0.18PT crystal structure under in situ electric fields by utilizing XRD-RSM for the sample used for M−E and ESR measurements. The (113) and (103) diffraction peaks of the pseudocubic unit cell in PMN-0.18PT with crystal planes shown in Figure 3a were selected to study the effect of the electric field on the FE phase as the in-plane lattice parameters can be deduced by the in-plane components of reciprocal vectors for diffractions around the (113) and (103) which are parallel to the [110] and [100] directions, respectively. A sequence of electric fields were applied to investigate the electric-field-induced FE phase change (Supporting Information S6) with the coordinate values defined by the cubic crystal structure (lattice parameter a0 = 0.403 nm).24 Interestingly, the peak positions of the diffraction pattern stay almost unchanged when the electric field turns out to be negative, whereas they show remarkable differences upon applying positive electric fields, especially large electric fields (e.g., +8 kV/cm), which makes the diffraction spots become a single one. The asymmetry of the effects on the phase of FE for the positive and negative electric fields was also indicated by the XRD diffractions of the θ−2θ scan around the (002) peak (Supporting Information S7). The change of the crystal phase under electric fields revealed by XRD-RSM can be divided into three regions, and the representative cases of −0, +3, and +8 kV/cm were selected for careful analysis, owing to their relatively pure phase nature. According to the positions of the diffraction spots around (113) and (103), the R distortions defined in Figure 3a can be deduced for −0 kV/cm, as shown in Figure 3b,c, respectively.25 By comparing Figure 3b,c, it can be deduced that there are only distortions of r1−, r2−, and r4−, with the ratio of r1−/r2− & r4− around 1:2, close to the ratio of two kinds of FM domains revealed by ESR results (Supporting Information S8). Considering that the distortions of r2− and r4− induce the same in-plane strain, we can take them as a whole. The distortions of r1− and r2− & r4− bring about the same value of in-plane strain around 0.22 ± 0.07% on the axes of [110] and [−110], respectively (Supporting Information S8), which induces the uniaxial magnetic anisotropy for CoFeB with perpendicular magnetic easy axes, consistent with the ESR results. The constant of uniaxial magnetic anisotropy was calculated by K1 = 3λsYε/2 = (10.5 ± 3.4) × 103 J/m3, with the magnetostrictive constant λs = 2 × 10−5 and Young’s modulus Y = 160 GPa for the CoFeB film,14 and the decrease of strain in the FM film can be neglected because of the long transfer 25573

DOI: 10.1021/acsami.9b06532 ACS Appl. Mater. Interfaces 2019, 11, 25569−25577

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ACS Applied Materials & Interfaces

Figure 4. Effect of ratio ξ for FE domains in the FE substrate on the behavior of the M−E curve. (a,b) In-plane lattices for domain A and domain B. Magnetization along the [100] [110] [010] [−110] directions under −0 and +8 kV/cm determined by domain A and domain B, respectively, calculated by minimizing free energy density. (c) M/MS−ξ phase diagram for the cases of −0 and +8 kV/cm. The solid lines and the dashed line are the magnetizations of the four directions for the cases of −0 and +8 kV/cm, respectively. (d−f) Three types of M−E behavior deduced from (c) for samples 1, 2, and 3 with different values of ξ obtained from RSM data.

positive and negative electric fields was also shown in the PFM measurements (Figure S12, Supporting Information). Hence, electric-field-induced R−MA−T phase transitions should contribute to the unusual phenomena in the M−E behaviors of CoFeB/PMN-0.18PT. The ratio of r1− and r2− & r4− distortions under negative electric fields also plays an important role in the details of M−E behaviors. We can analyze the magnetizations along the [110], [−110], [100], and [010] directions using semiquantitative calculations for the cases of −0 and +8 kV/cm, respectively, and then compare the results with Figure 1c. The remnant magnetizations for the [100], [010], [110], and [−110] directions under −0 and +8 kV/cm, respectively, can be deduced roughly as follows. The key point is that for the cases of −0 kV/cm, there are mainly r1− (domain B) and r2− & r4− (domain A) distortions with their in-plane strains perpendicular to each other (corresponding to two kinds of FM domains) (Figure S9c, Supporting Information), and the portion ratio of r1−/r2− & r4− is 1:2 (Tables S1 and S2). However, for the case of +8 kV/cm, the electric-field-induced T phase has an in-plane isotropic strain state. A schematic diagram of the FE, FM domains and effect of a 5 Oe magnetic field is shown in Figure S13. Considering the ratio of the two kinds of FM domains for the case of −0

kV/cm, the total free energy density of the sample can be described by the following equation. f=

F V

= (1 − ξ)(K1 sin 2 θA − MSH cos φA ) + ξ(K1 sin 2 θB − MSH cos φB)

(1)

where (1 − ξ) and ξ are the volume ratios of FE domains A and B, respectively, the aforementioned magnetic anisotropy constant K1 = 7.5 × 103 J/m3, the saturation magnetization of the CoFeB film MS = 1200 emu/cm3 as deduced from Figure 2b, θA and θB are the angles between the magnetization and the magnetic easy axis for each kind of FM domains, and φA and φB are the angles between the magnetization and the 5 Oe magnetic field. For the case of +8 kV/cm, the total free energy is just the Zeeman energy because of the absence of magnetic anisotropy so that the magnetization tends to align along the magnetic field with a net magnetization Mr (Figure S13f, Supporting Information) of about 0.67MS (Supporting Information S13). Besides the electric-field-induced R−MA− T phase transition in the FE substrate which brings about the absence of magnetic anisotropy under +8 kV/cm, the ratios of domain A and domain B related to ξ are also crucial for 25574

DOI: 10.1021/acsami.9b06532 ACS Appl. Mater. Interfaces 2019, 11, 25569−25577

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ACS Applied Materials & Interfaces

directions should increase for the electric field switched from −0 to +8 kV/cm (Figure 4e), which is supported by the experimental M−E curves (Figure S15a, Supporting Information). For type III behavior, the M−E curves along the [110] and [−110] directions are complementary (Figure 4f), which matches the experimental M−E curves (Figure S15b, Supporting Information). Moreover, for all three types of M−E curves, they should show similar behavior along the [100] and [010] directions (Figure 4d−f), which is supported by the experimental M−E curves (Figure 1c, Figure S15a,b, Supporting Information). It should be mentioned that the nonvolatile electric-field control of magnetization in the present work is originated from the electric-field-induced FE phase transition, which occurs in the whole PMN-0.18PT and is deterministic, in contrast to the nonvolatile EFCM in the CoFeB/PMN-0.30PT heterostructures, which only occurs in some regions of the sample with the 109° FE domain switching in PMN-0.30PT.14,15 It may be noted that the magnetizations for the cases of positive electric fields are not the same along the four aforementioned directions especially along the [100] direction, inconsistent with the calculated results. This can be understood by considering the ESR result for +8 kV/cm (Figure 2f), which shows weak magnetic anisotropy. The aforementioned residual R phase can account for the easy axis on [110], whereas the hard axis for [100] can be attributed to induced magnetic anisotropy during film growth, which is supported by the ESR result of the as-grown sample in the same batch as samples 1, 2, and 3 (Figure S4c, Supporting Information). Therefore, the magnetization along [100] is relatively lower than that along other directions. When changed from +8 to −0 kV/cm, the strain-induced magnetic anisotropy dominates in the FM film. In this process, [100] and [010] are symmetric, leading to the same variation of magnetization for them. Moreover, the interactions between FM domains on domain A and domain B are neglected in the calculations. Therefore, the difference between experiment results and theoretical calculations can be explained. The aforementioned internal field in the PMN-0.18PT crystal is unique and needs careful consideration, because there is no report, to our best knowledge, on the internal bias field in bulk PMN-PT without doping. We explore its origin based on the properties of PMN and PMN-xPT with quite low x reported in the literature, and current understanding in this field. For PMN (ABO3), the arrangement of Mg2+ and Nb5+ on the B site brings out a dynamic polar nanoregion (PNR), pinned to the regions of quenched chemical short-range order below Burns temperature.27 PNR possesses short-range polarorder with their polarizations along the eight R phase ⟨111⟩ directions,28 which can team up and form large domains in the FE state at low temperature or under electric field.29 For the regions of chemical short-range order, namely chemically order region (COR), Mg2+ and Nb5+ on the B site are ordered with a 1:1 ratio and the resultant charge imbalance (negative) is compensated by an Nb-rich disordered perovskite matrix (positive) adjacent to the PNR, forming electric dipoles at the COR boundaries.30 The size for this kind COR can be of a nanometer level.31 Considering the charge imbalance in both the COR and Nb-rich disordered region, the existence of oxygen vacancies in the COR and the vacancies of Nb5+ or Pb2+ in the Nb-rich disordered region was proposed, which still needs to be confirmed by experiments.32 These defects have a remarkable influence on the electric dipoles at the boundaries between the COR and the Nb-rich disordered region. The

magnetizations along the in-plane directions under −0 kV/cm, which determine the behaviors of the M−E curves. To make it clear, we first calculate magnetizations under −0 and +8 kV/ cm for multiferroic heterostructures with only domain A or domain B in FE substrates and analyze the M−E behaviors. For the multiferroic heterostructure with only domain A in the FE substrate, as shown in Figure 4a, magnetization along the [−110] under −0 kV/cm is saturated because of the large equivalent magnetic field (125 Oe, Supporting Information S5) of magnetic anisotropy related to the strain induced by domain A, whereas magnetizations along [100] and [010] are around 0.73MS and it is around 0.04MS along [110] (details about the calculation are presented in Supporting Information S14 with the [100] direction as an example). As mentioned before, the magnetization along each direction is around 0.67MS for the case of +8 kV/cm. Therefore, the M−E curves along the four directions should show loop-like behavior with magnetization increasing only along the [110] and decreasing for the other three directions (Figure 4a) when electric field is switched from −0 to +8 kV/cm. For the multiferroic heterostructure with only domain B in FE substrates, the calculation results (Figure 4b), as expected, show reverse M−E behaviors for the [110] and [−110] directions compared with that of Figure 4a. In PMN-0.18PT substrates used in the present work, domain A and domain B coexist as shown by the RSM data (Supporting Information S8). So the behavior of electric-field control of magnetization is determined by the volume ratios of FE domains A and B with a combination of behaviors shown in Figure 4a,b. The variations of magnetizations along the four directions under −0 and +8 kV/cm are shown in Figure 4c, which exhibits three regions, that is domain-A-dominated, crossover, and domain-B-dominated. In the domain-A-dominated region (ξ < 0.33) and domain-Bdominated region (ξ > 0.67), the behaviors of M−E curves for the four directions are similar to that of the case with only domain A (Figure 4a) and only domain B (Figure 4b), respectively. However, in the crossover region (0.33 < ξ < 0.67), the magnetizations along both the [110] and [−110] directions increase when the electric field is switched from −0 to +8 kV/cm. The value of ξ for the aforementioned sample (sample 1) is around 0.33 (Table S1), and the M−E curves (named type I) of the four directions for the cases of −0 to +8 kV/cm can be deduced as shown in Figure 4d. It indicates that M−E curve along [−110] is butterfly-like (magnetization does not change for −0 and +8 kV/cm), whereas the magnetization along [110] increases for the electric field switched from −0 to +8 kV/cm and decreases along the [100] and [010] directions, which is consistent with the experimental results (Figure 1c). Besides the type I behavior, we also observed two other kinds of M−E behaviors (named type II and type III) in other samples (samples 2 and 3) (Figure S15, Supporting Information). XRD-RSM results around the (113) peak indicate that the values of ξ for these samples are 0.44 and 0.85, corresponding to the crossover and domain-B-dominated regions, respectively (Supporting Information S15). On the basis of these values and Figure 4c, we can deduce the M−E curves of the four directions for the cases of −0 to +8 kV/cm as shown in Figure 4e (type II) and Figure 4f (type III). Careful comparisons between these analysis results (Figure 4e,f) and the experimental M−E curves (Figure S15, Supporting Information) suggest they are consistent in terms of change for −0 and +8 kV/cm. For example, for type II behavior, the magnetizations along the [110] and [−110] 25575

DOI: 10.1021/acsami.9b06532 ACS Appl. Mater. Interfaces 2019, 11, 25569−25577

Research Article

ACS Applied Materials & Interfaces

vacuum magnetron sputtering system with a base pressure of 1 × 10−6 Pa without a magnetic field. Au layers with a thickness of 300 nm were sputtered on the bottom of the structures as electrodes. Magnetic Measurement. Voltages were applied in situ through the cap layer and bottom electrodes across the CoFeB/PMN-0.18PT by a Keithley 6517A electrometer, and a Keithley 2400 ammeter together with a 16 MΩ protecting resistor were series-wound in the circuit to monitor the current during measurements. Magnetic properties of the samples were measured with a magnetic property measurement system (MPMS 7 T; Quantum Design). The sample was first magnetized with a magnetic field of 1000 Oe before the M−E curve measurements. PFM Measurement. PFM was carried out with the cantilever along the diagonal of the FE substrate, namely in-plane ⟨110⟩. The ±10 V dc voltage biased by the tip on PMN-0.18PT with a thickness of 0.5 mm can polarize the skin layer of the FE crystal because of the high electric field around the tip. ESR Measurement. The ESR data at different angles under various electric fields were gathered by JEOL FA-200 with the magnetic fields in the plane of the sample to investigate the electricfield control of in-plane magnetic anisotropy in the FM film. XRD-RSM Measurement and Diffraction θ−2θ Scan. XRD patterns of the samples were obtained using a Rigaku SmartLab X-ray diffractometer with a Ge(220) × 2 incident beam monochromator, and the wavelength of Cu Kα1 is 1.5406 Å. Polarization Versus Electric-Field Measurement. The P−E hysteresis loops for PMN-0.18PT were measured by the commercial precision FE tester.

polarizations of dipoles at the COR boundaries gradually reorient themselves parallel to the spontaneous polarizations after letting the sample alone for a long time (aging process), and in turn, stabilize and pin the FE polarization,22 leading to the internal bias field. The spontaneous polarizations in PMN0.18PT are along ⟨111⟩ and have eight equivalent directions. After poling by an electric field along [001] as we did in our experiments, four spontaneous polarization directions with positive projection on [001] can form engineering domains, possessing the same energy. Thus, the ratios of r1−, r2−, r3−, and r4− can be influenced by many factors with nondeterminism, such as the FE domain nucleation rate, the slight tilt of the surface of the measured PMN-PT crystal from the (001) crystal face, and internal field, which is along the four ⟨111⟩ directions possessing positive projection on [001]. FE domains in different samples are discussed in the Supporting Information S15. For sample 1, the XRD-RSM measurement for +8 kV/cm exhibits the weak diffraction spots (Figure 3e,f), whose positions indicate that some pinned nonswitched negative R phase FE domains embed in PMN-0.18PT, which is consistent with the internal bias field discussed above. The positive electric-field-induced R−MA−T phase transition occurs with the appearance of nonswitched negative FE domains, which can be demonstrated with the proposal that the FE domain wall surrounding the nonswitched FE domains can be influenced by the polarization electric field induced by nonswitched regions so that it becomes easier to form the MA phase and this phase coherently rotates to the T phase with an increasing positive electric field. However, further work is needed to clarify this issue by theoretical calculations, for example microscopic phase-field simulation. The Ti 4+ composition of PMN-0.18PT is much lower compared to PMN-0.30PT, which brings out much stronger random local fields because of the larger charge difference27 and induces larger ratio of COR. This makes the internal field much larger or remarkable compared to that in PMN-0.30PT.

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S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.9b06532. Magnetic hysteresis loops along the [−110] direction under different electric fields, P−E loops measured with different ranges of electric field, ME coupling coefficient, raw ESR signals for some typical angles along the inplane ⟨110⟩ directions, Kittel formula for fitting ESR data, XRD-RSM results and the analysis of polarization history under a sequence of electric fields, XRD results around the (002) peak under a sequence of electric fields, ratio of various R phase distortions and the inplane strains under −0 kV/cm, analysis of the in-plane strains under the sequence of electric fields, MA phase distortion model and phase transition induced by the sequence of electric fields, FE domain switching in the PMN-0.18PT measured with PFM, schematic diagram of the FE, FM domains and effect of a 5 Oe magnetic field, comparison of M S and M r , magnetization calculations along the [100] direction under −0 and +8 kV/cm, M−E curves and RSM measurements on samples of type II and type III, and determination of composition for the PMN-xPT crystal (PDF)

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CONCLUSIONS In summary, the electric-field-induced FE phase transition in PMN-0.18PT at RT is observed. Moreover, the breaking of complementary symmetry on M−E behaviors for corresponding directions, which has never been reported for FM/FE heterostructures and cannot be explained by the existing mechanisms, is observed for EFCM in CoFeB/PMN0.18PT(001). Through systematic investigation of magnetic anisotropy, FE domains and crystal structures under in situ electric fields, respectively, we demonstrate that the unusual behaviors of EFCM in CoFeB/PMN-0.18PT are related to the cooperation of electric-field-induced FE phase transitions, internal electric field in PMN-0.18PT, and the competition of different FE domains. This work opens a new avenue for exploring EFCM in FM/FE multiferroic heterostructures, and is significant for understanding the mechanism of EFCM in multiferroic heterostructures as well as applications.

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ASSOCIATED CONTENT

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (S.Z.). *E-mail: [email protected] (Y.Z.).

METHODS

Sample Preparation. Samples with the PMN-0.18PT (0.5 mm)/ CoFeB (20 nm)/Ta (5 nm) structure were investigated. Pb(Mg1/3Nb2/3)0.82Ti0.18O3 (PMN-0.18PT) substrates whose composition was determined by dielectric measurements (Supporting Information S16) were (001)-cut and one-side-polished with a size of 2.5 mm × 2.5 mm × 0.5 mm. Co40Fe40B20 soft magnetic films with a Ta (5 nm) protective cap-layer were deposited in an ultrahigh

ORCID

Haoliang Huang: 0000-0002-5686-5519 Jinxing Zhang: 0000-0001-8977-5678 Sen Zhang: 0000-0002-1751-0731 Yonggang Zhao: 0000-0002-7803-7378 25576

DOI: 10.1021/acsami.9b06532 ACS Appl. Mater. Interfaces 2019, 11, 25569−25577

Research Article

ACS Applied Materials & Interfaces Notes

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The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the Science Center of National Science Foundation of China (grant no. 51788104), the 973 project of the Ministry of Science and Technology of China (grant no. 2015CB921402), the National Science Foundation of China (grant nos. 51831005, 51572150, 11704388), and State Key Laboratory of Low-Dimensional Quantum Physics (grant nos. ZZ201701, KF201717).

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DOI: 10.1021/acsami.9b06532 ACS Appl. Mater. Interfaces 2019, 11, 25569−25577