Magnetoelectric Coupling in Well-Ordered Epitaxial BiFeO3/CoFe2O4/SrRuO3 Heterostructured Nanodot Array Guo Tian,† Fengyuan Zhang,† Junxiang Yao,† Hua Fan,† Peilian Li,† Zhongwen Li,† Xiao Song,† Xiaoyan Zhang,† Minghui Qin,† Min Zeng,† Zhang Zhang,† Jianjun Yao,‡ Xingsen Gao,*,† and Junming Liu*,†,§ †
Institute for Advanced Materials and Guangdong Provincial Key Laboratory of Quantum Engineering and Quantum Materials, South China Normal University, Guangzhou 510006, China ‡ Asylum Research, Santa Barbara, California 93117, United States § Laboratory of Solid State Microstructures and Innovation Center of Advanced Microstructures, Nanjing University, Nanjing 210093, China S Supporting Information *
ABSTRACT: Multiferroic magnetoelectric (ME) composites exhibit sizable ME coupling at room temperature, promising applications in a wide range of novel devices. For high density integrated devices, it is indispensable to achieve a well-ordered nanostructured array with reasonable ME coupling. For this purpose, we explored the wellordered array of isolated epitaxial BiFeO3/CoFe2O4/ SrRuO3 heterostructured nanodots fabricated by nanoporous anodic alumina (AAO) template method. The arrayed heterostructured nanodots demonstrate well-established epitaxial structures and coexistence of piezoelectric and ferromagnetic properties, as revealed by transmission electron microscopy (TEM) and peizoeresponse/magnetic force microscopy (PFM/MFM). It was found that the heterostructured nanodots yield apparent ME coupling, likely due to the effective transfer of interface couplings along with the substantial release of substrate clamping. A noticeable change in piezoelectric response of the nanodots can be triggered by magnetic field, indicating a substantial enhancement of ME coupling. Moreover, an electric field induced magnetization switching in these nanodots can be observed, showing a large reverse ME effect. These results offer good opportunities of the nanodots for applications in high-density ME devices, e.g., high density recording (>100 Gbit/in.2) or logic devices. KEYWORDS: multiferroic composite, nanodot array, magnetoelectric coupling, AAO template, CoFe2O4, BiFeO3 multilayers,15−17 (1−3) columnar films,18−24 core−shell nanofibers,25 and so on. The (2−2) multilayers have been the most well investigated ME materials, in which different constitution phases are combined at atomic level yielding a near-perfect mechanical coupling. Nonetheless the ME response of these multilayers is limited to quite a low level due to the clamping from the stiff substrates.26 The (1−3) nanostructures with selfassembled vertical epitaxial columnar films can greatly enhance the ME coupling, enabling an electric-field induced magnetization switching.20,22 However, they suffer from large leakage
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ith the current miniaturization and multifunctional technological trends in microelectronic industry, there is a growing interest in a family of multiferrroic magnetoelectric (ME) materials, which accommodate both electric and magnetic orders and enable the mutual control of magnetic and electric properties.1−8 These superior properties promise a wide range of applications, such as new concept ME sensors, actuators, logical devices, and in particular low power consumption and high density ME memory devices.9−13 Among the multiferroic family, the nanoscale heterostructured composites are of particular appealing due to their relatively high ME performance and high operation temperatures with respect to their single phase counterparts. There are several kinds of multiferroic heterosturctures being investigated, including (0−3) nanocomposites,14 (2−2) type © XXXX American Chemical Society
Received: October 8, 2015 Accepted: December 9, 2015
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DOI: 10.1021/acsnano.5b06339 ACS Nano XXXX, XXX, XXX−XXX
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Figure 1. Fabrication process and microstructures for the BFO/CFO/SRO nanodot array. (a) A schematic flowchart for the fabrication procedure. (b) A top-view SEM image of the nanodot array. (c) The θ − 2θ XRD diffraction pattern and (d) a reciprocal space mapping (RSM) adjacent to the STO (002) plane.
conductor process, with the ability to scale down to ultrahigh density and diminish the cross-take among the neighboring dots. However, up to now there is only an indirect ME coupling driven by phase transition observed in BaTiO3/CoFe2O4 heterostructured nanodots,30,31 while a direct ME coupling at room temperature in this kind of lateral (0−0) type multiferroic heterostructured nanodots has not yet been achieved. In this work, we explored the possibility of improving the ME coupling in a (0−0) heterostructure using the ordered BiFeO3/ CoFe2O4/SrRuO3 (BFO/CFO/SRO) epitaxial nanodot array fabricated by pulsed laser deposition (PLD) of the three components through a ultrathin nanoporous anodic aluminum oxide (AAO) membrane template.29,33 We chose BFO as an active ferroelectric layer, because it not only is a well-known room temperature single phase multiferroic, but also possesses large ferroelectric polarization and antiferromagnetic nature which may bring more coupling mechanisms than mechanical coupling alone, e.g., exchange bias and charge mediated couplings, thus adding to ME effect.34,35 To ensure its good electric properties, a SRO layer was introduced as top electrode to form a solid state capacitor structure. This can much improve the electrode contact and produce more uniform electric field than that using an AFM tip alone as top electrode, and thus a much enhanced piezoelectric property can be achieved. Interestingly, we are able to observe both apparent magnetic-alternation of piezoelectric properties (ME effect) and electro-switching of magnetization (reverse ME effect).
due to the low resistivity of the constitutional magnetic phase or conductive interfaces.27 There are also other structures like quasi (0−3) structure proposed to combine the features of the above two types of architectures.28 Nevertheless, it requires rather complicated process and is difficult to choose component distribution due to the self-assembly nature. On the other hand, for multiferroic nanostructures in high density microelectronic devices such as memories and logical devices, the ability to place the nanostructures into predetermined ordered arrays is highly demanded. There exist several efforts, such as templated fabrication of selfassembled ordered (1−3) type composites,19,23,24 and film covered nanodots.29 However, it is still a challenge to maintain both sizable ME coupling and well-ordered architectures simultaneously. In 2011, Lu et al. proposed an array of lateral (0−0) epitaxial heterostructured nanodots, which may offer several apparent advantages over previously mentioned structures.30,31 First, such an architecture allows atomic level mechanical coupling, and concurrently suppresses the substrate clamping that hampers the ME coupling in multiferroic composite films. Moreover, this also makes it easier to tune the strain and domain structure, and substantially improve the ME coupling by properly adjusting their geometric sizes and architecture combinations. For instance, a theoretic work predicted that by properly tailoring the shape anisotropy of nanomagnets, an electro-controllable magnetization reversal can be achieved in a system of nanomagnets plus piezoelectric substrate.32 Finally, it is also rather compatible with semiB
DOI: 10.1021/acsnano.5b06339 ACS Nano XXXX, XXX, XXX−XXX
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ACS Nano These results pave a way for applying this type of nanostructure in high density devices.
RESULTS AND DISCUSSION The detailed fabrication process can be found in the Methods section. A schematic of the fabrication process is illustrated in Figure 1a, which involves three steps: AAO mask transfer (i), materials deposition by PLD (ii), and mask removal (iii). In prior to the multiferroic nanodots deposition, atomically flat conductive SRO layer (∼20 nm) as bottom electrode was first epitaxially grown on (100)-oriented SrTiO3 (STO) substrate. The epitaxial BFO/CFO/SRO nanodot array was sequentially prepared by deposition of CFO and BFO layers through the AAO template at a temperature of 590 °C. Finally, the AAO mask was removed by mechanical or chemical liftoff, thus leaving the nanodot array alone. As shown in Figure 1b, the nanodots exhibit an average lateral size of ∼70 nm and neighboring dot−dot distance of ∼110 nm. A further examination of the nanodots found a squarish structure of each nanodot (see Figure S1 in the Supporting Information), indicating the well-developed crystalline faces. The XRD spectrum shown in Figure 1c confirms the (001)-oriented heterostructured nanodots on STO, as reflected by the diffraction peaks from STO (002), SRO (002), BFO (002), and CFO (004). The epitaxial structure was further examined by the reciprocal space map (RSM) of the nanodots close to the STO (002) diffraction peak, as shown in Figure 1d, indicating well-defined reciprocal diffraction peaks for the different layers. The (002) spot of BFO exhibits some extent of broadening along k⊥, implying some variation in lattice constant c. For the (004) spot of CFO, there exists some broadening along k// too, implying the in-plane lattice variation. From the XRD and RSM, we can derive the BFO lattice constant c of ∼0.405 nm and CFO lattice constant c of ∼0.829 nm, indicating an in-plane compressive strain of the BFO layer and an in-plane tensile strain of the CFO layer, respectively. The microstructures of the as-deposited nanodot array were examined by scanning electron microscopy (SEM), transmission electron microscopy (TEM), and atomic force microscopy (AFM). Both the dots and AAO mask are visible within the observation scope. It is worth to mention that the top-view shape of the nanodots is trapezoid-like, which indicates well-developed (011) and (001) faces, agreeing with the three-dimensional (3D) surface topological map (Figure S1a,b of the Supporting Information). The cross section TEM images for a nanodot are presented in Figure 2. A clear threelayer structure with a stacking of BFO (20 nm)/CFO (25 nm)/ SRO (20 nm) was observed in Figure 2a. The BFO, CFO and SRO layers in the nanodot show quite good single crystallinity with the same orientation as the substrate, although lattice defects like dislocations are also visible. The epitaxial quality was also examined by the selected area electron diffraction (SAED) along the ⟨010⟩ direction (Figure 2b), where apparent diffraction spots of BFO and CFO can be identified. The TEM (HRTEM) analysis reveals an epitaxial growth over the whole nanodot, with sharp SRO/BFO/CFO interfaces (Figure 2d). The compositions of the nanodots were inspected by energydispersive X-ray spectroscopy (EDS), and the distributions of different elements are given in Figure S2 in the Supporting Information. In spite of some uncertainties, the Ru, Bi, and Co elements are indeed located at the corresponding SRO, BFO, and CFO layers respectively, while element Sr distributes in both the STO and SRO layers, and element Fe in BFO and
Figure 2. TEM cross section images and diffraction spots for a BFO/CFO/SRO nanodot. (a) The cross section TEM image, (b) selected area electron diffraction alone the ⟨010⟩ direction showing diffraction spots of BFO and CFO planes, (c) the lattice plan image for a selected area adjacent to the BFO/CFO interface (the rectangle region in d) in which only (200) planes of BFO and (400) planes of CFO are visible after a fast Fourier transformation(FFT)filtering of the original HRTEM image, and (d) a high resolution TEM magnification image for a selected area illustrating the variations in lattice spaces over the cross section of the heterostructured nanodot.
CFO layers. A weak overlapping among elements Bi, Co, and Ru across the adjacent interfaces is detected. This issue is probably more due to the low EDS mapping resolution than to the interface-crossing diffusion, which, if any, is quite weak. It was also worth to mention that the spatial dependence of the lattice parameter in these layers. For the BFO layer, one sees that the c-lattice constant drops down from 0.411 to 0.401 nm, while the a-lattice constant increases from 0.391 to 0.397 nm over a 20 nm-thick BFO layer from the BFO/STO interface side to the BFO/CFO interface side within a single nanodot, as demonstrated in HRTEM image (Figure 2d). Considering the clamping effect from the STO substrate, this lattice parameter variation indicates a very fast lattice strain relaxation in the nandot, compared to the coherent compressive strain of the BFO film below a critical thickness (∼70 nm) on the STO substrate.36 From our previous XRD results, the average clattice constant of the BFO layer is ∼0.405 nm, only a little smaller than the reported value of ∼0.408 nm for a fully strained epitaxial BFO film.36 Nonetheless, at the top surface of BFO nanodot (near the BFO/CFO interface), the c-lattice constant is much smaller (∼0.401 nm), implying that over 60% of the in-plane compressive strain induced by the substrate clamping is released at the BFO/CFO interface as compared to the fully strained BFO film. Nevertheless, what seems inconsistent with the above discussion is that we have identified an in-plane tensile strain in the CFO layer, even though its in-plane lattice constant is larger than that of BFO layer and expected to obtain a compressive misfit strain from the BFO layer. This discrepancy can be interpreted by the occurrence of high density of interface dislocations (as shown in Figure 2c), which not only fully balances the misfit strain but also produces a tensile strain. Similar phenomenon was also reported in our previous study on CFO thin film on bare STO substrate, which exhibits a C
DOI: 10.1021/acsnano.5b06339 ACS Nano XXXX, XXX, XXX−XXX
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Figure 3. Magnetic and piezoelectric performances of the BFO/CFO/SRO nanodot array. (a) The topologic image, (b) the corresponding MFM micrograph, and (c) the room temperature magnetization reversal hysteresis loops for both in-plane and out-of-plane directions. (d) The vertical piezoresponse amplitude, (e) the correspondent phase images for an area electrically prepoled by ±6 V bias voltages, respectively, using a scanning conductive AFM probe, and (f) the piezoresponse hysteresis loops acquired on a selected nanodot exhibiting the phase hysteresis loop (upper panel) and amplitude butterfly loop (lower panel). The scale bars in all the image panels are 150 nm.
tensile strain. On the other hand, it converts to an in-plane compressive strain by inserting a SRO buffer layer, which was interpreted by the occurrence of high density of interface dislocations that leads to completely different strain states.37 Subsequently, we investigated the magnetic and ferroelectric properties of these nanodots. The magnetic force microscopy (MFM) and piezoresponse force microcopy (PFM) were utilized for the characterizations. Panels a and b of Figure 3 present the topologic and MFM images for the nanodots, respectively. The dark- and bright-contrast scales in Figure 3b reflect the perpendicular components of the stray field and thus the out-of-plane magnitudes of opposite magnetizations. The image shown in Figure 3b is on the virgin state, demonstrating an irregular out-of-plane striped-like contrast. This implies the significance of the magnetostatic dipolar interaction among the nanodots, which tends to drive the individual magnetization into stripe-like clusters. The overall magnetization reversal of the nanodot array was examined by magnetic hysteresis loops measured using a vibrating sample magnetometer (VSM) integrated in a Quantum Design physical properties measurement system (PPMS). Figure 3c shows the M−H loops along the in- and out-of-plane directions. Both the loops are very slim with small coercive fields of
