Tunable Magnetoelectric Nonvolatile Memory Devices Based on

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Tunable Magnetoelectric Nonvolatile Memory Devices Based on SmFeO3/P(VDF-TrFE) Nanocomposite Films Anju Ahlawat,*,† S. Satapathy,*,†,‡ Mandar M. Shirolkar,§,∥ Jieni Li,∥ Azam Ali Khan, Pratik Deshmukh,† Haiqian Wang,∥ R. J. Choudhary,⊥ and A. K. Karnal†,‡ †

Laser Materials Section, Raja Ramanna Centre for Advanced Technology, Indore 452013, India Homi Bhabha National Institute, Training School Complex, Anushakti Nagar, Mumbai-400094, India § Department of Physics, Tamkang University, Tamsui, 251, Taiwan ∥ Hefei National Laboratory for Physical Sciences at the Microscale, University of Science and Technology of China, Hefei, Anhui 230026, China ⊥ UGC DAE, Consortium for Scientific Research, Indore 452001, India

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‡

S Supporting Information *

ABSTRACT: Utilization of magnetoelectric effects in multiferroic materials hold great potential to fabricate nonvolatile memory devices with outstanding characteristics. In particular, organic thin memories are favorable because of their environment friendly nature, mechanical flexibility, and low fabrication cost. In this work, we have demonstrated a room temperature paradigm two level nonvolatile memory operation by exploiting the nonlinear magnetoelectric effects in flexible SmFeO3/P(VDF-TrFE) nanocomposite films using organic ferroelectric polymer (P(VDF-TrFE)) as a host matrix. Strong strain mediated interfacial interactions between ferromagnetic and ferroelectric phases in SmFeO3/P(VDFTrFE) nanocomposite films allow electric field controlled magnetic switching. The maximum magnetoelectric coefficient (α) obtained is 45 mV cm−1 Oe1− at Hbias= 1 kOe and 16 mV cm−1 Oe1− at Hbias= 0 in electrically poled composite films (30% SmFeO3). The experiments demonstrate that during seven operative cycles for 1500 s, the applied positive and negative electric fields can repeatedly switch states of α. Binary information is stored by using the states of α, rather than resistance, magnetization, and electric polarization, which is advantageous to overcome the drawback of destructive reading of polarization of ferroelectric random access memory. The magnetoelectric response and the required voltage for switching of α can be tuned by varying the magnetic phase fraction (SmFeO3 nanoparticles) in nanocomposite films. Hence, the kind of nonvolatile memory using organic, flexible magnetoelectric SmFeO3/P(VDF-TrFE) nanocomposite films has excellent practical characteristics, that is, compactness, easy and fast speed reading/writing operation, and reduced power consumption. KEYWORDS: nonvolatile memory, multiferroics, P(VDF-TrFE), SmFeO3, nanocomposite films, magnetic ordering, electric poling, magneteoelectric coupling

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INTRODUCTION The modern generation of data storage technology demands for low power consuming high performance memory storage devices. In past few decades, volatile and nonvolatile memories have been well explored1−6 The typically used fast speed memory storage devices (dynamic and static random access memory etc.) suffer from many disadvantages during the operation. For example, they consume high power due to leakage and their volatile nature results in loss of data after removal of power supply.7−10 In contrast, nonvolatile memories can store information in the absence of power supply and hence they are promising for durable and persistent storage. 11 However, in spite of having lower energy consumption, the conventional nonvolatile memories (magnetic random access memory (MRAM) and ferroelectric © XXXX American Chemical Society

random access memory (FRAM)) are facing certain challenging hurdles to become mainstream in industry.12 For instance, FRAM devices suffer from their limited storage density and destructive read operations.13 In this respect, artificial multiferroic composites based on ferromagnetic (FM) and ferroelectric (FE) phases that exhibit strong magnetoelectric (ME) coupling, hold promise for designing new generation memory devices with several advantages.14 Nonvolatile memory devices based on multiferroic composites offer innovative approaches rather than semiconductor transistorbased devices. In the past decade, various nonvolatile Received: March 12, 2018 Accepted: July 5, 2018

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DOI: 10.1021/acsanm.8b00401 ACS Appl. Nano Mater. XXXX, XXX, XXX−XXX

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of casting blade at ∼110 °C and kept them at same temperature (∼110 °C) for 10 min. To peel off the SFO/P(VDF-TrFE) nanocomposite films from the glass plate, they were put into water bath for 30 min. The crystalline phase of the composite films was achieved by cooling them down to room temperature, at the rate of 5 degrees per minute. The schematic illustration of fabrication of SmFeO3/P(VDF-TrFE) nanocomposite films is shown in Scheme 1.

multiferroic/magnetoelectric RAMs, such as four state resistive memory device using multiferroic tunnel junction (by integrating ferroelectric/ferroelectric nanostructures as tunnel barrier) and ME coupling based memories have been explored.10,14−16 Indeed, the read-write operations in FRAM and MRAMs can be improved by employing ME coupling in multiferroic composites.10,17,18 The multiferroic composites show effective piezoelectric control of magnetic states and have a distinctive physical quantity; ME coupling coefficient (α) (in addition to electric polarization and magnetization). The quantity α can be defined for direct ME effect and converse ME effect, as αD = dP/dH and αC = dM/dE, respectively, where P, M, H, and E are electric polarization, magnetization, magnetic field, and electric field, respectively. The value of α can be used to encode digital data, instead of using M and P. The innovative concept of using α for memory storage has many advantages10 over previously known nonvolatile RAMs, such as FRAM,19,20 resistive switching random access memory (RRAM),21,22 or phase-change memory (PRAM),23−25 where the direction of M, P, and resistance (R) are used to store the binary information. Hence, it is of great interest to design nonvolatile memories using α in magnetoelectric multiferroics. On the basis of this new concept of memory storage, few multiferroic composite structures comprising magnetostrictive materials (Terfenol-D, Metglas) and piezoelectric materials (e.g PMN−PT,26 Poly(vinylidene fluoride-trifluoroethylene) (P(VDF-TrFE))) have been used for nonvolatile memory applications.8,9,27 However, search on the choice of materials for creating the multiferroic heterostructures is highly required for performance optimization of the memory devices. ME composites consisting of environment-friendly, cost-effective, and easily processable materials are favorable compared to the inorganic piezoelectric materials (e.g., PMN−PT, PZT, etc.) and magnetostrictive materials (e.g NiFe2O4, Terfenol-D, and Metglas), which need highly complicated synthesis process. In the present work, we fabricated SmFeO3/P(VDF-TrFE) nanocomposite films and studied ME effects. Nonvolatile magnetization switching controlled by electric field has been realized in the composite films at room temperature. P(VDFTrFE)28−30 (highly piezoelectric) and SmFeO331,32 (magnetostrictive) are exceptional materials to exhibit strong ME coupling in the form of composites. These materials can be simply fabricated at low temperature as described in Experimental Section. (P(VDF-TrFE)) is well-known ferroelectric copolymer of PVDF and its major advantage is the ability to act as host matrix for nano particles and can be easily cast in the form of thin films at low temperatures (∼100−130 °C).33−35 The P(VDF-TrFE) based composite films are cost efficient, highly flexible and environment-friendly. Potential use of the SmFeO3/P(VDF-TrFE) composite nanostructures as a nonvolatile memory element has been demonstrated.

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Scheme 1. Schematic Illustration of Fabrication of SmFeO3/ P(VDF-TrFE) Nanocomposite Films

For electric field poling the gold electrodes measuring 5 mm × 5 mm were coated by thermal evaporation under vacuum of order of 10−5 mbar. Electric filed Poling was performed at room temperature by applying DC voltage of 100 kVcm−1 across two electrodes of composite films in a silicon oil container. All composite films were poled for 1 min. Characterization Techniques. The particle size was calculated using transmission electron microscopy (TEM) and the surface morphology of the films were determined using Zeiss field emission scanning microscope (FESEM). Magnetic measurements were performed by SQUID-vibrating sample magnetometer equipped with 7-T magnet (Quantum Design Inc., USA). For magnetoelectric measurements, a varying bias magnetic field of 1 T along with ac magnetic field of 0.5 Oe (resonating frequency-1 kHz) was generated using Helmholtz coils. A lock-in amplifier (Stanford, SR532) was used to generate output voltage from the composite and signal generator was used to produce the reference signal.

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RESULT AND DISCUSSION The size of SFO nanoparticles and microstructure of the composite films is studied using TEM. The average diameter of the nanoparticles obtained from FESEM was ∼100 nm (Figure 1a). FESEM images shown in Figure 1c−1f reveal that all the SFO/P(VDF-TrFE) composite films have a chainlike structure corresponding to pure P(VDF-TrFE36), as shown in Figure 1b. The SEM images indicate that the SFO nanoparticles are trapped inside the chains, therefore the individual particles are not clearly visible in the FESEM images. The dispersion is nearly uniform in the lower SFO content composite films (up to 10%). There might be some agglomeration at higher concentration (20%−30%) of SFO nanoparticles. Moreover, the chain structure is slightly disrupted with increasing concentration of SFO nanoparticles in composite films. The disrupted chains indicate reduction in polar phase (β) (responsible for chainlike structure) of P(VDF-TrFE) in composite films. This infers that crystalline structure of the nanocomposite films can be controlled by the particle loading. With the increasing concentration of SFO nanoparticles in polymer, the flexibility of the films reduces gradually and become brittle.

EXPERIMENTAL SECTION

Preparation of Composite Films. P(VDF-TrFE) with 30% TrFE was used without further purification. P(VDF-TrFE) solution was prepared (density: 1.82 g/cm3 procured from Arkema) using N,N-dimethyl formamide (DMF). The nanoparticles of SmFeO3 (SFO) were prepared by sol−gel autocombustion.21,22 To develop SFO/P(VDF-TrFE) composite films, varying volume percentages of SFO nanoparticles (5%, 10%, 20% and 30%) were added to P(VDFTrFE) solution and the mixture was ultrasonicated for 3 h by using ultrasonic probe vibrator. After that the solutions with SFO fraction of 5% (S1), 10% (S2), 20% (S3), and 30% (S4) were poured on glass plate. The thin films of thickness ∼30 μm were prepared with the help B

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Figure 1. TEM images of pure SmFeO3 (a). FESEM images of pure PVDF-TrFE (b). SmFeO3/PVDF-TrFE composite films with 5% (c), 10% (d), 20% (e), and 30% (f) volume fraction of SmFeO3.

Further, the crystallinity of composite films is characterized using X-ray diffraction as shown in Supporting Information. XRD patterns also indicate that higher concentration (20% and 30%) of SFO nanoparticles in polymer matrix reduce the content of β- phase of P(VDF-TrFE) in composite films. This might be due to nanoparticle doping-induced change in crystalline structure of P(VDF-TrFE), as observed in other reports also.37 In order to measure the spontaneous polarization of SFO/ P(VDF-TrFE) composite films with varying the SFO content, polarization versus electric field (P-E) loop measurements

were performed. Figure 2a shows the P-E loops of the pure P(VDF-TrFE) and SFO/P(VDF-TrFE) composite films S1, S2, S3, and S4. The composite films S1 and S2 show ferroelectric loop with polarization and remnants slightly less than that of the pure P(VDF-TrFE). The increasing SFO nanoparticles content leads to the reduction in ferroelectric polarization and reduced dramatically for 20% and 30% volume fraction of SFO in composite films. This is due to the disrupted polymer chain with heavy loading of SFO nanoparticles in composite films, as shown in the SEM images (Figure 1c−1f). Indeed the crystalline beta phase (consists of C

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PVDF−multiwalled carbon nanotubes composites when properly drawn and electrically poled.42 However, as compared to pure PVDF, its copolymer PVDF-TrFE can be easily crystallized in polar β-phase without any post processing treatments. In the present case also, β-phase of PVDF-TrFE is easily formed in pure P(VDF-TrFE) films as well as in SFO/ P(VDF-TrFE) composite films without post-treatment methods (mechanical stretching and electrical poling). The huge reduction in β-phase of SFO/P(VDF-TrFE) composite films beyond 10% concentration of SFO nanoparticles (as shown in XRD patterns) is attributed to the change in crystalline structure, as reflected in SEM images also. However, the poling process could significantly enhance β-phase in PVDF and its copolymers which provide the necessary piezoelectric properties for various technologically applications, In the next section, we describe the consequences of electric filed poling on magnetization and magnetoelectric output voltage (which depend on the piezoelectric coefficient of the P(VDF-TrFE) and piezomagnetic coefficient of the SFO) of SFO/P(VDFTrFE) composite films. The electric field (E) poling induced manipulation of magnetization was measured in SFO/P(VDF-TrFE) nanocomposite films. The magnetization versus magnetic field (M− H) curves were measured for unpoled and poled samples, as shown in Figure 2b and 2c. Interestingly, the M−H curves behavior of poled films reveal a significant change as compared to that of the unpoled films. Poled composite film S1 (5% SFO content) showed well saturated M−H loop at H = 5 T, while unpoled films were not saturated up to high H = 7 T. Moreover, remnant magnetization (MR) and coercivity was reduced by half after poling (insets of (Figure 2b). In contrast, the M−H loop for poled composite film S4 (5% SFO content) was not saturated up to applied magnetic field of 7 T. The MR was reduced by half and coercivity increased nearly by a factor of 2 (insets of (Figure 2c). However, the discrepancy in the magnetization behavior of poled samples S1 and S4 can be attributed to the distribution geometry of SFO nanoparticles in the polymer matrix. Higher SFO content, (especially 30%) in the composite films leads to agglomeration of nanoparticles and hence it may be difficult to switch the magnetic domains completely. Whereas, at low concentration of SFO the nanoparticles are distributed far away from each other and forming core−shell like structure (nanoparticles covered with polymer), which makes easy magnetic domains switching due to strain transfer between consecutive phases. The electric field poling induces strain in ferroelectric phase (P(VDF-TrFE)) because of the converse piezoelectric effect, which is transferred to SFO nanoparticles via interfacial interactions and hence magnetization switches through the inverse magnetostrictive effects.43,44 The observed modifications in magnetic properties of composite films are caused by changes in magnetic anisotropy. Direct and converse ME coupling was examined by measuring ME coefficient α for electrically poled and unpoled SFO/P(VDF-TrFE) nanocomposite films, as shown in Figure 3. Figure 3a and 3b shows Hdc (in-plane) dependence of ME coefficient (α) at an off-resonance frequency of 10 kHz in the presence of Hac (in-plane) = 0.5 Oe. To switch direction of P, the E field of 100 kVcm−1 is applied across two electrodes in SFO/P(VDF-TrFE) composite films (S1−S4). Electric filed poled composite films show enhanced α value (nearly double) as compared to the unpoled samples. Interestingly, poled composite films display hysteretic behavior with nonzero α

Figure 2. (a) Polarization vs electric filed loop for pure PVDF-TrFe and SmFeO3/PVDF-TrFe composite films with varying volume fraction of SmFeO3, that is, 5% (S1), 10% (S2), 20% (S3), 30% (S4). (b and c) Magnetization vs magnetic field curves for unpoled and poled (at electric field of 100 kV/cm) SmFeO3/PVDF-TrFe composite films S1 and S4, respectively. The upper left inset of figure shows percentage drop in magnetization after poling and lower right inset indicate enlarge view.

− ((−CF2−CH2)x−(−CF2−CHF−)1‑x)n− chains arranged in a pseudohexagonal lattice38) is responsible for ferroelectric polarization in P(VDF-TrFE) and incorporation of filler (SFO nanoparticles), might have modified its crystalline structure. Therefore, the increased content of SFO reduces the ferroelectric phase of P(VDF-TrFE) in composite films. Further, increase in the SFO concentrations (>30%) leads to increase the dielectric losses of composite films, (observed in preliminary measurements but not shown in the manuscript). Therefore, detailed measurements were made on samples up to 30% filler content. It has been well studied in previous reports39 that the addition of fillers in polymers strongly affect the crystallization kinetics and the resulting polymer morphology.40 This behavior depends on many factors including the filler type, their size, content, filler and polymer chains interface, dispersion and processing conditions.41 The change in crystallization of ferroelectric polymers with different types of fillers, for example, graphite, carbon fibers, carbon nanotubes, ceramic nanoparticles, and metal fillers have been widely studied.40 For instance, few additives (e.g., carbon nanotubes, BTO nanoparticles, silver nanoparticles) and their increasing concentration act as nucleating agent for ferroelectric polymers PVDF by increasing the degree of crystallinity, while the presence of potassium, cesium ions, nickel and gold nanowires etc. has the opposite effect.40 Recently, Kim et al. report the enhancement of piezoelectric β-phase of the PVDF polymer in D

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Figure 3. (a) Magnetic DC field dependent ME coefficient (α) of unpoled and (b) poled SmFeO3/PVDF-TrFe composite films with varying volume fraction of SmFeO3, that is, 5% (S1), 10% (S2), 20% (S3), 30% (S4). (c) Configuration of the ME coefficient (α) measurement. (d) αmaximum and αremnant values as a function of SmFeO3 content in composite SmFeO3/PVDF-TrFe films.

values at Hdc = 0 (self-biased CME effect), whereas unpoled sample shows zero α values under zero bias field (Hdc). The nonzero values of ME coefficient α at zero Hbias establishes selfbiased ME effect20 in poled composite films. It is worth noticing that the operation of devices without a dc bias (Hdc) is advantageous for practical applications. The αmaximum and αremnant (αR) values increases with increasing the SFO nanoparticles concentration in composite films (Figure 3d) and the maximum ME response is optimized at 30% SFO content (S4). Indeed, the value of ME coupling coefficient α can be written as27 α=

dp ∝ qd dh

Figure 4. DC-bias magnetic field dependence of ME coefficient (α) for SmFeO3/P(VDF-TrFE) composite films (a) S1, (b) S2, (c) S3, and (d) S4. Red and black curves indicate the state of α depends on the relative orientation of M and P. Insets of panel a illustrate schematically the change of relative orientation between M and P during the measurement. Repeatable switch of α by applying electric fields E as a function of time measured for samples (e) S1, (f) S2, (g) S3, and (h) S4.

(1)

where q = dλM/dH and d = dP/dλP, λ represent strain, and q and d correspond to the piezomagnetic coefficient of the SFO and piezoelectric coefficient of the P(VDF-TrFE), respectively. Hence, increase in SFO content leads to increase in piezomagnetic coefficient of composite films and allows stronger ME coupling between consecutive phases.31 Further higher concentration (>30%) of SFO nanoparticles in composite films might lead to the disruption of the ferroelectric PVDF copolymer phase. We further demonstrate use of SFO/P(VDF-TrFE) nanocomposite films as nonvolatile memory element, as shown by schematic structure in Figure 2c. To develop a memory cell, a multiferroic medium with ME effects, is sandwiched between two electrodes. Because of the strong ME effects in the SFO/ P(VDF-TrFE) composite films, switching the direction of inplane M would change vertical direction of P and vice versa. However, in case of constant M, the direction of P decides the sign of αD= dP/dH: αD > 0 for +P (up) and αD < 0 for −P (down). For ME measurements, the direction of P was set by applied positive and negative E = 4 kV cm−1 on composite films, as shown in Figure 4a−4d. In case of upward P (the black curves), α increased rapidly with increasing Hdc, and reached to a maximum value at a particular Hdc, where the magnetostriction coefficient of SFO reaches a peak and then

decreases at high Hdc (5 kOe) because of saturated M and nearly zero magnetostriction coefficient of SFO. In the SFO/ P(VDF-TrFE) nanocomposite films, the Hdc value at which α reached its maximum, depends on the SFO concentration. For 5% SFO content, maximum α is observed at Hdc = 0.7 kOe (Figure 4a), while for higher SFO concentrations (10%, 20%, 30%) maximum α reached at Hdc= 0.45 kOe (Figure 4b−4d). However, maximum α values gradually increases with SFO concentration in composite films. The sign of ME-coupling coefficient α switches from positive to negative when Hdc scans from positive to negative. Contrarily, for downward direction of P (the red curves), α dependence on Hdc is completely reversed, being α < 0 for + Hdc and α > 0 for −Hdc. It is worth noticing that the relative orientation of M and P decides the sign of α. When the direction of P is fixed, switching of α state is possible by reversal of M with Hdc. On the other hand when the direction of M is fixed, the α state can be switched by reversal of P with E. Thus the direction of M is not influenced with P reversal, and this phenomenon can be used for E

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ACS Applied Nano Materials nonvolatile memory operation, employing the ME coupling coefficient α of SFO/P(VDF-TrFE) nanocomposite films. In the following, we show repeatable switching of α for the device structure (Figure 2c) made up of SFO/P(VDF-TrFE) nanocomposite films, as shown in Figure 4e−4h. For each case, α is measured for ∼100 s after applying an positive electric pulse then a negative E pulse is applied and again α is measured for ∼100 s. For a memory device operation, repeated writing and reading of binary information is required. Therefore, the process is repeated for seven cycles. When the applied E reverses direction of P, the state of α also reverses and retains its same state until the next E pulse is applied. Figure 4a−4d illustrate switching of α states between two levels (α > 0 and α < 0). For nonvolatile memory applications, the states of α can be encoded in terms of binary information of “0” and “1” (e.g., α > 0 as “0” and α < 0 as “1”). It should be noted that the required switching voltage is different for different composite films (S1−S4). Electric pulse E of +120, 90, 70, and 60 kV cm−1 is applied for switching αE of magnitude 46, 39, 35, 30 mV cm−1 Oe1− for the samples S1, S2, S3, and S4, respectively. This infers that the applied E can be modulated according to different αE values, as observed for multilevel nonvolatile memory.3 The mechanism of switching of α with P reversal in SFO/ P(VDF-TrFE) nanocomposite films can be explained via strain induced ME coupling across the interface between SFO and P(VDF-TrFE), as shown schematically in Figure 5. The electric field poling of composite films in z direction (Figure 5b) leads to the alignment of ferroelectric domains in applied field direction and induces strain (dλP) in ferroelectric matrix in x and y direction and consequently fix a fraction of magnetic moments in these directions. Effect of electric filed poling on magnetization is clearly reflected in M−H curves (Figure 2), when the applied magnetic field is in-plane direction of composite films. The electric filed modulated magnetization establishes magneto electric coupling in the composite system. As mentioned above in eq 1, α can be expressed as follows: α = dP/dλP) × (dλM/dH). The terms dP/dλP and dλM/dH represent piezoelectric coefficient of the ferroelectric matrix and magnetostriction coefficient of the magnetic phase fraction, respectively. When P is reversed in the presence of applied magnetic field (Hdc + Hac) in the in-plane direction of composite films, the ferroelectric domains switch (as shown schematically in the Figure 5c and 5d) and the sign of dP/dλP is reversed but resultant M remains unchanged. Indeed, the magnetostriction in SFO depends on the rotation angle of the M.28 Hence, in case of constant dλM/dH, the sign of α is just determined by dP/dλP. However, the increases in concentration of SFO nanoparticles have substantial influence on the magnetoelectric coupling as well as required voltage for switching of α. Certainly, the higher concentration (20− 30%) of SFO nanoparticles in composite films with 0−3 type configuration leads to large interface interaction between polymer chain and SFO nanoparticles. In this scenario, a large number of SFO nanoparticles are connected to the polymer chain and the magnetic moments are locked in a particular direction when P is upward and applied magnetic field Hdc is perpendicular to it. Hence, switching of the ferroelectric domains with P reversal becomes difficult and relatively higher voltage is required to switch the ferroelectric domains and to change the sign of α because M remains unchanged. Figure 5e and 5f represent possible ferroelectric domains switching with P reversal in 0−3 type configuration for higher concentration

Figure 5. Schematic illustration of possible switching mechanism of α with P reversal. (a) Random arrangement of ferroelectric and magnetic domains within the SmFeO3/P(VDF-TrFE) composite films in unpoled condition. (b) alignment of ferroelectric domains after electric poling of composite film. (c, d) Switching of ferroelectric domains with P reversal ((c) when P is upward and (d) P is upward) in the presence of in plane applied ac magnetic field in prepoled composite films with low concentration (5−10%) of magnetic SmFeO3 nanoparticles. (e, f) Switching of ferroelectric domains with P reversal ((c) when P is upward and (d) P is upward) in the presence of in plane applied ac magnetic field in prepoled composite films with high concentration (20−30%) of magnetic SmFeO3 nanoparticles.

(20−30%) of SFO nanoparticles in composite films. The switching operation is possible in all the composite films. We have shown that the effect of content on the magnitude of magneto electric output voltage. The experiments demonstrate that composite films can be used as a memory element. Hence, for the device operation, the composite films can be prepared with appropriate SFO content. Compared to the previously known non volatile memories, ME coefficient based memories have numerous exceptional advantages.12,13 For instance, FRAMs possess readout process via polarization switching and then for rewriting process, it is required to switch the polarization back. Hence they suffer from destructive readout process.12,13 However, in the present device structure the readout of ME coefficient α does not require polarization switching. The writing of digital information occurs via polarization switching, similar to FRAMs. While the read out process is completed by simply measuring the sign of α, employing a low magnetic field and the induced voltage is evaluated across the two electrodes. Hence, this process offer convenient reading operation as compared to the FRAM and magnetic random-access memory F

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ACS Applied Nano Materials (MRAM).12,13 The read out process need independent coil to generate a small reading magnetic field (∼1 Oe). Thus, a single reading coil can be shared by all the memory elements and parallel read out is possible for all the stored information.12 In this way, the power consumption for a high-density memory can be significantly reduced. Thus this process proposes convenient and efficient way of the writing and reading operations. Although, the principle of using ME coefficient α to store digital information has been recently used in the study of ME composites toward nonvolatile memories.12,13 However, new materials need to be explored for nonvolatile memories. In this context, the SFO/P(VDF-TrFE) composite films made up of lead free, cost-effective and easily processable materials hold great promise for flexible memories applications.

(2) Jeong, D. S.; Thomas, R.; Katiyar, R. S.; Scott, J. F.; Kohlstedt, H.; Petraru, A.; Hwang, C. S. Emerging Memories: Resistive Switching Mechanisms and Current Status. Rep. Prog. Phys. 2012, 75, 076502−076533. (3) Shirolkar, M. M.; Hao, C.; Yin, S.; Li, M.; Wang, H. Influence of Surface Null Potential on Non-Volatile Bistable Resistive Switching Memory Behavior of Dilutely Aluminum Doped ZnO Thin Film. Appl. Phys. Lett. 2013, 102, 243501−243505. (4) Balliou, A.; Papadimitropoulos, G.; Skoulatakis, G.; Kennou, S.; Davazoglou, D.; Gardelis, S.; Glezos, N. Low-Dimensional Polyoxometalate Molecules/Tantalum Oxide Hybrids for Non-Volatile Capacitive Memories. ACS Appl. Mater. Interfaces 2016, 8, 7212− 7220. (5) Cho, D. Y.; Luebben, M.; Wiefels, S.; Lee, K. S.; Valov, I. Interfacial Metal-Oxide Interactions in Resistive Switching Memories. ACS Appl. Mater. Interfaces 2017, 9, 19287−19295. (6) Bae, I.; Hwang, S. K.; Kim, R. H.; Kang, S. J.; Park, C. WaferScale Arrays of Non-Volatile Polymer Memories with Microprinted Semiconducting Small Molecule/Polymer Blends. ACS Appl. Mater. Interfaces 2013, 5, 10696−10704. (7) Yang, Y. C.; Pan, F.; Liu, Q.; Liu, M.; Zeng, F. Fully Roomtemperature-Fabricated Non-Volatile Resistive Memory for Ultrafast and High-Density Memory Application. Nano Lett. 2009, 9, 1636− 1643. (8) Rozenberg, M.; Inoue, I.; Sanchez, M. Non-Volatile Memory with Multilevel Switching: A Basic Model. Phys. Rev. Lett. 2004, 92, 178302−178304. (9) Shirolkar, M. M.; Hao, C.; Dong, X.; Guo, T.; Zhang, L.; Li, M.; Wang, H. Tunable Multiferroic and Bistable/Complementary Resistive Switching Properties of Dilutely Li-Doped BiFeO3 Nanoparticles: an Effect of Aliovalent Substitution. Nanoscale 2014, 6, 4735−4744. (10) Shirolkar, M. M.; Li, J.; Dong, X.; Li, M.; Wang, H. Controlling the Ferroelectric and Resistive Switching Properties of a BiFeO3 Thin Film Prepared Using Sub-5 nm Dimension Nanoparticles. Phys. Chem. Chem. Phys. 2017, 19, 26085−26097. (11) Al-Haddad, A.; Wang, C.; Qi, H.; Grote, F.; Wen, L.; Bernhard, J.; Vellacheri, R.; Tarish, S.; Nabi, G.; Kaiser, U.; Lei, Y. HighlyOrdered 3D Vertical Resistive Switching Memory Arrays with Ultralow Power Consumption and Ultrahigh Density. ACS Appl. Mater. Interfaces 2016, 8, 23348−23355. (12) Shen, J. X.; Cong, J. Z.; Chai, Y. S.; Shang, D. S.; Shen, S. P.; Zhai, K.; Tian, Y.; Sun, Y. Non-Volatile Memory Based on Nonlinear Magnetoelectric Effects. Phys. Rev. Appl. 2016, 6, 021001. (13) Shen, J. X.; Cong, J. Z.; Shang, D. S.; Chai, Y. S.; Shen, S. P.; Zhai, K.; Sun, Y. Sci. Rep. 2016, 6, 34473. (14) Bibes, M.; Barthélémy, A. Towards a Magnetoelectric Memory. Nat. Mater. 2008, 7, 425−426. (15) Heron, J. T.; Bosse, J. L.; He, Q.; Gao, Y.; Trassin, M.; Ye, L.; Clarkson, J. D.; Wang, C.; Liu, J.; Salahuddin, S.; Ralph, D. C.; Schlom, D. G.; Iñ́ iguez, J.; Huey, B. D.; Ramesh, R. Deterministic Switching of Ferromagnetism at Room Temperature Using an Electric Field. Nature 2014, 516, 370−373. (16) Baek, S. H.; Jang, H. W.; Folkman, C. M.; Li, Y. L.; Winchester, B.; Zhang, J. X.; He, Q.; Chu, Y. H.; Nelson, C. T.; Rzchowski, M. S.; Pan, X. Q.; Ramesh, R.; Chen, L. Q.; Eom, C. B. Ferroelastic Switching for Nanoscale Non-Volatile Magnetoelectric Devices. Nat. Mater. 2010, 9, 309−314. (17) Lu, X.; Kim, Y.; Goetze, S.; Li, X.; Dong, S.; Werner, P.; Alexe, M.; Hesse, D. Magnetoelectric Coupling in Ordered Arrays of Multilayered Heteroepitaxial BaTiO3/CoFe2O4 Nanodots. Nano Lett. 2011, 11, 3202−3206. (18) Erdem, D.; Bingham, N. S.; Heiligtag, F. J.; Pilet, N.; Warnicke, P.; Vaz, C. A. F.; Shi, Y.; Buzzi, M.; Rupp, J. L. M.; Heyderman, L. J.; Niederberger, M. Nanoparticle-Based Magnetoelectric BaTiO3− CoFe2O4 Thin Film Heterostructures for Voltage Control of Magnetism. ACS Nano 2016, 10, 9840−9851. (19) Waser, R.; Aono, M. Nanoionics-Based Resistive Switching Memories. Nat. Mater. 2007, 6, 833−840.

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CONCLUSION In summary, our results demonstrate the possibility of the SmFeO3/P(VDF-TrFE) nanocomposite films in nonvolatile memory devices. A very simple paradigm device structure is presented for high-density storage memory element. At room temperature, repeatable nonvolatile switching of ME coupling coefficient αE between high and low states is realized by applying positive and negative E pluses. This kind of memory based on α results in low energy consumption. A low cost, easy and flexible device structure using P(VDF-TrFE)/SmFeO3 nanocomposite films, has great potential for use in nonvolatile memories.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsanm.8b00401. X-ray diffraction patterns of parent compounds (SmFeO3 and P(VDF-TrFE)) and SmFeO3/P(VDFTrFE) nanocomposite films (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. ORCID

Anju Ahlawat: 0000-0001-5317-1514 Author Contributions

A.A. designed and formuated the set of experiments. A.A. analyzed the experimental data and wrote the manuscript. S.S. and A.K.K. mentored the research work. M.M.S, J.L., and H.W. performed the magnetoelectric measurements and its analysis. A.A.K. and P.D. helped in sample preparation and contributed in discussion. R.J.C. carried out magnetic measurement. All authors contributed through scientific discussion. All authors have given approval to the final version of the manuscript. Funding

This work was supported by Department of Science and Technology (DST), New Delhi, India. Notes

The authors declare no competing financial interest.

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REFERENCES

(1) Yang, J. J.; Strukov, D. B.; Stewart, D. R. Memristive Devices for Computing. Nat. Nanotechnol. 2013, 8, 13−24. G

DOI: 10.1021/acsanm.8b00401 ACS Appl. Nano Mater. XXXX, XXX, XXX−XXX

Article

ACS Applied Nano Materials (20) Yang, J. J.; Pickett, M. D.; Li, X.; Ohlberg, D. A. A.; Stewart, D. R.; Williams, R. S. Memristive Switching Mechanism for Metal/oxide/ Metal Nanodevices. Nat. Nanotechnol. 2008, 3, 429−433. (21) Kolobov, A. V.; Fons, P.; Frenkel, A. I.; Ankudinov, A. L.; Tominaga, J.; Uruga, T. Understanding the Phase-Change Mechanism of Rewritable Optical Media. Nat. Mater. 2004, 3, 703−708. (22) Wuttig, M.; Yamada, N. Phase-Change Materials for Rewriteable Data Storage. Nat. Mater. 2007, 6, 824−832. (23) Scott, J. F.; Paz de Araujo, C. A. Ferroelectric Memories. Science 1989, 246, 1400−1405. (24) Garcia, V.; Fusil, S.; Bouzehouane, K.; Enouz-Vedrenne, S.; Mathur, N. D.; Barthélémy, A.; Bibes, M. Giant Tunnel Electroresistance for Non-Destructive Readout of Ferroelectric States. Nature 2009, 460, 81−84. (25) Guo, R.; You, L.; Zhou, Y.; Shiuh Lim, Z.; Zou, X.; Chen, L.; Ramesh, R.; Wang, J. Non-Volatile Memory Based on The Ferroelectric Photovoltaic Effect. Nat. Nat. Commun. 2013, 4, 1990−1995. (26) Ahlawat, A.; Satapathy, S.; Deshmukh, P.; Shirolkar, M. M.; Sinha, A. K.; Karnal, A. K. Electric Field Poling Induced Self-Biased Converse Magnetoelectric Response in PMNPT/NiFe2O4 Nanocomposites. Appl. Phys. Lett. 2017, 111, 262902. (27) Lu, P.; Shang, D.; Shen, J.; Chai, Y.; Yang, C.; Zhai, K.; Cong, J.; Shen, S.; Sun, Y. Non-Volatile Transtance Change Random Access Memory Based on Magnetoelectric P(VDF-TrFE)/Metglas Heterostructures. Appl. Phys. Lett. 2016, 109, 252902−252907. (28) Martins, P.; Lopes, A.; Lanceros-Mendez, S. Electroactive Phases of Poly (vinylidene fluoride): Determination, Processing and Applications. Prog. Polym. Sci. 2014, 39, 683−706. (29) Martins, P.; Lasheras, A.; Gutierrez, J.; Barandiaran, J.; Orue, I.; Lanceros-Mendez, S. Optimizing Piezoelectric and Magnetoelectric Responses on CoFe2O4/P(VDFTrFE) Nanocomposites. J. Phys. D: Appl. Phys. 2011, 44, 495303−495310. (30) Martins, P.; Lanceros-Mendez, S. Polymer-Based Magnetoelectric Materials. Adv. Funct. Mater. 2013, 23, 3371−3385. (31) Ahlawat, A.; Satapathy, S.; Choudhary, R. J.; Shirolkar, M. M.; Singh, M. K.; Gupta, P. K. Tunable Room Temperature Magnetoelectric Response of SmFeO3/poly(vinylidene fluoride) Nanocomposite Films. RSC Adv. 2016, 6, 44843−44850. (32) Abe, M.; Kaneta, K.; Gomi, M.; Mori, Y.; Nomura, S. Magnetostriction of SmFeO3 and YFe1‑xCox/2Tix/2O3 in the Spin Reorientation Region. Jpn. J. Appl. Phys. 1977, 16, 1799−1804. (33) Brito-Pereira, R.; Ribeiro, C.; Lanceros-Mendez, S.; Martins, P. Magnetoelectric Response on Terfenol-D/P(VDFTrFE) Two-Phase Composites. Composites, Part B 2017, 120, 97−102. (34) Gonçalves, R.; Larrea, A.; Zheng, T.; Higgins, M. J.; Sebastian, V.; Lanceros-Mendez, S.; Martins, P. Synthesis of Highly Magnetostrictive Nanostructures and Their Application in A Polymer-based Magnetoelectric Sensing Device. Eur. Polym. J. 2016, 84, 685−692. (35) Prabhakaran, T.; Hemalatha, J. Ferroelectric and magnetic studies on unpoled poly (vinylidine- fluoride)/Fe3O4 magnetoelectric nanocomposite structures. Mater. Chem. Phys. 2013, 137, 781−787. (36) Hahm, S. W.; Khang, D. Y. Crystallization and MicrostructureDependent Elastic Moduli of Ferroelectric P(VDF−TrFE) Thin Films. Soft Matter 2010, 6, 5802−5806. (37) Andrew, J. S.; Clarke, D. R. Effect of Electrospinning on the Ferroelectric Phase Content of Polyvinylidene Difluoride Fibers. Langmuir 2008, 24, 670−672. (38) Gil, H. A. C.; Faria, R. M.; Kawano, Y. Structural Modifications of Polyvinylidene fluoride- trifluoroethylene (70−30) Copolymer Induced by X-ray Irradiation. Polym. Degrad. Stab. 1998, 61, 265− 273. (39) Subhash, C. K.; Valiyaneerilakkal, U.; Singh, K.; Varghese, S. Device Level Optimization of Poly(vinylidene fluoride-trifluoroethylene)−Zinc Oxide Polymer Nanocomposite Thin Films for Ferroelectric Applications. J. Appl. Phys. 2015, 118, 204102−204108. (40) Prateek; Thakur, V. K.; Gupta, R. K. Recent Progress on Ferroelectric Polymer-Based Nanocomposites for High Energy

Density Capacitors: Synthesis, Dielectric Properties, and Future Aspects. Chem. Rev. 2016, 116, 4260−4317. (41) Cho, K. Y.; Lee, Y. J.; Kim, H. J.; Yoon, H. G.; Hwang, S. S.; Han, Y. K.; Baek, K. Y. Interfacial Control of PVDFTrFE/SWCNT Nanocomposites Using P3HTPMMA block Copolymer for UltraLow Percolation Threshold. Polymer 2015, 77, 55−63. (42) Kim, G. H.; Hong, S. M.; Seo, Y. Piezoelectric Properties of Poly(vinylidene fluoride) and Carbon Nanotube Blends: β-phase Development. Phys. Chem. Chem. Phys. 2009, 11, 10506−10512. (43) Liu, Y.; Zhan, Q.; Dai, G.; Zhang, X.; Wang, B.; Liu, G.; Zuo, Z.; Rong, X.; Yang, H.; Zhu, X.; Xie, Y.; Chen, B.; Li, R. W. Thermally Assisted Electric Field Control of Magnetism in Flexible Multiferroic Heterostructures. Sci. Rep. 2015, 4, 6925−6931. (44) Wang, Y.; Hu, J.; Lin, Y.; Nan, C. W. Multiferroic Magnetoelectric Composite Nanostructures. NPG Asia Mater. 2010, 2, 61−68.

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DOI: 10.1021/acsanm.8b00401 ACS Appl. Nano Mater. XXXX, XXX, XXX−XXX