Anomalous Magnetoelectric Coupling Effect of CoFe2O4âBaTiO3

Article Cite This: ACS Appl. Electron. Mater. XXXX, XXX, XXX−XXX

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Anomalous Magnetoelectric Coupling Effect of CoFe2O4−BaTiO3 Binary Mixed Fluids Rongli Gao,*,†,‡ Qingmei Zhang,§ Zhiyi Xu,∥ Zhenhua Wang,†,‡ Gang Chen,†,‡ Chunlin Fu,†,‡ Xiaoling Deng,†,‡ and Wei Cai*,†,‡ †

School of Metallurgy and Materials Engineering, Chongqing University of Science and Technology, Chongqing 401331, China Chongqing Key Laboratory of Nano/Micro Composite Materials and Devices, Chongqing 401331, China § School of Applied Science, Taiyuan University of Science and Technology, Taiyuan 030024, China ∥ National Institute of Metrology, Beijing 100029, China

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ABSTRACT: We report an anomalous magnetoelectric coupling effect of CoFe2O4−BaTiO3 binary mixed fluids, which were prepared by distributing surfactant treated (oleic acid) magnetic CoFe2O4 and ferroelectric BaTiO3 nanoparticles into highly insulating base fluid (silicone oil). A strong coupling effect in this mixed fluid has been observed, in which both the remnant polarization/magnetization and coercive electric/magnetic field are found to be enhanced visibly in the presence of an external magnetic/electric field. Colossal converse and direct magnetoelectric coupling coefficients are estimated to be αH = 1.22 × 10−4Oe·cm/V and αE = 2.52 × 104 V/(cm·Oe), respectively. Moreover, this coefficient clearly depends on the volume fraction of the two kinds of particles and the applied external magnetic/electric field. Even more amazing, CoFe2O4 also shows a strong magnetoelectric coupling effect with a coefficient as high as 4.2 × 10−5 Oe·cm/V. Further analysis indicates that the aggregated, chain-like structure of the particles under the action of external fields is believed to be responsible for the clamping effect, which in turn induces the observable anomalous coefficient. The field generated chain-like structure shows intense relaxation behavior with a relaxation time of several minutes. These results provide valuable information for enhancing the coupling effect of mixed fluids and may be important for practical applications in magnetoelectric devices. KEYWORDS: multiferroic materials, coupling effect, binary, mixed fluids, clamping effect

1. INTRODUCTION Multiferroic magnetoelectric materials, which simultaneously exhibit ferroelectricity and ferromagnetism, have recently stimulated a sharply increasing number of research activities because of their scientific interest and significant applications.1−3 In addition, the magnetic properties of multiferroic materials can be manipulated by external electric field or vice versa, which is called the magnetoelectric coupling effect. Such coupling effects in multiferroics can be exploited to implement additional functionalities on existing devices or to circumvent bottlenecks that impede the device performances.4 Although magnetoelectric effects have been extensively explored in single-phase multiferroic compounds, it is still challenging to achieve controllable switching/modulation of a substantial net polarization (magnetization) with a magnetic (electric) field, mostly because the magnetic or (and) the ferroelectric Curie temperature is usually far below room temperature; thus, their application to magnetoelectric devices remains a long-term goal.5 Therefore, how to enhance the coupling effect further is at present a very serious challenge. Driven by this worldwide goal, researchers have begun to investigate a broad spectrum of candidate materials. Among them, multiferroic composites, © XXXX American Chemical Society

which are composed of a magnetic phase and ferroelectric phase, offer an exciting opportunity to explore new materials. Multiferroic composites take advantage of the specific coupling between the individual components and thus show significant magnetoelectric coupling as opposed to the single phase.6−8 It is well-known that the probable route to obtain a strong magnetoelectric effect is to couple the interfacial strain between a piezoelectric effect in the ferroelectric phase and a magnetostrictive effect in the ferromagnetic phase.9−11 It is well-known that the coupling effect is obviously dependent on the content of the ferroelectric and magnetic phases, the geometric parameter of the surface, and the connective patterns 12−15. Until now, many works have tried different combinations of ferroelectric and magnetic phases; nevertheless, the coupling effect in composite multiferroics is not destined to be strong because of the sharply limited magnetostrictive/electrostrictive coefficient of magnetic/ferroelectric phases. Received: March 6, 2019 Accepted: June 19, 2019 Published: June 19, 2019 A

DOI: 10.1021/acsaelm.9b00140 ACS Appl. Electron. Mater. XXXX, XXX, XXX−XXX

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ACS Applied Electronic Materials

Figure 1. (a) XRD patterns of CoFe2O4 and BaTiO3 particles, (b) TEM images of CoFe2O4 particles, (c) TEM images of BaTiO3 particles, (d) TEM images of CoFe2O4−BaTiO3 mixed particles after ball-milling for 15 h, (e) magnetic hysteresis loops of CoFe2O4 particles, and (f) ferroelectric hysteresis loop of the BaTiO3 ceramic.

can be tuned by electric field and the microstructure of ferroelectric particles can also be affected by magnetic field; as a result, a magnetoelectric coupling effect can be obtained. Here, we report the observation of controlling magnetization/ polarization under an external electric/magnetic field in CoFe2O4 (CFO)−BaTiO3 (BTO) binary mixed fluids, from which another coupling mechanism, the so-called clamping effect, but not the conventional interface stress, has been proposed, and an enhancement of the coercive field as well as the improved residual magnetization/polarization have been observed.

In previous works, we have found that when core−shell structured CoFe2O4(CFO)@BaTiO3 (BTO) composite particles were distributed into fluids, strong coefficients (αH = 8.16 × 10−4 (Oe cm) V−1 and αE = 1.58 × 104 V (cm Oe)−1) can be observed.16,17 Nothing has ever been proven, but we believe that CFO@BTO composite particles can move and then further form a chain-like structure when an external magnetic or (and) electric field is applied. Because the CFO phase and BTO phase are an integral part of the core−shell structure composite particles, once the particle moves under the action of a field, both the magnetization direction and the direction of polarization can be changed; therefore, magnetoelectric coupling effect is formed. On the basis of this assumption, if the magnetic and ferroelectric particles can respectively move and form a chainlike structure under the actions of a magnetic/electric field, the magnetic chains and ferroelectric chains will interact with each other. As a consequence, the kinestate of magnetic particles

2. EXPERIMENTAL PROCESS 2.1. Materials. All chemical reagents were purchased from Sinopharm Chemical Regent Beijing Co., Ltd., including Fe(NO3)3· 9H2O (ferric nitrate), Co(NO3)2·6H2O (cobalt nitrate), NaOH (sodium hydroxide), HNO3 (nitric acid), Ba(NO3)2 (barium nitrate), C6H8O7 (citric acid), C2H5OCH2CH2OH (glycol ether), CH3COOH B


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with the applied field and in the normal direction of the sheet. The measurements were all performed at room temperature. The transmitted light was measured by a photoelectric cell detector that outputs a voltage signal proportional to the light intensity.

(acetic acid), C18H34O2 (oleic acid), C6H18OSi2 (silicone oil), and CH3COCH3 (acetone). All of these reagents are of analytical purity and were used without further purification. Deionized water was used throughout the synthesis and treatment processes. 2.2. Synthesis of CFO Particles. Co(NO3)2·6H2O and Fe(NO3)3·9H2O with the molar ratio of 1:2 were individually dissolved in 50 mL of deionized water and stirred for several minutes to form a clear solution; at the same time, NaOH was dissolved in 200 mL of deionized water to form 2 mol/L clear solution, which was then maintained at 90 °C. Before Co(NO3)2·6H2O and Fe(NO3)3·9H2O were mixed, 5 mL of HNO3 was individually added to the two solutions and then mixed evenly. Subsequently, this mixture was cautiously added into NaOH solution, allowing the reaction to subside with heating again between drops. Following all of the solution being mixed, it was heated to a boil and maintained for 1 h with frequent stirring, followed by a natural cooling to room temperature. After the reaction, the obtained product was washed with deionized water and acetone several times, and then, it was dried in a vacuum oven at 100 °C for 8 h followed by sintering in a microwave oven at 1100 °C for 30 min. Then, the calcined powders were prepared through ball-milling, and the final products were dried at 80 °C for 12 h. 2.3. Synthesis of BTO Particles. Ba(CH 3 COO) 2 and TiC12H28O4 with the molar ratio of 1:1 were mixed with moderate 2-ethoxyethanol and acetic acid and stirred constantly, and then, the mixture was heated at 70 °C for 30 min. The pH value of the precursor solution is about 3 by adding the appropriate amount of HOCH2CH2NH2. Then, CH3COCH2COCH3 was added to control the hydrolysis rate. The obtained gels were dried at 100 °C for 10 h followed by sintering in microwave oven at 1100 °C for 30 min. Then, the calcined powders were prepared through ball-milling, and subsequently, the final products were dried at 80 °C for 12 h. As a contrast, some dried gels were annealed at 600 °C for 10 min in oxygen atmosphere and then ground gently and pressed into slices with diameters of 10 mm; afterward, they were sintered at 1100 °C for 30 min by microwave to obtain BTO ceramics. To measure the electric properties, all sintered samples were polished and then coated with silver electrodes on both sides. Subsequently, the ceramics were sintered at 500 °C for 30 min, and then, they were cooled down to room temperature naturally. 2.4. Synthesis of CFO−BTO Binary Mixed Fluids. CFO and BTO powders with the mass ratio 1:1 were dispersed into oleic acid and prepared by the ball-milling dispersion method, followed by sonication for 2 h, and then drying at 100 °C for 24 h. Subsequently, silicone oil was added to the surfactant (oleic acid) treated CFO− BTO mixed nanoparticles, and this mixture was ball-milled for 15 h to ensure the particles dispersed completely into the silicone oil, forming a pure CFO−BTO binary mixed fluid. Prior to measuring the electric and magnetic properties, the prepared fluids were sonicated for several minutes immediately prior to filling the lamellar vessels. 2.5. Characterization. The crystalline structure of CFO and BTO particles was confirmed by a X-ray diffractometer (Smart Lab, Rigaku, Japan) using a Cu Kα radiation source (λ = 1.5406 Å) in the 2θ range of 20−80°. The morphology of the prepared particles was investigated using transmission electron microscopy (TEM, JEOL, JEM-2100F). The ferroelectric hysteresis loops of the prepared BTO ceramics were measured with a ferroelectric test system (TF2000, aix-ACCT Inc., Germany) while the ferroelectric properties of CFO−BTO mixed fluids were characterized by combining a signal generator (HP33120A), oscilloscope (TEKTRONIX, TDS2012C), and voltage amplifier (PINTECH, HA-205). Combined with a high voltage DC power supply (HVDC, DW-P503-1ACD, Vmax = 30 kV), magnetic hysteresis (M−H) loops were carried out at room temperature using a vibrating sample magnetometer (VSM, VSM100-500) with a maximum magnetic field of ±1 T. Using the typical ER rheometer (MCR301 Anton Paar, Germany) equipped with a high voltage power supply, the shear stress was measured under various electric field strengths. The light source for transmittance measurement under the action of magnetic and electric field is a semiconductor laser with a 650 nm wavelength and 3 mW power. The incident light is parallel

3. RESULTS AND DISCUSSION Figure 1(a) shows the XRD patterns of the prepared CoFe2O4 and BaTiO3 particles; it can be seen that there is no evident impurity presence in the composites with this resolution limit of XRD. Figure 1(b) is a typical TEM image of the synthesized CoFe2O4 particles; all the particles present a spherical-like structure, and the particle size is about 25 nm. Figure 1(c) is a typical TEM image of the synthesized BaTiO3 particles; all the particles present a spherical-like structure, and the particle size is about 35 nm. The TEM images of CFO−BTO mixed particles that have been ball-milled for more than 15 h are shown in Figure 1(d). The results indicate that after ballmilling for more than 15 h, the size of the two particles is about 15 nm, which is slightly less than that without ball-milled particles. And the shape has become more sphere-like after milling. Magnetic hysteresis loops of CoFe2O4 particles were measured at room temperature, as shown in Figure 1(e). A clear hysteresis loop has been observed, indicating ferromagnetic behavior; the remnant magnetization (Mr) and coercive field (Hc) are about 21 emu/g and 800 Oe, respectively. The spontaneous magnetization (Ms) was determined by the extrapolation of the magnetization curve to zero applied field and was ∼51 emu/g, which is almost consistent with previous reports.18−20 Figure 1(f) shows room temperature ferroelectric hysteresis loops of the BaTiO3 ceramic measured at 1 kHz. It is observed that the spontaneous magnetization (Ps) and remnant polarization (Pr) are respectively ∼27 and ∼14 μC/ cm2, and the coercive field (Ec) is about 6.5 kV/cm, indicating excellent ferroelectric properties. In order to investigate the magnetic and magnetoelectric coupling effect of the prepared mixed fluids, a lamellate device with the inner dimensions of 15 × 15 × 0.5 mm3 was manufactured, as shown in Figure 2(a). The top and bottom

Figure 2. Schematic of the measurement of CoFe2O4−BaTiO3 mixed fluids, (a) the thin container, and (b) the schematic diagram of the measurement process. The electric field is perpendicular to the magnetic field.

indium tin oxide (ITO) electrodes were gummed together, except for two opposite corners, by using highly insulating ethoxyline; the glass plates were displaced with respect to each other by 5 mm for an electrical connection with the ITO layers. Before measurement, the container was filled with the mixed fluids, and then, it was secured around the edges with epoxy resin so that all four sides were sealed. Next, it was fixed C


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Figure 3. (a) Magnetic hysteresis loops of CFO−BTO mixed fluids with different applied electric fields, (b) the magnified part of the coordinates near the origin of (a), (c) electric field dependence of the remnant magnetization, (d) magnetoelectric coefficient as a function of applied electric field, (e) remnant magnetization as a function of volume fraction, and (f) magnetoelectric coefficient as a function of volume fraction.

general, the superparamagnetism of solid is usually attributed to the reduction in size of magnetic particles below a critical particle size. Nevertheless, this property in fluids can be ascribed to the combined action of Brownian motion of particles in fluids and the change of the magnetic moments under the influence of magnetic field. Actually, rather than the Brownian motion of particles in fluids, the superparamagnetism of CFO in fluid may arise from the reduction in particle size after ball-milling and/or the change in particle morphology (relevant to shape anisotropy). As it has been presented in Figure 1(d), after ball-milling for more than 15 h, the size of the two particles was decreased. This will result in the decreased coercive field, and thus, superparamagnetism behavior may be caused. The Langevin equation can be used to describe the paramagnetic behavior. In addition, Ms is ∼15.5 emu/g, which is much smaller than that of solid particles, which is ∼51 emu/g, as shown in Figure 1(e). The reason is that the magnetization of fluids is mainly from the ferromagnetic CFO particles is because BTO and silicone oil are nonmagnetic (paramagnetic and diamagnetic). In view of

on the sample holder of the VSM with the container surface perpendicular to the magnetic field direction. The two ITO electrodes of the container are connected to a high voltage source, as shown in Figure 2(b). Figure 3(a) shows the magnetic hysteresis loops of CFO− BTO mixed fluids measured at room temperature under different electric fields with the volume fraction (Φv) of 20%. Φv is an important parameter of fluids, which is defined as ϕv =

Vparticle Vparticle + Vcarrier

=

mCFO/ρCFO + mBTO/ρBTO mCFO/ρCFO + mBTO/ρBTO + Vcarrier (1)

wherein, Vparticle is the total volume of CFO and BTO particles; mCFO and mBTO are the weights of CFO and BTO particles, respectively; ρ CFO and ρBTO are respectively the densities of the CFO and BTO particles; Vcarrier is the volume of carrier fluid (silicone oil in this experiment) in the fluids. It is obvious that the fluids show superparamagnetic behavior without application of an electric field, which is quite different from the solid particles shown in Figure 1(c). In D


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the remnant magnetization and magnetoelectric coefficient was measured at 40 kV/cm, and the results are shown in Figure 3(e,f). It should be pointed out that although Mr increases with the increase of applied field E, the coupling coefficient αH decreases with E, as shown in Figure 3(c). Therefore, in order to obtain maximal value of Mr, the measured electric field is selected as 40 kV/cm. However, to observe the apparent coupling coefficient, the measured electric field is 5 kV/cm. The curve shape is very similar when the electric fields are 5 and 40 kV/cm; the biggest difference is the value. It can be seen that Mr increases monotonically from 0 to about 10 emu/ g when the volume fraction Φv increases from 0.01 to 50% (the maximal value in this experiment). One can speculate from this variation tendency that Mr can be enhanced continuously with the increased Φv. In addition, the results indicate that the coercivity increases monotonically as the volume fraction increases; thus, this increase in coercivity may enhance the interactions among the particles suspended in fluid. Similarly, the coupling coefficient αH increases monotonically from near zero to about 2.52 × 10−4 Oe·cm/V when the volume fraction Φv increases from 0.01 to 50%, and αH may increase further if Φv continues to increase. These results presented above show a colossal coupling coefficient in the CFO−BTO mixed fluids. This large coefficient is absent and has never been reported in solid magnetoelectric composites including one-, two-, and threedimensional magnetoelectric composites, let alone single-phase multiferroics. It can be speculated that the anomalous coupling effect is due to the influence of some force on the magnetic particles under electric field. How does this force happen in the mixed fluids that we presented above? In mixed fluids, the distribution of the magnetic and ferroelectric particles was random without an external electric and magnetic field applied, as shown in Figure 4(a). In this way, the mixed fluids can much more easily show superparamagnetic behavior, because the magnetic particle can rotate and thus change its magnetization direction under magnetic field, whereas there is only one way for the magnetic particles to change their magnetization direction in bulk nanomaterials (i.e., the Neel movement, because the magnetic particles cannot move and rotate in the bulk state). Indeed, whether the magnetic particle can rotate or not rests with the particle size to a large extent. When applying electric field, ferroelectric particles in fluids can rotate and move; then, they are aligned, and their dipole moments become parallel to the field. As a result, chain-like structures may be formed. In contrast, the polarization of magnetic particles under the action of electric field is far weaker than that of ferroelectric particles. Therefore, the magnetic particles were still distributed randomly due to the weaker interaction between the electric moment, as shown in Figure 4(b). Compared with the condition with no electric field applied, these magnetic particles show smaller free space, and thus, it is hard for them to rotate freely and change their directions of the magnetic moment because the existence of ferroelectric chains; as a consequence, a larger force (magnetic field) is needed to switch the magnetization directions of the magnetic particles. Furthermore, when applying a magnetic field in the process of measuring the M−H curves, the magnetic particles were aligned because of their magnetic properties, such that their magnetic moments became parallel to the field, and thus, chain-like structures were formed. Finally, increased Mr can be

the fact that the volume fraction is 20%, the densities of the CFO particle, BTO particle, and silicone oil are approximately 5.2, 6.1, and 1.0 g/cm3, and the mass ratio of CFO and BTO is 1:1; therefore, the weight percentage of CFO in the fluids is roughly estimated to be 28.8%, and hence, the magnetization of the CFO particles in the fluids should be 15.5/0.288 = 53.8 emu/g. This value is slightly greater than the experimentally obtained value of 51 emu/g for the pure CFO particles, which is mainly due to the nonmagnetic BTO and silicone oil, because all the magnetization has been deducted from the influence of the container. Interestingly, the fluids have been switched from superparamagnetism to ferromagnetism when electric fields are applied, both coercive field (Hc) and remnant magnetism (Mr) increase distinctly with increasing the applied field, as shown in Figure 3(b). When the electric field is 0, 10, 20, and 40 kV/cm, the corresponding Hc is 3, 154, 402, and 563 Oe, respectively, and the values of Mr are 0, 2.18, 3.12, and 4.19 emu/g, respectively, revealing a direct magnetoelectric coupling effect. In order to investigate the magnetoelectric coupling coefficient in CFO−BTO mixed fluids, Mr and Hc as functions of external electric field were extracted from Figure 3(b), as shown in Figure 3(c). Clearly, both Mr and Hc mainly show a nonlinearly increasing trend with the increase of electric field, and then, they reach saturation. The coupling coefficient can be expressed as αH = dH /dE

(2)

where αH, H, and E are the converse magnetoelectric coupling coefficient, electric field induced magnetic field, and the corresponding electric field, respectively. According to the relation 1emu/cm3 = 4π × 10−4 T, the relationship between M (emu/g) and H (Oe) can be expressed as M = 4π × ρ × H

(3)

where ρ is the density (g/cm ) of the multiferroic fluids, which can be concluded by 3

ρ=

ρparticle · Vparticle + ρcarrier · Vcarrier mparticle + mcarrier m = = V Vparticle + Vcarrier Vparticle + Vcarrier (4)

Therefore, the coupling coefficient can be rewritten as αH = dH /dE = 1/(4π × ρ) × dM /dE

(5)

When the volume fraction is 20%, the density of the CFO− BTO mixed fluids is about 1.5 g/cm3, which is concluded according to eq 4. The relation between the remnant magnetization Mr and the applied electric field E was fitted with a sixth-order polynominal function (not shown). According to experimental data and fitting curves from Figure 3(c), the coupling coefficient can be concluded based on eq 5. The magnetoelectric coefficient as a function of applied electric field is shown in Figure 3(d). As can be seen, αH decreases monotonically with increasing electric field, and it is close to zero when the field is larger than 30 kV/cm, while the maximal αH is 1.22 × 10−4 Oe·cm/V when the field is approximately zero. As the coercive field increases with the applied electric field, it can be speculated that the magnetic particles are hard to be moved and are rotated under the action of the electric field. In addition to the electric field E, volume fraction φv is an important factor that can affect the value of the coupling coefficient αH. Therefore, the volume fraction dependence of E


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Also, one piece of backstairs evidence that can prove the chain structure under electric field is the shear stress (Ts); the stronger the interaction between the chains, the larger the shear stress.21−23 The mechanism of this shear stress has attracted much more investigation, and we will not discuss it here. Field dependence of the shear stress of the CFO−BTO mixed fluids with different volume fractions is shown in Figure 5 a). It is quite clear that the shear stress shows a linearly increasing trend on the whole with the increased electric field and volume fraction, indicating the enhanced interactions of the chain-like structures, which can indirectly prove the improved clamping effect between the ferroelectric and magnetic particles under external fields. However, a new problem that has emerged here is whether the shear stress can be entirely ascribed to the chain-like structure formed by BTO particles. As a matter of fact, CFO is regarded as a material with high resistance that can be polarized and thus form a chain-like structure under the action of an electric field.24−26 As a consequence, CFO particles can present a magnetoelectric coupling effect with themselves even in the absence of BTO ferroelectric particles. For this reason, M−H curves of pure CFO ferrofluids prepared by the same method as CFO−BTO mixed fluids were measured with and without applying electric field, as shown in Figure 5(b). Interestingly, CFO ferrofluids can also show a coupling effect, i.e., both Mr and Hc increase obviously under the influence of an electric field, similar to the result of CFO−BTO mixed fluids. For the purpose of investigating the magnetoelectric coupling coefficient in CFO ferrofluids, the field dependence of Mr was measured and shown in Figure 5(c). The coupling coefficient can be concluded according to the fitted curve from Figure 5(c), and it is shown in Figure 5(d). As can be seen, it decreases monotonically with the increase of electric field, and the maximal value of αH obtained when the field is approximately zero is about 0.42 × 10−4Oe·cm/V, one-third of the value of the CFO−BTO mixed fluids shown in Figure 3(d). These results indicate that roughly half of the enormous coupling coefficient is likely to come from the pure CFO ferrofluids, while the other half came from the clamping effect between ferroelectric and magnetic particles under external field. In addition to the converse magnetoelectric coupling effect, i.e., how the magnetism is regulated by electric field, the direct converse magnetoelectric coupling effect is also very important. The transient current method17,18 was usually adopted to investigate the intrinsic ferroelectric properties of mixed fluids under different magnetic/electric fields, as shown in Figure 6. The ferroelectric parameters in the mixed fluids can be determined by the measurement of displacement current. When an electric field is applied to the device, both ferroelectric and magnetic particles can form a chain-like structure, and their dipolemoments are parallel to the applied field. A periodic AC electric field can only cause unceasing rotation of the ferroelectric nanoparticles, which gives rise to displacement current in the circuit, while the sudden charging and discharging current as well as the conduction current resulting from the capacitor can be deducted by using triangular wave voltages. If the applied electric field is high enough, the reorientation of all the particles should be completed, and thus, a displacement charge quantity 2Q can be obtained by the integration of the positive (or negative) part of the displacement current density, i.e.,

Figure 4. Schematic of magnetoelectric coupling effect in mixed fluids. (a) The particles are of random distribution without an external field applied, (b) ferroelectric particles will form a chain-like structure as a result of the application of electric field while magnetic particles are still distributed randomly, (c) ferroelectric particles form a long chain-like structure when electric field is applied while magnetic particles form a short chain-like structure, and (d) both ferroelectric particles and magnetic particles form a long chain-like structure when electric field is applied.

obtained because of the clamping effect between the ferroelectric and mgnetic chains. With the increase of electric field or (and) the enhancement of volume fraction, the interaction between ferroelectric chains and magnetic particles could be strengthened; therefore, these magnetic particles can form short chain-like structures in order to reduce the free energy despite the fact that no magnetic field was applied, as is shown in Figure 4(c). At this stage, the magnetization directions of the short chains may be different due to the lower volume fraction and electric field, so the net remnant magnetization is zero. Similarly, when applying magnetic field in the process of measuring the M−H curves, the magnetic particles were aligned and formed chain-like structures; therefore, increased Mr can be obtained because of the increasing clamping effect between the ferroelectric and mgnetic chains induced by the enhancement of electric field and volume fraction. In the same way, when the electric field and/or the volume fraction increase further, obviously, the interaction between the ferroelectric chains and the magnetic particles could be strengthened. Therefore, in the situation of E ≠ 0, H = 0, magnetic particles are clamped by the chain-like structure formed by ferroelectric particles, and the magnetic moments prefer to be aligned in order to reduce the free energy due to their interactions, as shown in Figure 4(d). In this case, Hc and Mr should increase apparently compared with the state of E = 0. Finally, when H ≠ 0 and E ≠ 0, i.e., in the course of measuring the M−H curves under the effect of external electric field, the changing magnetic field would force the magnetic particles to oscillate, and then, they are aligned along the magnetic field followed by the action of clamping. Therefore, the strongest coupling effect can be obtained, and both Mr and Hc are the maximal values. F


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Figure 5. (a) Shear stress as a function of the electric field of CFO−BTO mixed fluids with different volume fractions, (b) M−H curves of CFO ferrofluids with and without electric field applied, (c) remnant magnetization of CFO ferrofluids as a function of applied electric fields with a volume fraction of 20%, (d) converse magnetoelectric coefficient of both CFO ferrofluids as a function of applied electric fields with a volume fraction of 20%.

because the displacement charge is only contributed by ferroelectric particles, Q is the displacement charge quantity, and S is the inner effective area of the device. Eq 5 can be rewritten as Ps =

∫0

T /2

I(t )dt =

∫0

T /2

U (t ) dt Rs

(6)

Herein, T is the period of the applied triangular wave voltage in this experiment, and I is the displacement current in the circuit, which is equal to the current flow through the series resistor Rs, because Rs and the sample used in this experiment are in series, and U is the voltage drop across the series resistor (100 kΩ in this experiment), which is monitored by the oscilloscope. The polarization can be expressed as27,28 P=

Q 2S

ϕBTO

=

Q 2S ·ϕBTO

(8)

Ps is the spontaneous polarization of the BTO particle, and ΦBTO is the effective volume fraction of BTO particles in the mixed fluids. Figure 7(a) shows typical P−E curves of CFO−BTO mixed fluids with a volume fraction of 20% measured by the transient current method of the positive part. One can see that the polarization first increases rapidly from zero and then approaches saturation when the field is larger than 10 kV/ cm; the saturation polarization is ∼0.78 μC/cm2. The inset of Figure 7(a) is the entire hysteresis curve; the result indicates that the specimen shows superparaelectric behavior, i.e., without remnant polarization, and a coercive electric field can be observed. This curve is entirely different from that of the ceramic shown in Figure 1(d). This superparaelectric behavior can be attributed to the free-to-rotate electric dipoles system in an external electric field, which can also be described by the Langevin function. It is quite clear that the intrinsic spontaneous polarization Ps of BTO particles is different from the polarization calculated from the transient current method, because the current is strongly affected by the rotation of the electric dipoles under external electric field. Although the rotational complexity of BTO particles is connected with the

Figure 6. Schematic of measuring ferroelectric properties of CFO− BTO binary mixed fluids.

Q=

P

(7)

where P is the equivalent polarization of the mixed fluids, which is equal to the effective polarization of BTO particles, G


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Figure 7. P−E curves (a), P/Φv−E curves (b), P/Φv−H curves (c), Pr/Φv−H curves (d), αE−H curves (e), and Ts−H curves (f) of CFO−BTO mixed fluids.

concentration (volume fraction), with further increase of concentration, interaction between BTO particles is enhanced due to the reduced distance, which brought about the formation of new particle−particle aggregates and chain-like structures, increasing the difficulty of rotation. In consequence, Ps can be calculated by P/Φv if the interaction among the solid particles can be negligible. To research the spontaneous polarization of BTO particles, the volume fraction dependence of polarization was measured, and the results are shown in Figure 7(b). As expected, the experimental Ps decreases monotonically with increased Φv, which can be attributed to the aggregation of BTO particles. The distance between particles increases with the decrease of volume fraction. Therefore, with the decrease of volume fraction, the interaction between particles was decreased; thus, the degree of aggregation was declined. As a consequence, the polarization

should be increased with decreasing the volume fraction. When the volume fraction Φv approaches zero but is not zero, the maximal saturated polarization (the intrinsic Ps of BTO particles) can be obtained. The effective volume fraction of BTO particles ΦBTO in the mixed fluids is about half of Φv; therefore, the intrinsic Ps of BTO particles is about 22 μC/cm2 when Φv is near zero, which is extrapolated from the P/Φv−Φv curves. This high spontaneous polarization is more than twice the experimental values of the ceramic shown in Figure 1(e) and previously reported values.16,18 This enhanced spontaneous polarization can be attributed to the size effect; a spontaneous polarization of BaTiO3 as high as 120 μC/cm2 nanoparticles has been reported when the particle size is less than 10 nm.27,28 In contrast, one can deduce from Figure 7(b) that the smallest experimental polarization is about ∼15 μC/ cm2 when Φv is 100% (i.e., turning into a solid); this value is H


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ACS Applied Electronic Materials roughly equal to that of the bulk material such as is shown in Figure 1(f). We have illustrated before that when the concentration of the mixed fluids is high, particle aggregation is more serious. For one hand, higher concentrations can lead to a larger amount of aggregates, which will increase the difficulty of rotation, and thus, smaller polarization can be obtained; on the other hand, these aggregates are more likely to induce a partial compensation of their dipole moments because of the chain-like structure formed by little particles and that their dipole moments are parallel to the external electric field. As thus, the transferred charge density is naturally smaller than the spontaneous polarization of single nanoparticles significantly.27 Therefore, the experimental spontaneous polarization is very close to that of solid materials theoretically when the concentration is high enough. In addition to the concentration of ferroelectric particles in CFO−BTO mixed fluids, the formed structure of magnetic CFO particles is also another dominant factor that can affect the polarization because of the interaction between magnetic and ferroelectric particles. As previously stated, the rotation and aggregation of BTO particles can determine the value of polarization; therefore, the polarization should depend on the properties of magnetic properties. In order to investigate how the magnetic particles affect the ferroelectric properties, the spontaneous polarization with different concentrations was measured as a function of applied magnetic field, as shown in Figure 7(c). Interestingly, the magnetic response of the mixed fluids with different concentrations is quite different; the spontaneous polarization decreases slightly with increased magnetic field when the concentration is very low, such as 0.2%. On the contrary, when the volume fraction is high, for example, Φv > 2%, the polarization shows an increased trend with the external field. These results reveal a direct magnetoelectric coupling effect in the mixed fluids, which can be ascribed to the changes in the microstructure of magnetic particles under magnetic field. To intuitively present the coupling effect, magnetic field dependent intrinsic remnant polarization (Pr/Φv) under different magnetic field was measured, as shown in Figure 7(d). What needs illustration is that the remnant polarization (Pr) is calculated by the difference of the positive polarization (P+) and negative polarization (P−) obtained by the integration of the positive (negative) part of the displacement current density. For example, when the particles are in a natural state in the absence of an external magnetic/electric field, the positive and negative polarization should be the same, and the remnant polarization is supposed to be zero, because the direction of dipole moments is random; thus, Pr can be expressed as Pr = P+− P− = 0. Another particular case is that when the dipole moments are entirely parallel to the applied field, the maximal value can be observed, because P+(or P−) is zero, while P−(or P+) is the largest. It can be seen from Figure 7(d) that the remnant polarization increases rapidly first, and then, its growth speed slows down and finally approaches a saturation value. Furthermore, the remnant polarization increases with the enhanced concentration monotonously, which is different from the spontaneous polarization. This aberrant behavior may be connected with the structural change of ferroelectric particles under electric/magnetic field, indicating an anomalous direct magnetoelectric coupling effect, and the direct magnetoelectric coefficient can be expressed as

dE dH

αE =

(9)

where αE is the direct magnetoelectric coefficient (V/m·Oe), E is the induced electric field (V/m) by applied magnetic field, and H is the applied magnetic field (Oe). The sheet device (with the size of 10 × 10 × 0.1 mm3) can be regarded as a plate capacitor in this experiment, and therefore, the local electric field can be estimated to be E=

σ P = ε ε0εr

(10)

where σ is the density of the surface charge (C/m2), P is the magnetic field induced polarization, which is equal to the remnant polarization of the samples (C/m2), ε0 is the vacuum permittivity (∼8.854 × 10−12 F/m), and εr is the relative dielectric constant of the mixed fluids (∼11 at 1 kHz). Therefore, the direct magnetoelectric coupling coefficient can be rewritten as αH =

dE dP = dH ε · dH

(11)

The relationship between Pr and H can be expressed by the fitting curves of Figure 7(d); thus, the coupling coefficient can be concluded by combining the fitting curves and eq 9, which is shown in Figure 7(e). It can be seen that the magnetoelectric coupling coefficient per unit volume increases with the increased concentration, indicating stronger interaction between the two kinds of particles in high concentration. In addition, the coefficient of the prepared CFO−BTO mixed fluids slowly decreases monotonically with increasing applied magnetic field, and then, it suddenly drops to zero. The largest coefficients per unit volume αH/Φv for the sample with Φv = 20 and 0.02% are about 2.8 × 104 and 6.3 × 102 V/(cm·Oe), respectively, and the corresponding direct coupling coefficients αH are 5.6 × 104 and 0.126 V/(cm·Oe). This enormous difference between the coefficients can be attributed to the different interaction between CFO and BTO particles under the action of field. Most worthy of mention is that the coefficient in the mixed fluids with high concentration (on the order of ∼kV/(cm·Oe)) is a few orders of magnitude larger than what has been obtained in magnetoelectric composite ceramics, films, and nanotubes (about ∼mV/(cm·Oe)) previous reports,9−14 such as CoFe2O4/BaTiO3, Fe3O4/ BaTiO3, (Ni,Zn)Fe2O4/BaTiO3, and ferrite/Pb(Zr,Ti)O3. How is the strong coupling effect generated in the mixed fluids that we presented above? Analysis of the movement of the two kinds of nanoparticles in the mixed fluids under external field enables us to reveal the direct magnetoelectric coupling effect. The effect of magnetic field on the ferroelectric properties is similar to that electric field on the magnetic properties investigated above. Likewise, the shear stress (Ts) is also circumstantial evidence that can prove the chain structure under magnetic field. It was reported in a magnetorheological fluid (MRF) that the stronger the interaction between the chains, the larger the shear stress.29,30 The field dependence of the shear stress of the CFO−BTO mixed fluids with different volume fractions is shown in Figure 7(f). It is quite clear that the shear stress shows a linearly increasing trend on the whole with the increased magnetic field and volume fraction, indicating the enhanced interactions of the chain-like structures, which can indirectly prove the improved clamping I


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a large spontaneous polarization will be concluded. On the contrary, when the concentration is high, the distance between particles is so small that the aggregation is serious, and the compensated electric dipole is high; thus, a small spontaneous polarization will be observed, which is in accordance with the result shown in Figure 7(c). In contrast, the magnetic particles in fluids are much easier to rotate when external field is applied because of the relatively smaller viscous resistance. When a magnetic field is applied to the mixed fluids, for example, it is perpendicular to the surface of the sheet, and the magnetic CFO particles will move, rotate, and then form a chain-like structure along the magnetic field direction. because the magnetic moments prefer to be aligned in order to reduce the free energy by reason for their interactions, as shown in Figure 8(b). On this occasion, the free space of ferroelectric particles will decrease, because these particles are restricted to the space between two magnetic chains. Therefore, the measured polarization of the mixed fluids with low concentration will decrease when magnetic field is applied, because the ferroelectric particles are hard to rotate, and the compensated electric doublet increases compared with the initial state. However, when the concentration of the fluids is high, the compensated electric dipole may decrease compared with the state, which is severely agglomerated; as a consequence, the polarization increases with the increase magnetic field. With the increase of magnetic field and (or) concentration, the ferroelectric particles can form chain-like structures even without the application of electric field because of the clamping effect caused by the magnetic chains, as shown in Figure 8(c). The strong clamping effect can give rise to the enhanced interaction between the ferroelectric particles, and thus, chainlike structures can be formed in order to reduce the total energy, while the direction of the electric moment may be random. Similarly, with the further increase of concentration and magnetic field, the interaction between the ferroelectric particles increases gradually; as a result, the randomly distributed short chains will evolve into a long chain-like structure, as shown in Figure 8(d). When there no electric field exists, the direction of the electric moments is also random. In the process of measuring the P−E curves, a high negative or positive electric field was applied beforehand, and the dipole moments of ferroelectric particles are parallel to the electric field whatever they used to be, and this direction can be retained to some extent because of the strong clamping effect generated by the magnetic chains. Therefore, an enhanced Pr can be obtained compared with that without an external magnetic field applied. Simultaneously, compared with single nanoparticles, the chain is difficult to rotate, and thus, the coercive field Hc must increase. Obviously, Pr should increase with the increase of magnetic field and concentration because of the enhanced clamping effect. When the field increases further, the Pr must be saturated, because the chain-like structures formed by the particles will be entirely clamped. This can explain the results in Figure 7(d,e). As we have mentioned above, the ferroelectric/magnetic chain-like structures were induced by an external electric/ magnetic field; therefore, these chain-like structures should disappear when the external field revolves. Taking the direct magnetoelectric coupling effect as an example, as a magnetic field can increase the remnant polarization because of the clamping effect, if the applied magnetic field was removed, the magnetic chains should transform into short chains and finally

effect between the ferroelectric and magnetic particles under external fields. The magnetoelectric coupling mechanism can be depicted in Figure 8, which is very similar to that of Figure 4. Essentially

Figure 8. Schematic of magnetoelectric coupling effect (a) in the original state, (b) when a magnetic field is applied, (c) short chains are formed with increase of magnetic field and (or) concentration, (d) long chains are formed with the further increased magnetic field and (or) concentration, (e) short chains can be maintained when removing the applied electric field, and (f) both ferroelectric and magnetic short chains can be formed when removing external fields.

speaking, the behavior of magnetic particles under the action of a magnetic field is similar to that of a ferroelectric particle when applying electric field. In the mixed fluids, the magnetic and ferroelectric particles should be randomly distributed without applying external electric/magnetic field, and Pr is supposed to be zero, as shown in Figure 8(a). In the process of measuring the P−E curves, these particles can move and then form a chain-like structure, because the electric dipole moment prefers to be aligned under the action of an electric field in order to reduce the free energy induced by their interactions, and thus, the polarization should increase with increasing applied electric field, while Pr ought to be zero, because the chain-like structure can be destroyed, and these particles can recover to the initial free state once the external field was canceled. As a consequence, the mixed fluids show superparaelectric behavior, without Hc, and Pr can be observed. Further more, when the concentration is low, the distance between particles is so large that the aggregation can be neglected, and the number of compensated electric dipoles is also small; thus, J


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Figure 9. (a) Time dependence of remnant magnetization, (b) time dependence of remnant polarization, (c) converse ME coupling coefficient as a function of time, (d) direct ME coupling coefficient as a function of time.

different switching time that the time to establish the chain-like structure is very short, far less than 1 min, while the disappearance needs more time, at least a few minutes. The reason is that under application of an magnetic/electric field, the particles move quickly because of the small viscous resistance and strong electric field force. On the contrary, when external field was canceled, the electric field force disappears, the motion of the chains or particles is considered as diffusion motion, and a longer time is needed. It is observed that the converse and direct coefficient can be enhanced further if increasing the magnetization/polarization of the magnetic/ferroelectric particles, because the clampling effect can be enforced. In addition, the response speed can also been improved if the magnetization/polarization increased and the viscosity of the base fluid decreased. Moreover, the particle size is an important element to affect the coupling effect, because the movement, formation, and disappearance of the chain-like structure depends strongly on the particle size. Finally, the concentration, the coercive field of the magnetic/ ferroelectric phase, and the proportion of the two phases can affect the coupling result. Therefore, how to improve the ferroelectric and magnetic properties of the mixed fluids, how to enhance both the converse and direct coupling coefficient, and how to develop the response speed are needed to be determined in practical applications. Although it is direct evidence for the proposed mechanisms, as a result of technological and equipment limits, we are unable to directly measure and observe in situ the movement and microstructure of the particles by using TEM. Previous reports indicated that the light transmittance of ferrofluid (or magnetic fluids, magnetic liquid) as a function of time under the action of an external field can reflect the movement and chain-like structure of magnetic particles to some extent.31−33 Therefore,

disappear. Afterward, the ferroelectric chains should also switch to short chains and even vanish in the absence of magnetic chains, as shown in Figure 8(e,f). Actually, both the formation and disappearance of the chain-like structure should be a relaxation process. In order to present the relaxation behavior as well as the response speed of magnetoelectric coupling effect, the time dependence of Mr and Pr was measured, as shown in Figure 9(a,b). Now obviously, both Mr and Pr decrease rapidly with time after an external electric field (converse magnetoelectric coupling effect) and magnetic field (direct magnetoelectric coupling effect) is removed. When the time is longer than 6 and 10 min, they reach a minimum and gradually approach zero; the minor difference between the magnetic and electric field response time may be due to the different interaction force between ferroelectric particles or magnetic particles. As pointed out above, after some time, the highly ordered chain-like structures have been completely destroyed. These particles become more disorderly, and no net polarization and magnetization can be left in the mixed fluids. A straightforward method to evaluate the relaxation time of particles under external field is the light transmission under the same conditions. In situ TEM observation is a direct technique to estimate the response time, and therefore, further studies are needed to be done. In order to evaluate the coupling speed, the time dependence of the converse coupling coefficient αH and direct converse coupling coefficient αE were measured, as shown in Figure 9(c,d). On one hand, both αH and αE show a pronounced switching effect. The coefficient can be stable in two states. On another hand, the time of turn on and off is quite different, indicating that the process of formation and disappearance of the chain-like structure with applied or canceled external field is dissimilar. One can deduce from the K


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Figure 10. (a) Time dependence of T−t curve for CoFe2O4−BaTiO3 binary mixed fluids with particle volume fraction ϕv = 20% in an applied magnetic field H = 500 Oe (a) and E = 20 kV/cm (b). The field is applied at t = 0 and removed at t = 1200 s.

CoFe2O4 is as high as 4.2 × 10−5Oe·cm/V. The field induced chain-like structure is attributed to the clamping effect, which in turn produces the strong coupling effect, and the coupling effect shows a typical switching effect with a response time of several minutes.

in order to further support the interpretation of the observed ME behaviors and the proposed mechanisms, light transmittance of the fluids with and without applying external field was measured. Figure 10 shows a typical curve for the variation of the relative transmittivity T with time t during the application of a magnetic field to a ferrofluid film. T is defined as31 ij I ′ yz ij I yz I′ T = jjj zzz/jjj zzz = j I0 z j I0 z I k { k {

■

AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. (R.G.) *E-mail: [email protected]. (W.C.)

(12)

ORCID

where I0 is the intensity of incident light, and I′(I) is the intensity of transmitted light after (before) the application of the field. The relaxation behavior, giving a valley in the T−t curve, is called nonlinear relaxation.32 In such a T−t curve, the minimum value with its corresponding time and the maximum value and its corresponding time are known as the characteristic parameters, which can describe the relaxation behavior.32 The relaxation variation of light transmitted through the binary mixed system results mainly from the motion of the chains, and the polarized gas will contribute a modulation effect. The single unchained particle system does not produce any microstructure transition in relation to the variation of transmitted light. Generally, the variation of transmitted light only exhibits a valley in the T−t curve, which can be called the simple nonlinear relaxation process, and an unchained particle system can produce a range modulation and time modulation.33 Therefore, similar to the relaxation process of ferrrofluids, magnetielectric binary mixed fluids can show the same relaxation phenomenon, because the magnetic particles in fluids can present the same characteristics as those of magnetic particles in ferrofluids. The nonmagnetic BTO particles in the binary mixed fluids play similar role in tuning the magnetic behaviors of CFO in the fluids.

ACKNOWLEDGMENTS This work has been supported by the National Natural Science Foundation of China (Grant No. 11704274), the Chongqing Research Program of Basic Research and Frontier Technology (CSTC2018jcyjAX0416, CSTC2016jcyjA0175, CSTC2016jcyjA0349), the Young Scientific and Technological Research Program of Chongqing Municipal Education Commission (KJQN201801509), the Excellent Talent Project in University of Chongqing (Grant No. 2017-35), the Science and Technology Innovation Project of Social Undertakings and Peoples Livelihood Guarantee of Chongqing (Grant No. cstc2017shmsA0192), the Program for Innovation Teams in University of Chongqing, China (Grant No. CXTDX201601032), the Latter Foundation Project of Chongqing University of Science & Technology (CKHQZZ2008002), and the Scientific & Technological Achievements Foundation Project of Chongqing University of Science & Technology (CKKJCG2016328).

4. CONCLUSION In summary, CoFe2O4−BaTiO3 mixed fluids have been prepared. and the magnetoelectric coupling effect was researched. The experimental results have demonstrated that the interaction between CFO and BTO particles can generate an anomalous magnetoelectric coefficient. An enhancement of coercive field and remnant magnetism appearing under external field can be obtained, and large converse and direct magnetoelectric coupling coefficients are estimated to be αH = 1.22 × 10−4 Oe·cm/V and αE = 2.52 × 104 V/(cm·Oe), respectively. The magnetoelectric coupling coefficient of

(1) Ma, J.; Shi, Z.; Nan, C. W. Magnetoelectric Properties of Composites of Single Pb(Zr,Ti)O3 Rods and Terfenol-D/Epoxy with a Single-Period of 1−3-Type Structure. Adv. Mater. 2007, 19, 2571− 2573. (2) Dong, S.; Liu, J.-M.; Cheong, S.-W.; Ren, Z. Multiferroic Materials and Magnetoelectric Physics: Symmetry, Entanglement, Excitation, and Topology. Adv. Phys. 2015, 64, 519−626. (3) Gao, R. L.; Yang, H. W.; Chen, Y. S.; Sun, J. R.; Zhao, Y. G.; Shen, B. G. Oxygen Vacancies Induced Switchable and Nonswitchable Photovoltaic Effects in Ag/Bi0.9La0.1FeO3 /La0.7Sr0.3MnO3 Sandwiched Capacitors. Appl. Phys. Lett. 2014, 104, 031906.

Rongli Gao: 0000-0001-7255-9944 Notes

The authors declare no competing financial interest.

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Anomalous Magnetoelectric Coupling Effect of CoFe2O4âBaTiO3

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