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Magnetoelectric coupling in multiferroic BiFeO3 nanowires
Accepted Manuscript Magnetoelectric coupling in multiferroic BiFeO3 nanowires Gurmeet Singh Lotey, NK Verma PII: DOI: Reference:
S0009-2614(13)00774-4 http://dx.doi.org/10.1016/j.cplett.2013.06.016 CPLETT 31318
To appear in:
Chemical Physics Letters
Received Date: Accepted Date:
20 April 2013 10 June 2013
Please cite this article as: G.S. Lotey, N. Verma, Magnetoelectric coupling in multiferroic BiFeO3 nanowires, Chemical Physics Letters (2013), doi: http://dx.doi.org/10.1016/j.cplett.2013.06.016
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Magnetoelectric coupling in multiferroic BiFeO3 nanowires Gurmeet Singh Lotey*, NK Verma Nano Research Lab, School of Physics and Materials Science, Thapar University, Patiala, 147 004, India *Corresponding author E-mail Id: [email protected], [email protected] T. +91-175-239-3343, F. +91 175 236 4498
Abstract Nanowires are the building blocks of future nanoelectronic and spintronic devices. Multiferroic BiFeO3 nanowires of 20 nm size with rhombohedral structure (R3c space group) have been synthesized by template-assisted technique. Magnetic and electrical measurements reveal their ferromagnetic and ferroelectric behavior having high saturation magnetization and polarization, 3.82 emu/g and 54 µC/cm2, respectively, with small leakage-current. Room temperature magnetoelectric longitudinal (L-αME) and transverse (T-αME) coupling coefficients have been found to be 10.738 and 6.866 mV/cmOe, respectively, using dynamic lock-in technique. The observed magnetoelectric coupling properties have been explained on the basis of their nanosize, phase purity and defect free nature.
Keywords: Nanowires, multiferroics, magnetoelectric coupling coefficient, ferromagnetism, ferroelectric
1. Introduction One-dimensional nanowires are fanatical because of their potential to test fundamental concepts; in what ways the dimensionality and the size influence the magnetic, electric, electronic and dielectric properties at nanoscale; to serve as both interconnections as well as the basic functional building components to assemble new generation’s nanoscale devices [1-2]. The ability to engineer the desire properties in nanowires as well as their easiness in fabrication, and, integration on ICs, than other nanostructures make these the most favorable candidate for future nanoelectronics [3-4]. It becomes highly desirable to integrate multifunctions in a single material because we are approaching to the fundamental limits of device integration (such as transistors)
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on ICs towards the device miniaturization [4-9]. Multiferroic materials with strong magnetoelectric coupling in different ferroics orders viz., electric and magnetic are requisites ones, and, have the potential to explore their applications in spintronic and nanoelectronics [4-9]. The enthusiasm to work in this research area has been arisen due to their multifunctionality and unprecedented integration density possible at nanoscale [1-9]. BiFeO3 is one of the scariest single phase multiferroic material [10] has attracted extraordinary attention, because it shows weak ferromagnetism and magnetoelectric coupling with high-Curie (TC~830oC) and Nèel (TN~370oC) temperature [5, 7, 10-12]. However, it has been not yet exploited for device application due to poor spontaneous polarization, weak ferromagnetism (antiferromagnetic), high leakage current density, and, low magnetoelectric coupling at bulk scale [5, 7, 10-17]. The incommensurate spin-spiral structure of BiFeO3, with long cycloidal period of 62 nm, results in antiferromagnetic ordering and weak magnetoelectric coupling. It has been found that the multiferroic properties of BiFeO3 are size-dependent at nanoscale, and, its spin-spiral cycloidal structure can be broken to achieve ferromagnetism and high magnetoelectric coupling by synthesizing BiFeO3 nanoparticle with size less than the cycloidal period [12-17]. A few reports are available on the synthesis and characterization of BiFeO3 nanowires, nanotubes and nanofibers [17-26]. Prashanthi [20] reported the applied electric field induced magnetization in BiFeO3 nanowires of diameter, 150 nm using magnetic force microscopy and confirmed the presence of magnetoelectric coupling. In our previous reports magnetoelectric coupling is study indirectly by evaluating magnetodielectric coefficients [17, 26]. Nanowires of multiferroics possessing smaller size, large surface to volume ratio; therefore excellent magnetoelectric coupling in the BiFeO3 nanowires can be expected and this would lead to envisage new paths for designing future nanoelectronic and spintronic devices. In this communication, the magnetoelectric coupling coefficients of BiFeO3 nanowires synthesized by colloidal dispersion template-assisted technique have been measured as well as the effect of size and phase purity on their multiferroic properties investigated.
2 Experimental 2.1 Materials and synthesis
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BiFeO3 nanowires have been synthesized by colloidal dispersion capillary force-induced filling of nanochannels of anodic alumina oxide (AAO) template [17, 26]. The commercially available AAO templates (Anodisc 25, Whatman, UK) of average pore diameter – 20 nm, have been used. In typical synthesis process, appropriate amounts of Bi(NO3)2.5H2O, and Fe(NO3)3.9H2O are dissolved in 2-methoxyethanol. The concentration of resultant solution has been adjusted to 0.3 M and the pH to around 3-4 by adding 2-methoxyethanol and nitric acid, respectively. The resultant solution is transparent, brownish, and clear. After stirring the mixture above for 4 h at 60oC sol is formed. The excess amount of synthesized sol has been dropped onto indium tin oxide (ITO) glass substrate by injection, and, subsequently the AAO template is placed on top of the solution under ambient conditions of temperature and pressure for 5 h to allow complete filling of AAO channels and solidification of sol [17]. The filled AAO template on ITO substrate is dried under vacuum at 80oC for 12 h, and subsequently fired at 600oC for 1 h to get BiFeO3 nanowires. The detailed mechanism of formation of BiFeO3 in AAO template has been explained elsewhere [17, 26].
2.2 Characterization For the morphological study, the synthesized nanowires have been first removed from the AAO matrix by dissolving it in 0.1 M NaOH solution at 27oC for 2 h, followed by washing with deionized water and then ethanol, and finally drying in hot air oven at ambient temperature (60oC) for overnight. For scanning electron microscopy (SEM) study, a layer of gold–palladium alloy using (JEOL, FINE SPUTTER JFC-1100) sputter coating unit has been deposited and viewed under SEM (JEOL, JSM-6510LV) at 25 kV accelerating voltage. The quantitative elemental composition analysis has been carried out by energy dispersive x-ray spectroscopy (EDAX) of OXFORD analytical system, attached with SEM. The crystallographic study of synthesized nanowires embedded in AAO have been carried out using x-ray diffractometer of PANalyticalX’PertPRO MRD with Cu-Kα (λ = 1.54060Å) is recorded for 2θ value from 20o to 60o. X-ray photoelectron spectroscopy (XPS) has been carried out to know oxidation state and phase purity of BiFeO3 nanowires using VG Microtech MultiLab ESCA 3000 System. The magnetic study has been carried out using Superconducting Quantum Interface Device (SQUID) of Quantum Design. For ferroelectric and dielectric measurements of the synthesized nanowires,
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a special testing system has been designed, which is attached to the respective measuring instruments as discussed elsewhere [17, 26]. To conduct these measurements, the surface of as grown BiFeO3 nanowires embedded in AAO templates is first mechanically polished, and, then Au electrodes are deposited on both sides of the filled template for electrical contacts. The measurement using specific design has been carried out using the fact that the synthesized BiFeO3 nanowires embedded in AAO could be considered as a columnar composite and their ferroelectric properties are controlled by the BiFeO3 due to the non-polarization nature and high resistance of AAO [17, 26]. Therefore, there is no contribution of AAO template towards the ferroelectric behavior, and likewise, for the dielectric measurement. The polarization versus applied electric field (P-E) loop and, leakage current density versus applied electric field (J-E) of the nanowires have been measured using Precision Premier II Workstation (Radiant Technology, USA). Relative dielectric constant (ε) versus frequency study has been carried using an impedance analyzer (Agilent HP 4294A).
2.3 Magnetoelectric coupling coefficients measurement system The dynamic lock-in technique has been employed to measure the direct magnetoelectric coupling (ME) coefficients in the synthesized nanowires as shown in Figure 1. Similar experimental set up has been used for ME measurement as reported by [27, 28, 36]. In this technique the DC magnetic bias field up to 20 kOe has been produced using electromagnetics (E) and measured with Hall probe (HDC), while a time varying DC magnetic field is produced by DC power source (programmable DC power source of Siemens NTN 35000-200 has been employed)), and, is measured using Hall probe (HAc). AC magnetic bias field up to 20 Oe with frequency ranging from 1 to 1000 Hz has been produced using the amplified current signal from the internal function generator (F) of the lock-in amplifier (Stanford Research, model SR850) and, it is fed to the Helmholtz coils (H). The Gauss meter, GAC and GDC have also been employed to measure AC and DC magnetic field, respectively. The amplitude of AC field is measured using Keithley-2700 multimeter. Capacitive structures with gold (Au) electrodes (AuBiFeO3-Au) has been fabricated to measure the ME effect. To measure ME coupling coefficients, the synthesized nanowires have been placed in between the Helmholtz coils; these coils generate an AC magnetic field. A direct current (DC) bias magnetic field is superimposed on the AC magnetic field in parallel. The reorientation of the electrical dipoles in the BiFeO3
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nanowires by an AC magnetic field induces an AC voltage on the top and bottom surfaces of the sample through magnetoelectric coupling. The induced voltage has been measured using lock-in amplifier. To remove errors in signals due to Faraday induction or inductive contribution (eddy currents) the lock-in amplifier is operated in the differential mode [27, 28]. The ME coupling coefficient, (αME), has been calculated using the following relation: [27, 28]
where, t is thickness of the sample, Vout is the AC magnetoelectric voltage appearing across the sample surface (as measured by the lock-in-amplifier), ho is the amplitude of the AC magnetic field. For the comparison, the ME coupling coefficients have been measured in two modes viz., longitudinal (L-αME) and transverse (T-αME) with DC magnetic field parallel with and perpendicular to the direction of induced voltage, respectively. For longitudinal magnetoelectric coupling coefficient measuring mode, i.e., L-αME, the longitudinal magnetization of sample take place (direction of the applied field is along the sample surface) and the output voltage is measured in transverse direction, and, vice-versa is true for the T-αME mode. Data acquisition has been conducted by using a LABVIEW™ program
3. Results and discussion 3.1Morpholgical, crystallographic and elemental compositional analyses Figure 2 shows SEM micrograph of the synthesized nanowires revealing their dense and uniform growth as well as they are found to be homogeneous, parallel, and well aligned. The diameter of BiFeO3 nanowires has been found to be around 20 nm. To determine the structural features of BiFeO3 nanowires, Rietveld refinement of XRD patterns have been performed using the FullProf program. The observed, calculated and the different refined x-ray diffraction (XRD) patterns of the synthesized nanowires are shown in Figure 3. It has been found that the XRD patterns of synthesized BiFeO3 nanowires embedded in AAO matrix are well agreement with hexagonal phase of rhombohedral structure with R3c space group (JCPDS file no. 86–1518). The lattice parameters, bond angle, bond length, R-factors and microstrain of the nanowires have been calculated and are shown in Table 1. The microstrain has been found to be 0.034. No additional peaks related to any impurity or other phases have been detected in XRD patterns revealing their defect-free nature without the formation of any secondary phase. The high intense peaks of XRD
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patterns demonstrate the high crystalline nature of nanowires. No peak related to anodic alumina oxide (AAO) Al2O3 template has been seen in XRD patterns because of their amorphous nature. To check the phase purity and oxidation state of Fe in BiFeO3, enlarged view of XPS spectrum in the range of 705 to 725 eV has been carefully examined as shown in Figure 4a. The background correction has been done using Shirley background subtraction - non-linear least square fitting and using mixed Gauss-Lorentz function. There are two main photo emission peaks positioned around 711 and 721.7eV assigned to Fe3+. These represent the spin orbit doublet, 2p3/2 and 2p1/2 of Fe3+ and affirm the 3+ oxidation state of Fe. No peak corresponding 2+ oxidation state of Fe or any other impurity phase has been observed in the XPS spectrum; this endorses the dominant role of Fe3+ ions for the observed ferromagnetism (to be discussed in magnetic analysis ahead).Three additional peaks around 706.5, 713.7 and 716 eV have been also seen; these are known as pre-peak, surface peak and satellite peak, respectively [17, 26]. The surface peak is related to decrease in co-ordination number of Fe3+ ions located at the surface of the nanowires. The satellite peak is associated with shake-up process. Figure 4b shows the EDAX spectrum of BiFeO3 nanowires revealing the presence of Bi, Fe and O. No peak related to any other element or oxide has been observed in EDAX spectrum, which confirms the highly pure nature of BiFeO3 nanowires.
3.2 Magnetic analysis The magnetization versus applied magnetic field (M-H) hysteresis loop of synthesized BiFeO3 nanowires at room temperature is shown in Figure 5a; this indicates that the synthesized nanowires exhibit ferromagnetic behavior with high saturation magnetization, 3.82 emu/g. Similar results of magnetic behavior has been also presented for ultrafine fibers and nanowires of BiFeO3 [17-18, 26]. The XRD, EDAX and XPS analyses rule out the presence of any impurity, or secondary phase or Fe2+ ions; it confirms that the observed ferromagnetic behavior is not associated with the presence of ferromagnetic, Fe2+ ions or any other impurity phases in the nanowires. The observed magnetic behavior can be described on the basis of following facts: First, bulk BiFeO3 (having size > 62nm) is antiferromagnetic and possesses spin-spiral incommensurate structure with long range cycloidal period of 62 nm. The antiferromagnetic axis rotates through the crystal with an incommensurate long-wavelength period of 62 nm; it cancels
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the macroscopic magnetization, results in anti-ferromagnetism with unsaturated magnetization. However, in the present study, the size of the synthesized nanowires is 20 nm, which is less than the cycloidal period of 62 nm; this destroys the cycloidal structure and, thus, leads to ferromagnetic ordering, as depicted in Figure 5a [11-17, 26, 29]. Second, the synthesized nanowires possess large surface-to-volume ratio because of their smaller size. This enhances the overall magnetization of nanowires due to uncompensated spins at the surface of BiFeO3 nanowires, and, results in the net magnetic moment produced due to non-exact compensation of the two magnetic sub-lattices [12, 15, 17, 26, 30-31]. There are the surface imperfections and surface strain anisotropies dominate magnetic properties of nanoparticles [12, 17, 26]. The increases of magnetization in the synthesized nanowires may be due to the contribution strain anisotropies, and non-collinear magnetic ordering. Third, the MH hysteresis loop is found to be symmetric on both sides of the axis, which establishes that the exchange interactions are not responsible for the magnetic behavior of nanowires [15, 17, 26]. Lastly, the synthesized nanowires have been found (Table 1) to possess high microstrain, 0.034; this may result in higher spin canting, and give rise to ferromagnetism [32].
Temperature-dependent magnetic study (zero field cooling (ZFC) and field cooling (FC) curves) have been employed to check the existence of inter-nanowire interactions. For the ZFC magnetization measurements, the sample is first cooled from room temperature to 5 K in zero field and 1000 Oe magnetic field is applied, and, then the magnetization has been measured in the warming cycle with applied field. To perform FC magnetization measurements, the sample is first cooled in the applied magnetic field of 1000 Oe, to 5 K, and, the FC magnetization has been measured in the warming cycle under the same field (Figure 5b). The broad peak has been observed in ZFC and FC curves at lower temperature due to nano-size as shown in Figure 5b. No such peaks are presented in bulk BiFeO3 having particle size higher than that of the critical spin spiral cycloidal period of 62 nm [14]. Due to nano-size of synthesized nanowires, Fe3+ spins orient towards the direction of applied magnetic field, thereby breaking the antiferromagnetic spiral ordering. The splitting between ZFC and FC curves around 80K has taken place. This is indicates the spin-glass transition temperature of the synthesized nanowires, representing their spin-glass behavior [14]. This can be attributed to the nano-size effect, inter-nanowire
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interactions, random distribution of anisotropy axes, microstrain, and the high packing volume fraction with complex interplay between finite size effects in the synthesized nanowires [12, 15, 20-33]. Liu [21] reported spin-glass transition temperature, 55K in hydrothermally synthesized BiFeO3 nanowires, that is smaller than as observed in the nanowires (80 K).
3.3 Electrical analysis 3.3.1 Ferroelectric study Figure 6a shows polarization as a function of applied electric field about 600 kV/cm with frequency (ν) = 10 kHz (PE) hysteresis loop of BiFeO3 nanowires at room temperature. The observed well-saturated rectangular like PE loop may be due to the presence of less oxygenrelated defects and phase purity of the nanowires. The high value of saturation polarization around 54 µC/cm2 has been observed at 535 kV/cm applied electric field. The smaller leakage current (to be discussed ahead) may result due to the absence of secondary phases or impurity (corroborates with XRD, EDAX and XPS analyses); this, consequently improves the domain pinning effects and the spontaneous polarization [15, 17, 26]. More importantly, the average grain size of the synthesized nanowires is small, which may result in lower space charge density, smaller leakage current density, and consequently resulting in spontaneous polarization.
3.3.2 Dielectric study The variation of relative dielectric constant (ε) of the BiFeO3 nanowires as a function of frequency has been shown in Figure 6b. A monotonous decrease of dielectric constant and decease in dielectric loss has been registered with increase in frequency. The observed trend in dielectric constant and dielectric loss with frequency indicates large dispersion due to Maxwell– Wagner [34] type interfacial polarization, and is in good agreement with Koops phenomenological theory [35]. The high value of dielectric constant, 492, at 1000 Hz is due to space charge polarization resulting from the inhomogeneous dielectric structure. In our earlier reports [17, 26] low value of dielectric constant as compare to present case is observed may be due to the presence of defects and secondary phases in undoped BiFeO3 nanowires. Zhang [19] reported dielectric constant around 90 at 100 kHz in BiFeO3 nanotubes. At low frequency, the dielectric constant is dependent on different types of polarization mechanisms, viz., electronic, atomic, interfacial and ionic, whereas at higher frequencies it arises due to electronic
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polarization. Synthesized polycrystalline nanowires consist of large number of grain boundary regions, uncompensated surfaces with dangling bonds; this influences the dielectric properties. The observed trend in dielectric constant can be explained on the basis of dipolar, oriental and interfacial polarization. It has been observed that at lower frequency (≤ 1 kHz), the migration of carriers stops due to their collection near the physical interface barrier, thereby producing localized polarization in the material; here, the dipoles track the frequencies of applied field. But in higher frequency range (~103–106 Hz), the dipolar polarization becomes effective, and, the diploes are completely formed due to high applied field, which finally gets saturated [17, 26, 37].
3.3.3 Leakage current density versus applied electric field (J-E) characteristics Leakage current density versus applied electric field (J-E) of the nanowires embedded in AAO templates have been studied to know the leakage current phenomenon associated with the BiFeO3. Figure 6 (c) displays J-E curve of the synthesized nanowires, revealing their excellent symmetry under positive and negative applied electric fields. The very small leakage current has been observed in the synthesized nanowires as compare to earlier reports [15, 17, 25-26] attributed to small defects, high crystallinity, absence of impurity or secondary phases, nanosized and small grain boundaries in the synthesized nanowires.
3.4 Magnetoelectric coupling analysis Figure 7 shows longitudinal (L-αME) and transverse (T-αME) magnetoelectric coupling coefficients of synthesized BiFeO3 nanowires measured at room temperature. The magnetoelectric coefficient versus DC bias magnetic field reveals the hysteretic behavior of the nanowires as depicted in the magnetic field cycles shown in Figure 7. It has been found that both L-αME, and T-αME increase quickly with increase in bias magnetic field, and, attain maximum value, L-αME = 10.738 mV/cmOe and T-αME = 6.866 mV/cmOe, respectively at 0.5 and 0.3 kOe. This shows that the longitudinal magnetoelectric coupling coefficient exceeds the transverse magnetoelectric coupling coefficient. The observed value of L-αME, and T-αME is around 17 and 24 times higher than as reported by Naik [28] for bulk, and, one order higher as reported by Caicedo [36] for thin film. The smaller magnetoelectric coupling coefficients observed in bulk and thin films [28, 36], may be due to presence of impurity or secondary
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phases, large grain size and charge imbalance of Fe, i.e.Fe2+↔Fe3+. The wide-range scattering in the magnetoelectric coefficients has been noticed may be attributed to the nano-size of the synthesized BiFeO3 nanowires. Bulk BiFeO3 has long range cycloidal period of 62 nm, and, is antiferromagnetic with Néel temperature 370oC [5, 11, 28]. However, in the present case, the size of the synthesized nanowires, 20 nm, being less than the cycloidal period, the antiferromagnetic ordering gets destroyed; it is reflected from the magnetic properties of the synthesized nanowires possessing ferromagnetism. This modifies the domain structure, and, thereby leading to high magnetoelectric coupling [12, 28, 38]. Therefore, the enhancement in magnetoelectric coupling coefficient in the synthesized nanowires can be attributed to their nano-size, and high microstrain. The nano-size of synthesized nanowires enhances the magnetic ordering, which strengthens the sub-lattice interactions and thereby results in the ME [17, 26]. The synthesized nanowires have been found (Table 1) to possess high microstrain, 0.034; this may result in higher spin canting, and give rise to ferromagnetism [32]. The contribution of magnetoresistance and Maxwell–Wagner effect for the observed ME cannot be ignored [39]. The observed intrinsic ME coupling may also be due to magnetostriction effect. The change in lattice parameters occurs with the application of magnetic field. This generates strain in the nanowires due to the coupling between ferroics domains, and, induces stress in it. This induces an electric field in the nanowires that orients the ferroelectric domains, and, subsequently leads to observed ME. Room temperature occurrence of ferromagnetism and ferroelectricity (Figure 5 and 6) further supports the observation of multiferrosim and magnetoelectric coupling in the BiFeO3 nanowires.
4. Conclusions Colloidal dispersion template-assisted technique has been successfully employed for the synthesis of BiFeO3 nanowires, having diameter 20 nm. Structural study confirms that the synthesized nanowires possess rhombohedral perovskite pure phase with R3c space group.
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Magnetic and electrical investigations confirm the ferromagnetic and ferroelectric behavior of synthesized nanowires, respectively, with low leakage-current density at room temperature. XRD, EDAX and XPS analyses reveal the phase purity of the synthesized nanowires, as well as rules out the presence Fe2+ or its oxide in the BiFeO3. Room temperature magnetoelectric coupling coefficient measurement confirms the strong magnetoelectric coupling in the synthesized nanowires. The high magnetoelectric coupling coefficients viz. longitudinal, L-αME =10.738 mV/cmOe, and transverse, T-αME = 6.866 mV/cmOe have been observed. The observed magnetoelectric coupling and multiferroism in the synthesized nanowires are attributed to their nano-size, and high microstrain. This study establishes that the strong magnetodielectric coupling in the BiFeO3 nanowires results in the best multifunctional multiferroic, and further have potential to explore its applications in nanoelectronics, spintronic, and multiferroic-based memory devices.
Acknowledgments One of the authors, Gurmeet Singh Lotey, gratefully acknowledges the Department of Science and Technology (DST), Government of India, for awarding him the INSPIRE (Innovation in Science Pursuit for Inspired Research) fellowship to carry out this research work. We are highly thankful to Professor Claudia Felser of Johannes Gutenberg University of Mainz, Germany, for the magnetic characterization.
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Figure captions Figure 1
Schematic diagram of the dynamic lock-in technique for magnetoelectric coefficient measurements
Figure 2
SEM micrograph of BiFeO3 nanowires
Figure 3
Rietveld refined XRD patterns of BiFeO3 nanowires
Figure 4
(a) Enlarge view of x-ray photoemission spectra of Fe 2p and (b) EDAX spectrum of BiFeO3 nanowires
Figure 5
(a) Magnetization versus applied field (MH) hysteresis loop and (b) temperature dependence of magnetization of BiFeO3 nanowires
Figure 6
(a) Polarization versus electric field hysteresis loop and (b) relative dielectric constant and dielectric loss vs. frequency traits of BiFeO3 nanowires (c) leakagecurrent density versus applied electric field traits of BiFeO3 nanowires
Figure 7
Room temperature DC bias magnetic field dependence (c) longitudinal (L-αME) and (b) transverse (T-αME) magnetodielectric coupling coefficients of BiFeO3 nanowires
Table caption
Table 1
Rietveld structural refinement parameters of BiFeO3 nanowires
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Table 1
Figure 1
Figure 2
Figure 3
Figure 4
Figure 5
Figure 6
Figure 7
Highlights
Multiferroic BiFeO3 nanowires of 20 nm size have been synthesized.
Nanowires are ferromagnetic with high saturation magnetization, 3.82 emu/g
Nanowires possess ferroelectric behavior with high polarization 54µC/cm2
Magnetoelectric coupling coefficients in synthesized BiFeO3 nanowires measured
Longitudinal and transverse coupling coefficients found 10.738 and 6.866 mV/cmOe
S0009-2614(13)00774-4 http://dx.doi.org/10.1016/j.cplett.2013.06.016 CPLETT 31318
To appear in:
Chemical Physics Letters
Received Date: Accepted Date:
20 April 2013 10 June 2013
Please cite this article as: G.S. Lotey, N. Verma, Magnetoelectric coupling in multiferroic BiFeO3 nanowires, Chemical Physics Letters (2013), doi: http://dx.doi.org/10.1016/j.cplett.2013.06.016
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Magnetoelectric coupling in multiferroic BiFeO3 nanowires Gurmeet Singh Lotey*, NK Verma Nano Research Lab, School of Physics and Materials Science, Thapar University, Patiala, 147 004, India *Corresponding author E-mail Id: [email protected], [email protected] T. +91-175-239-3343, F. +91 175 236 4498
Abstract Nanowires are the building blocks of future nanoelectronic and spintronic devices. Multiferroic BiFeO3 nanowires of 20 nm size with rhombohedral structure (R3c space group) have been synthesized by template-assisted technique. Magnetic and electrical measurements reveal their ferromagnetic and ferroelectric behavior having high saturation magnetization and polarization, 3.82 emu/g and 54 µC/cm2, respectively, with small leakage-current. Room temperature magnetoelectric longitudinal (L-αME) and transverse (T-αME) coupling coefficients have been found to be 10.738 and 6.866 mV/cmOe, respectively, using dynamic lock-in technique. The observed magnetoelectric coupling properties have been explained on the basis of their nanosize, phase purity and defect free nature.
Keywords: Nanowires, multiferroics, magnetoelectric coupling coefficient, ferromagnetism, ferroelectric
1. Introduction One-dimensional nanowires are fanatical because of their potential to test fundamental concepts; in what ways the dimensionality and the size influence the magnetic, electric, electronic and dielectric properties at nanoscale; to serve as both interconnections as well as the basic functional building components to assemble new generation’s nanoscale devices [1-2]. The ability to engineer the desire properties in nanowires as well as their easiness in fabrication, and, integration on ICs, than other nanostructures make these the most favorable candidate for future nanoelectronics [3-4]. It becomes highly desirable to integrate multifunctions in a single material because we are approaching to the fundamental limits of device integration (such as transistors)
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on ICs towards the device miniaturization [4-9]. Multiferroic materials with strong magnetoelectric coupling in different ferroics orders viz., electric and magnetic are requisites ones, and, have the potential to explore their applications in spintronic and nanoelectronics [4-9]. The enthusiasm to work in this research area has been arisen due to their multifunctionality and unprecedented integration density possible at nanoscale [1-9]. BiFeO3 is one of the scariest single phase multiferroic material [10] has attracted extraordinary attention, because it shows weak ferromagnetism and magnetoelectric coupling with high-Curie (TC~830oC) and Nèel (TN~370oC) temperature [5, 7, 10-12]. However, it has been not yet exploited for device application due to poor spontaneous polarization, weak ferromagnetism (antiferromagnetic), high leakage current density, and, low magnetoelectric coupling at bulk scale [5, 7, 10-17]. The incommensurate spin-spiral structure of BiFeO3, with long cycloidal period of 62 nm, results in antiferromagnetic ordering and weak magnetoelectric coupling. It has been found that the multiferroic properties of BiFeO3 are size-dependent at nanoscale, and, its spin-spiral cycloidal structure can be broken to achieve ferromagnetism and high magnetoelectric coupling by synthesizing BiFeO3 nanoparticle with size less than the cycloidal period [12-17]. A few reports are available on the synthesis and characterization of BiFeO3 nanowires, nanotubes and nanofibers [17-26]. Prashanthi [20] reported the applied electric field induced magnetization in BiFeO3 nanowires of diameter, 150 nm using magnetic force microscopy and confirmed the presence of magnetoelectric coupling. In our previous reports magnetoelectric coupling is study indirectly by evaluating magnetodielectric coefficients [17, 26]. Nanowires of multiferroics possessing smaller size, large surface to volume ratio; therefore excellent magnetoelectric coupling in the BiFeO3 nanowires can be expected and this would lead to envisage new paths for designing future nanoelectronic and spintronic devices. In this communication, the magnetoelectric coupling coefficients of BiFeO3 nanowires synthesized by colloidal dispersion template-assisted technique have been measured as well as the effect of size and phase purity on their multiferroic properties investigated.
2 Experimental 2.1 Materials and synthesis
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BiFeO3 nanowires have been synthesized by colloidal dispersion capillary force-induced filling of nanochannels of anodic alumina oxide (AAO) template [17, 26]. The commercially available AAO templates (Anodisc 25, Whatman, UK) of average pore diameter – 20 nm, have been used. In typical synthesis process, appropriate amounts of Bi(NO3)2.5H2O, and Fe(NO3)3.9H2O are dissolved in 2-methoxyethanol. The concentration of resultant solution has been adjusted to 0.3 M and the pH to around 3-4 by adding 2-methoxyethanol and nitric acid, respectively. The resultant solution is transparent, brownish, and clear. After stirring the mixture above for 4 h at 60oC sol is formed. The excess amount of synthesized sol has been dropped onto indium tin oxide (ITO) glass substrate by injection, and, subsequently the AAO template is placed on top of the solution under ambient conditions of temperature and pressure for 5 h to allow complete filling of AAO channels and solidification of sol [17]. The filled AAO template on ITO substrate is dried under vacuum at 80oC for 12 h, and subsequently fired at 600oC for 1 h to get BiFeO3 nanowires. The detailed mechanism of formation of BiFeO3 in AAO template has been explained elsewhere [17, 26].
2.2 Characterization For the morphological study, the synthesized nanowires have been first removed from the AAO matrix by dissolving it in 0.1 M NaOH solution at 27oC for 2 h, followed by washing with deionized water and then ethanol, and finally drying in hot air oven at ambient temperature (60oC) for overnight. For scanning electron microscopy (SEM) study, a layer of gold–palladium alloy using (JEOL, FINE SPUTTER JFC-1100) sputter coating unit has been deposited and viewed under SEM (JEOL, JSM-6510LV) at 25 kV accelerating voltage. The quantitative elemental composition analysis has been carried out by energy dispersive x-ray spectroscopy (EDAX) of OXFORD analytical system, attached with SEM. The crystallographic study of synthesized nanowires embedded in AAO have been carried out using x-ray diffractometer of PANalyticalX’PertPRO MRD with Cu-Kα (λ = 1.54060Å) is recorded for 2θ value from 20o to 60o. X-ray photoelectron spectroscopy (XPS) has been carried out to know oxidation state and phase purity of BiFeO3 nanowires using VG Microtech MultiLab ESCA 3000 System. The magnetic study has been carried out using Superconducting Quantum Interface Device (SQUID) of Quantum Design. For ferroelectric and dielectric measurements of the synthesized nanowires,
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a special testing system has been designed, which is attached to the respective measuring instruments as discussed elsewhere [17, 26]. To conduct these measurements, the surface of as grown BiFeO3 nanowires embedded in AAO templates is first mechanically polished, and, then Au electrodes are deposited on both sides of the filled template for electrical contacts. The measurement using specific design has been carried out using the fact that the synthesized BiFeO3 nanowires embedded in AAO could be considered as a columnar composite and their ferroelectric properties are controlled by the BiFeO3 due to the non-polarization nature and high resistance of AAO [17, 26]. Therefore, there is no contribution of AAO template towards the ferroelectric behavior, and likewise, for the dielectric measurement. The polarization versus applied electric field (P-E) loop and, leakage current density versus applied electric field (J-E) of the nanowires have been measured using Precision Premier II Workstation (Radiant Technology, USA). Relative dielectric constant (ε) versus frequency study has been carried using an impedance analyzer (Agilent HP 4294A).
2.3 Magnetoelectric coupling coefficients measurement system The dynamic lock-in technique has been employed to measure the direct magnetoelectric coupling (ME) coefficients in the synthesized nanowires as shown in Figure 1. Similar experimental set up has been used for ME measurement as reported by [27, 28, 36]. In this technique the DC magnetic bias field up to 20 kOe has been produced using electromagnetics (E) and measured with Hall probe (HDC), while a time varying DC magnetic field is produced by DC power source (programmable DC power source of Siemens NTN 35000-200 has been employed)), and, is measured using Hall probe (HAc). AC magnetic bias field up to 20 Oe with frequency ranging from 1 to 1000 Hz has been produced using the amplified current signal from the internal function generator (F) of the lock-in amplifier (Stanford Research, model SR850) and, it is fed to the Helmholtz coils (H). The Gauss meter, GAC and GDC have also been employed to measure AC and DC magnetic field, respectively. The amplitude of AC field is measured using Keithley-2700 multimeter. Capacitive structures with gold (Au) electrodes (AuBiFeO3-Au) has been fabricated to measure the ME effect. To measure ME coupling coefficients, the synthesized nanowires have been placed in between the Helmholtz coils; these coils generate an AC magnetic field. A direct current (DC) bias magnetic field is superimposed on the AC magnetic field in parallel. The reorientation of the electrical dipoles in the BiFeO3
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nanowires by an AC magnetic field induces an AC voltage on the top and bottom surfaces of the sample through magnetoelectric coupling. The induced voltage has been measured using lock-in amplifier. To remove errors in signals due to Faraday induction or inductive contribution (eddy currents) the lock-in amplifier is operated in the differential mode [27, 28]. The ME coupling coefficient, (αME), has been calculated using the following relation: [27, 28]
where, t is thickness of the sample, Vout is the AC magnetoelectric voltage appearing across the sample surface (as measured by the lock-in-amplifier), ho is the amplitude of the AC magnetic field. For the comparison, the ME coupling coefficients have been measured in two modes viz., longitudinal (L-αME) and transverse (T-αME) with DC magnetic field parallel with and perpendicular to the direction of induced voltage, respectively. For longitudinal magnetoelectric coupling coefficient measuring mode, i.e., L-αME, the longitudinal magnetization of sample take place (direction of the applied field is along the sample surface) and the output voltage is measured in transverse direction, and, vice-versa is true for the T-αME mode. Data acquisition has been conducted by using a LABVIEW™ program
3. Results and discussion 3.1Morpholgical, crystallographic and elemental compositional analyses Figure 2 shows SEM micrograph of the synthesized nanowires revealing their dense and uniform growth as well as they are found to be homogeneous, parallel, and well aligned. The diameter of BiFeO3 nanowires has been found to be around 20 nm. To determine the structural features of BiFeO3 nanowires, Rietveld refinement of XRD patterns have been performed using the FullProf program. The observed, calculated and the different refined x-ray diffraction (XRD) patterns of the synthesized nanowires are shown in Figure 3. It has been found that the XRD patterns of synthesized BiFeO3 nanowires embedded in AAO matrix are well agreement with hexagonal phase of rhombohedral structure with R3c space group (JCPDS file no. 86–1518). The lattice parameters, bond angle, bond length, R-factors and microstrain of the nanowires have been calculated and are shown in Table 1. The microstrain has been found to be 0.034. No additional peaks related to any impurity or other phases have been detected in XRD patterns revealing their defect-free nature without the formation of any secondary phase. The high intense peaks of XRD
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patterns demonstrate the high crystalline nature of nanowires. No peak related to anodic alumina oxide (AAO) Al2O3 template has been seen in XRD patterns because of their amorphous nature. To check the phase purity and oxidation state of Fe in BiFeO3, enlarged view of XPS spectrum in the range of 705 to 725 eV has been carefully examined as shown in Figure 4a. The background correction has been done using Shirley background subtraction - non-linear least square fitting and using mixed Gauss-Lorentz function. There are two main photo emission peaks positioned around 711 and 721.7eV assigned to Fe3+. These represent the spin orbit doublet, 2p3/2 and 2p1/2 of Fe3+ and affirm the 3+ oxidation state of Fe. No peak corresponding 2+ oxidation state of Fe or any other impurity phase has been observed in the XPS spectrum; this endorses the dominant role of Fe3+ ions for the observed ferromagnetism (to be discussed in magnetic analysis ahead).Three additional peaks around 706.5, 713.7 and 716 eV have been also seen; these are known as pre-peak, surface peak and satellite peak, respectively [17, 26]. The surface peak is related to decrease in co-ordination number of Fe3+ ions located at the surface of the nanowires. The satellite peak is associated with shake-up process. Figure 4b shows the EDAX spectrum of BiFeO3 nanowires revealing the presence of Bi, Fe and O. No peak related to any other element or oxide has been observed in EDAX spectrum, which confirms the highly pure nature of BiFeO3 nanowires.
3.2 Magnetic analysis The magnetization versus applied magnetic field (M-H) hysteresis loop of synthesized BiFeO3 nanowires at room temperature is shown in Figure 5a; this indicates that the synthesized nanowires exhibit ferromagnetic behavior with high saturation magnetization, 3.82 emu/g. Similar results of magnetic behavior has been also presented for ultrafine fibers and nanowires of BiFeO3 [17-18, 26]. The XRD, EDAX and XPS analyses rule out the presence of any impurity, or secondary phase or Fe2+ ions; it confirms that the observed ferromagnetic behavior is not associated with the presence of ferromagnetic, Fe2+ ions or any other impurity phases in the nanowires. The observed magnetic behavior can be described on the basis of following facts: First, bulk BiFeO3 (having size > 62nm) is antiferromagnetic and possesses spin-spiral incommensurate structure with long range cycloidal period of 62 nm. The antiferromagnetic axis rotates through the crystal with an incommensurate long-wavelength period of 62 nm; it cancels
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the macroscopic magnetization, results in anti-ferromagnetism with unsaturated magnetization. However, in the present study, the size of the synthesized nanowires is 20 nm, which is less than the cycloidal period of 62 nm; this destroys the cycloidal structure and, thus, leads to ferromagnetic ordering, as depicted in Figure 5a [11-17, 26, 29]. Second, the synthesized nanowires possess large surface-to-volume ratio because of their smaller size. This enhances the overall magnetization of nanowires due to uncompensated spins at the surface of BiFeO3 nanowires, and, results in the net magnetic moment produced due to non-exact compensation of the two magnetic sub-lattices [12, 15, 17, 26, 30-31]. There are the surface imperfections and surface strain anisotropies dominate magnetic properties of nanoparticles [12, 17, 26]. The increases of magnetization in the synthesized nanowires may be due to the contribution strain anisotropies, and non-collinear magnetic ordering. Third, the MH hysteresis loop is found to be symmetric on both sides of the axis, which establishes that the exchange interactions are not responsible for the magnetic behavior of nanowires [15, 17, 26]. Lastly, the synthesized nanowires have been found (Table 1) to possess high microstrain, 0.034; this may result in higher spin canting, and give rise to ferromagnetism [32].
Temperature-dependent magnetic study (zero field cooling (ZFC) and field cooling (FC) curves) have been employed to check the existence of inter-nanowire interactions. For the ZFC magnetization measurements, the sample is first cooled from room temperature to 5 K in zero field and 1000 Oe magnetic field is applied, and, then the magnetization has been measured in the warming cycle with applied field. To perform FC magnetization measurements, the sample is first cooled in the applied magnetic field of 1000 Oe, to 5 K, and, the FC magnetization has been measured in the warming cycle under the same field (Figure 5b). The broad peak has been observed in ZFC and FC curves at lower temperature due to nano-size as shown in Figure 5b. No such peaks are presented in bulk BiFeO3 having particle size higher than that of the critical spin spiral cycloidal period of 62 nm [14]. Due to nano-size of synthesized nanowires, Fe3+ spins orient towards the direction of applied magnetic field, thereby breaking the antiferromagnetic spiral ordering. The splitting between ZFC and FC curves around 80K has taken place. This is indicates the spin-glass transition temperature of the synthesized nanowires, representing their spin-glass behavior [14]. This can be attributed to the nano-size effect, inter-nanowire
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interactions, random distribution of anisotropy axes, microstrain, and the high packing volume fraction with complex interplay between finite size effects in the synthesized nanowires [12, 15, 20-33]. Liu [21] reported spin-glass transition temperature, 55K in hydrothermally synthesized BiFeO3 nanowires, that is smaller than as observed in the nanowires (80 K).
3.3 Electrical analysis 3.3.1 Ferroelectric study Figure 6a shows polarization as a function of applied electric field about 600 kV/cm with frequency (ν) = 10 kHz (PE) hysteresis loop of BiFeO3 nanowires at room temperature. The observed well-saturated rectangular like PE loop may be due to the presence of less oxygenrelated defects and phase purity of the nanowires. The high value of saturation polarization around 54 µC/cm2 has been observed at 535 kV/cm applied electric field. The smaller leakage current (to be discussed ahead) may result due to the absence of secondary phases or impurity (corroborates with XRD, EDAX and XPS analyses); this, consequently improves the domain pinning effects and the spontaneous polarization [15, 17, 26]. More importantly, the average grain size of the synthesized nanowires is small, which may result in lower space charge density, smaller leakage current density, and consequently resulting in spontaneous polarization.
3.3.2 Dielectric study The variation of relative dielectric constant (ε) of the BiFeO3 nanowires as a function of frequency has been shown in Figure 6b. A monotonous decrease of dielectric constant and decease in dielectric loss has been registered with increase in frequency. The observed trend in dielectric constant and dielectric loss with frequency indicates large dispersion due to Maxwell– Wagner [34] type interfacial polarization, and is in good agreement with Koops phenomenological theory [35]. The high value of dielectric constant, 492, at 1000 Hz is due to space charge polarization resulting from the inhomogeneous dielectric structure. In our earlier reports [17, 26] low value of dielectric constant as compare to present case is observed may be due to the presence of defects and secondary phases in undoped BiFeO3 nanowires. Zhang [19] reported dielectric constant around 90 at 100 kHz in BiFeO3 nanotubes. At low frequency, the dielectric constant is dependent on different types of polarization mechanisms, viz., electronic, atomic, interfacial and ionic, whereas at higher frequencies it arises due to electronic
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polarization. Synthesized polycrystalline nanowires consist of large number of grain boundary regions, uncompensated surfaces with dangling bonds; this influences the dielectric properties. The observed trend in dielectric constant can be explained on the basis of dipolar, oriental and interfacial polarization. It has been observed that at lower frequency (≤ 1 kHz), the migration of carriers stops due to their collection near the physical interface barrier, thereby producing localized polarization in the material; here, the dipoles track the frequencies of applied field. But in higher frequency range (~103–106 Hz), the dipolar polarization becomes effective, and, the diploes are completely formed due to high applied field, which finally gets saturated [17, 26, 37].
3.3.3 Leakage current density versus applied electric field (J-E) characteristics Leakage current density versus applied electric field (J-E) of the nanowires embedded in AAO templates have been studied to know the leakage current phenomenon associated with the BiFeO3. Figure 6 (c) displays J-E curve of the synthesized nanowires, revealing their excellent symmetry under positive and negative applied electric fields. The very small leakage current has been observed in the synthesized nanowires as compare to earlier reports [15, 17, 25-26] attributed to small defects, high crystallinity, absence of impurity or secondary phases, nanosized and small grain boundaries in the synthesized nanowires.
3.4 Magnetoelectric coupling analysis Figure 7 shows longitudinal (L-αME) and transverse (T-αME) magnetoelectric coupling coefficients of synthesized BiFeO3 nanowires measured at room temperature. The magnetoelectric coefficient versus DC bias magnetic field reveals the hysteretic behavior of the nanowires as depicted in the magnetic field cycles shown in Figure 7. It has been found that both L-αME, and T-αME increase quickly with increase in bias magnetic field, and, attain maximum value, L-αME = 10.738 mV/cmOe and T-αME = 6.866 mV/cmOe, respectively at 0.5 and 0.3 kOe. This shows that the longitudinal magnetoelectric coupling coefficient exceeds the transverse magnetoelectric coupling coefficient. The observed value of L-αME, and T-αME is around 17 and 24 times higher than as reported by Naik [28] for bulk, and, one order higher as reported by Caicedo [36] for thin film. The smaller magnetoelectric coupling coefficients observed in bulk and thin films [28, 36], may be due to presence of impurity or secondary
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phases, large grain size and charge imbalance of Fe, i.e.Fe2+↔Fe3+. The wide-range scattering in the magnetoelectric coefficients has been noticed may be attributed to the nano-size of the synthesized BiFeO3 nanowires. Bulk BiFeO3 has long range cycloidal period of 62 nm, and, is antiferromagnetic with Néel temperature 370oC [5, 11, 28]. However, in the present case, the size of the synthesized nanowires, 20 nm, being less than the cycloidal period, the antiferromagnetic ordering gets destroyed; it is reflected from the magnetic properties of the synthesized nanowires possessing ferromagnetism. This modifies the domain structure, and, thereby leading to high magnetoelectric coupling [12, 28, 38]. Therefore, the enhancement in magnetoelectric coupling coefficient in the synthesized nanowires can be attributed to their nano-size, and high microstrain. The nano-size of synthesized nanowires enhances the magnetic ordering, which strengthens the sub-lattice interactions and thereby results in the ME [17, 26]. The synthesized nanowires have been found (Table 1) to possess high microstrain, 0.034; this may result in higher spin canting, and give rise to ferromagnetism [32]. The contribution of magnetoresistance and Maxwell–Wagner effect for the observed ME cannot be ignored [39]. The observed intrinsic ME coupling may also be due to magnetostriction effect. The change in lattice parameters occurs with the application of magnetic field. This generates strain in the nanowires due to the coupling between ferroics domains, and, induces stress in it. This induces an electric field in the nanowires that orients the ferroelectric domains, and, subsequently leads to observed ME. Room temperature occurrence of ferromagnetism and ferroelectricity (Figure 5 and 6) further supports the observation of multiferrosim and magnetoelectric coupling in the BiFeO3 nanowires.
4. Conclusions Colloidal dispersion template-assisted technique has been successfully employed for the synthesis of BiFeO3 nanowires, having diameter 20 nm. Structural study confirms that the synthesized nanowires possess rhombohedral perovskite pure phase with R3c space group.
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Magnetic and electrical investigations confirm the ferromagnetic and ferroelectric behavior of synthesized nanowires, respectively, with low leakage-current density at room temperature. XRD, EDAX and XPS analyses reveal the phase purity of the synthesized nanowires, as well as rules out the presence Fe2+ or its oxide in the BiFeO3. Room temperature magnetoelectric coupling coefficient measurement confirms the strong magnetoelectric coupling in the synthesized nanowires. The high magnetoelectric coupling coefficients viz. longitudinal, L-αME =10.738 mV/cmOe, and transverse, T-αME = 6.866 mV/cmOe have been observed. The observed magnetoelectric coupling and multiferroism in the synthesized nanowires are attributed to their nano-size, and high microstrain. This study establishes that the strong magnetodielectric coupling in the BiFeO3 nanowires results in the best multifunctional multiferroic, and further have potential to explore its applications in nanoelectronics, spintronic, and multiferroic-based memory devices.
Acknowledgments One of the authors, Gurmeet Singh Lotey, gratefully acknowledges the Department of Science and Technology (DST), Government of India, for awarding him the INSPIRE (Innovation in Science Pursuit for Inspired Research) fellowship to carry out this research work. We are highly thankful to Professor Claudia Felser of Johannes Gutenberg University of Mainz, Germany, for the magnetic characterization.
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Figure captions Figure 1
Schematic diagram of the dynamic lock-in technique for magnetoelectric coefficient measurements
Figure 2
SEM micrograph of BiFeO3 nanowires
Figure 3
Rietveld refined XRD patterns of BiFeO3 nanowires
Figure 4
(a) Enlarge view of x-ray photoemission spectra of Fe 2p and (b) EDAX spectrum of BiFeO3 nanowires
Figure 5
(a) Magnetization versus applied field (MH) hysteresis loop and (b) temperature dependence of magnetization of BiFeO3 nanowires
Figure 6
(a) Polarization versus electric field hysteresis loop and (b) relative dielectric constant and dielectric loss vs. frequency traits of BiFeO3 nanowires (c) leakagecurrent density versus applied electric field traits of BiFeO3 nanowires
Figure 7
Room temperature DC bias magnetic field dependence (c) longitudinal (L-αME) and (b) transverse (T-αME) magnetodielectric coupling coefficients of BiFeO3 nanowires
Table caption
Table 1
Rietveld structural refinement parameters of BiFeO3 nanowires
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15
Table 1
Figure 1
Figure 2
Figure 3
Figure 4
Figure 5
Figure 6
Figure 7
Highlights
Multiferroic BiFeO3 nanowires of 20 nm size have been synthesized.
Nanowires are ferromagnetic with high saturation magnetization, 3.82 emu/g
Nanowires possess ferroelectric behavior with high polarization 54µC/cm2
Magnetoelectric coupling coefficients in synthesized BiFeO3 nanowires measured
Longitudinal and transverse coupling coefficients found 10.738 and 6.866 mV/cmOe