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Optical limiting and magnetoelectric coupling in multiferroic BiFeO3 nanoparticles
Accepted Manuscript Optical limiting and magnetoelectric coupling in multiferroic BiFeO3 nanoparticles P. Nisha Francis, S. Dhanuskodi, M.S. Jayalakshmy, M. Muneeswaran, J. Philip, N.V. Giridharan PII:
S0254-0584(18)30467-X
DOI:
10.1016/j.matchemphys.2018.05.062
Reference:
MAC 20677
To appear in:
Materials Chemistry and Physics
Received Date: 1 November 2016 Revised Date:
14 May 2018
Accepted Date: 23 May 2018
Please cite this article as: P.N. Francis, S. Dhanuskodi, M.S. Jayalakshmy, M. Muneeswaran, J. Philip, N.V. Giridharan, Optical limiting and magnetoelectric coupling in multiferroic BiFeO3 nanoparticles, Materials Chemistry and Physics (2018), doi: 10.1016/j.matchemphys.2018.05.062. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
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Optical Limiting and Magnetoelectric Coupling in Multiferroic BiFeO3 Nanoparticles
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P. Nisha Francis 1, S. Dhanuskodi1,∗, M. S. Jayalakshmy2, M. Muneeswaran3, J.Philip4, N. V. Giridharan3 1 Nonlinear Optical Materials Laboratory, School of Physics, Bharathidasan University, Tiruchirappalli 620024, India. 2 International and Interuniversity Centre for Nanoscience and Nanotechnology, M. G University, Kottayam 686560, India. 3 Department of Physics, National Institute of Technology, Tiruchirappalli 620015, India. 4 Amal Jyothi College of Engineering, Kanjirapally, Kottayam 686518, India *Corresponding author E-mail: [email protected]
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Abstract
Using citric acid as a chelating agent, Bismuth ferrite or BiFeO3 (BFO) nanoparticles
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have been synthesized by sol-gel method and formation of single phase rhombohedral BFO is confirmed by powder XRD. Three A1 and four E modes related to the different sites occupied by Bi atoms are found in Raman spectrum. From HRTEM analysis, the average particle size of the nanoparticles is evaluated as 49 nm. The prominent peaks in DTA at 822 and 980̊ C reveal α-β (rhombohedral-orthorhombic) and β-γ transitions (orthorhombic-cubic) respectively. The thermal conductivity and specific heat have been measured by a photopyroelectric (PPE) thermal wave
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technique. Transitions due to charge transfer excitation and d-d crystal field excitation of Fe3+ ions have been detected by the UV-Vis DRS and the energy bandgap is 2.05 eV. Nanoparticles exhibit reverse saturable absorption (RSA) in open aperture Z-scan technique (Nd: YAG, 532 nm, 5 ns, 150 µJ) with an optical limiting threshold of 2.44 x 1013 W/m2. Frequency and
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temperature dependent dielectric measurements reveal the ferromagnetic to paramagnetic phase transition at 375̊ C. Electrical conduction due to small polaron transport with activation energy of
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0.581 eV at 1 kHz is confirmed from AC conductivity (σac) measurements. Magnetization measurement indicates a weak ferromagnetism with the saturation magnetization (Ms) 1.177 emu/gm and coercive field (Hc) 151 kOe at room temperature. For applied fields 1500 – 6500 V, saturation polarization (Pr) varies from 0.082 to 1.13µC/cm2 with a corresponding increase of coercive field (Ec) from 2.32 to 23.01 kV/cm. The coexistence of ferroelectric and ferromagnetic orders is confirmed through magneto-electric (M-E) coupling measurement with a M-E coupling coefficient (α) of 0.011 Vcm-1Oe-1.
Keywords: Multiferroic, Bismuth ferrite, Optical limiting, Magneto-electric
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1. Introduction With the discovery of second harmonic generation in quartz crystal in 1961 by Franken et al. [1], nonlinear optics (NLO) has burgeoned into the fields of science and technology. This
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seminal work was followed by the discovery of a rich diversity of nonlinear optical effects like harmonic generation, optical parametric amplification, nonlinear absorption, four wave mixing etc. To spot the suitability of a material for nonlinear optical applications, it is necessary to determine its nonlinear absorption and refraction properties. A strong nonlinear absorption leads
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to good optical limiting whereas nonlinear refraction assists all optical switching applications. Zscan technique introduced by Bahae et al. is well known for its sensitivity and simplicity for the measurement of third order nonlinear optical properties like nonlinear refraction and absorption
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[2]. With the widespread use of optical detectors and sensors, the need for optical limiting devices for both pulsed and continuous wave regime has quickly arisen to protect the photosensitive elements. To safeguard the human eyes from the enfeebling laser effects, materials with a low threshold value are needed since the maximum permissible liability for eyes with a laser pointer is ~2.5 mW/cm2 in the visible spectral region [3]. Also, the development of lasers as the convenient and powerful sources for localized energy has led to the need for
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materials having large damage thresholds. Formation of a temperature gradient and the corresponding thermal expansion by an incident laser beam can rupture a material, which prevents it from many device applications. Materials with high values for specific heat capacity
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and thermal conductivity tend to have high damage threshold as only a smaller temperature gradient is formed in the material while absorbing thermal energy. Photopyroelectric (PPE) technique has developed as a well established method for the simultaneous determination of
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these thermal parameters such as the thermal conductivity and the specific heat capacity of crystalline samples which can be grown only to small sizes [4]. Perovskite oxides and related compounds have recently drawn much interest due to their
technological applications. Among them, bismuth ferric oxide or Bismuth ferrite (BFO) with a non-centrosymmetric structure has long been known to be an antiferromagnetic (TN = 370̊ C), ferroelectric (TC = 825̊ C) multiferroic [5]. Thus, this rhombohedrally distorted perovskite has received considerable attention as a multiferroic with room temperature magneto-electric (M-E) coupling though the weak magnetization and inhomogeneity induce a leakage current in the 2
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material making it difficult to observe the ferroelectric (FE) loops. When the particle size goes below ~ 62 nm, the periodicity of helical ordering, the modulated spin structure is suppressed and ferromagnetism (FM) is found to set in [6]. The coexistence of these ferroic orders finds applications in new multi-functional devices like data storage based on electric field control of
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magnetization and vice versa [7]. To avoid the volatilization of bismuth at high temperatures, it is necessary to follow a low temperature synthesis route. Also, as its practical applications are hampered by problems due to leakage current on account of non-stoichiometry, it is required to obtain nanosized stoichiometric single phase BFO. Nearly pure BFO nanostructures have been
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successfully synthesized via solid-state reaction route with a phase transition from rhombohedral to pseudo-cubic symmetry at 20 wt% Ba doping [8]. Multi-ferroic bismuth ferrite nanoparticles
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have been dispersed into the matrix of the ordered mesoporous silica in one pot synthetic protocol and the prepared materials have high specific surface area along with high crystallinity [9]. 3D mesoporous network of interconnected BFO nanoparticles with enhanced catalytic properties has been prepared by Papadas et al. through a facile nanoparticle templating process [10]. So far, BFO has been prepared by several physical and chemical methods like the solid state reaction [11], hydrothermal [12], sol-gel [13] etc. Though various properties of BFO have
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been reported earlier by previous workers, many of its properties such as nonlinear optical effects, M-E coupling etc. have not been reported so far. The present investigation deals with these properties as well as some associated properties of nanosized BFO synthesized via citrate sol-gel route.
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2. Materials and Methods
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2.1 Experimental Procedure
BFO nanoparticles were synthesized by metal ions complexing with citric acid (C6H8O7)
by sol-gel method. Starting materials bismuth nitrate, iron nitrate and citric acid (Merck, GR) were used without further purification.
The precursor solution was prepared by mixing
equimolar (0.015 M) ratio of Bi(NO3)3.5H2O and Fe(NO3)3.9H2O in double distilled water under constant magnetic stirring for 1 hr. To maintain the pH ~ 1, HNO3 was added. Citric acid was added as a chelating agent in the 1:2 molar ratio with metal ions and stirred at 50̊ C for another 1hr. Metal nitrate precursor solution was poured slowly to this and the colour of the solution changed from yellow to orange and turned brown upon further heating. The obtained light 3
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brownish solution was kept on a hot plate at 80̊ C till it became a viscous resin. During continuous heating, dried resin was auto-ignited and it released a good amount of ammonia gas and became a dark brownish powder which was further ground and calcined at 500̊ C for 2 hrs to
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get phase pure BFO [14]. 2.2 Characterizations
A powder X-ray diffractometer (XRD, Rigaku, D/Max Ultima III) with Cu Kα radiation was used for the structure analysis. Fourier transform infrared (FTIR) spectrum was taken using
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FTIR spectrometer (Jasco, 460 plus) for molecular confirmation. Raman spectrum was recorded at room temperature using a confocal laser Raman spectrometer (Witec, CRM200) with 488 nm
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output from an Ar+ laser. Morphology was analyzed using high resolution transmission electron microscope (HRTEM, Jeol/JEM 2100) equipped with selected area electron diffraction (SAED). In order to monitor the decomposition and pyrolysis of the powder, differential thermal analysis was carried out upto 1100̊ C in a DTA instrument (Netzsch, STA 449F3). Thermal parameters, thermal conductivity and specific heat capacity, were determined by the PPE technique, described elsewhere [4]. A He-Ne laser (632.8 nm, 20 mW), intensity modulated by a
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mechanical chopper, was used as the optical heating source. The sample was illuminated by the modulated beam of light which gave rise to periodic temperature variations by optical absorption. To enhance the optical absorption, a very thin layer of carbon black from a benzene flame was carefully coated on the sample having thickness 0.39 mm. Modulation frequency of
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light is kept above 40 Hz to ensure that the sample was thermally thick during measurements. The modulated radiations absorbed by the sample got converted into thermal waves at its surface, propagate through the sample and was detected with a pyroelectric detector
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(polyvinylidenediflouride film of thickness 28 µm, coated with Ni - Cr on both sides, with a pyroelectric coefficient P = 0.25x10-8 Vcm-1K-1). The output signals (amplitude and phase) were measured with a lock-in-amplifier (Stanford Research Systems, SR 830 DSP). Linear optical absorption was measured by recording the UV-Vis diffuse reflectance
spectrum (UV-Vis DRS) with a Shimadzu, UV – 2450 solid state laser with an integrated sphere assembly. The nonlinear optical absorption was measured by following the open aperture Z-scan technique. In this experiment, the 5 ns laser pulses from a frequency doubled Nd: YAG laser (532 nm, 10 Hz) were focused using a lens (f = 10 cm) and at each position ‘z’ along the beam 4
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axis, the transmittance was determined. The experiment was carried out at an average energy of 150 µJ for which the linear transmittance is tuned to be about 70%. By numerical fitting, the nonlinear optical parameters and the corresponding limiting threshold were calculated. The dielectric properties, dielectric constant and loss, were measured using a computer interfaced
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LCR meter (Hioki, 3535-50 LCR HiTester) in the frequency range 100 Hz - 5 MHz from ambient to 400̊ C. The magnetization M as a function of applied field H at room temperature was evaluated using a vibrating sample magnetometer (Lakeshore VSM, 7404). Room temperature ferroelectric hysteresis loops were assessed using a ferroelectric loop tracer (Radiant
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Technologies, USA). A dynamic lock-in-amplifier technique has been used for the measurement of M-E voltage. The constant dc magnetic field on ac scan and constant ac magnetic field on dc
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scan were fixed at 2000 Oe and 10 Oe respectively for the measurement. Electrical properties were studied on discs of sample made by pelletizing the ground powder, with thin layer of silver paste acting as an electrode.
3. Results and Discussion 3.1 Structural Analysis
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The XRD spectrum of BFO calcined at 500̊ C is presented in Fig. 1(a). The R3c rhombohedral structure is confirmed with the occurrence of (012), (104), (110), (202) and (024) planes and in an equivalent hexagonal representation of rhombohedral unit cell, the lattice parameters are a= 5.572 and c= 13.834 Å. The peak positions and the relative intensities are in
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accordance with JCPDS 86 - 1518 and no other crystal phases like Bi2Fe4O9 or Bi25FeO40 are observed. An average crystallite size D of ~ 48 nm is calculated from XRD data using Debye-
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Scherrer equation,
=
(1)
where K is the shape factor, λ is the wavelength of X-ray used, βhkl is the instrumental broadening
and is the Bragg angle.
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Fig.1 (a) XRD pattern of BFO nanoparticles calcined at 500̊ C (b) Magnified pattern of the characteristic peaks The magnified pattern of the characteristic peak of BFO in the vicinity of 2 = 32̊ is
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shown in Fig.1 (b) and it clearly shows the splitting of the doublet (104) and (110) confirming the rhombohedral R3c of BFO which are found to be merged together with the structural transitions from rhombohedral to either orthorhombic or triclinic while doping with rare earth or transition metals [13, 15]. It is possible to evaluate the peak broadening due to dislocation in terms of the crystallite size D and strain ε. A mathematical expression relating these two parameters,
4
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=
(2)
is proposed by Williamson and Hall (W-H). A graph is drawn with 4 sin along the x-axis and
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βcos along the y-axis (Fig.2). Using the intercept value of the linear fit (0.0027), D is derived as 51 nm, close to the value calculated from Debye-Scherrer equation (48 nm) and ε is found from
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the slope as 0.00104.
Fig.2 W - H plot for BFO nanoparticles 6
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3.2 Functional Group Analysis Formation of the perovskite structure is confirmed by the presence of metal- oxygen bands between 700 and 400 cm-1 in the FTIR spectrum. Inset of Fig. 3 displays the same in
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which two absorption peaks at 551 and 443 cm-1 are attributed to the Fe-O stretching and bending vibrations of octahedral FeO6 group respectively [16]. As both the BiO6 and FeO6 octahedral structures possess absorption bands in the same region, the observed ones are due to the overlapping of both Bi-O and Fe-O vibrations. The measured Raman spectrum, fitted and de-
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convoluted into individual Gaussian components is given in Fig.3. For the distorted rhombohedral BFO with 10 atoms in the unit cell, 18 Raman modes are summarized using the
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following irreducible representation [17], Γopt.R3c = 4A1 + 5A2+9E
(3)
where A1 and E are polar optical modes which are Raman and IR active while A2 modes are Raman inactive. In the polar optical modes, A1 symmetry phonons are longitudinal optical (LO) which is associated with the atomic motion along c axis while E symmetry phonons are
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transverse optical (TO) which are associated with the atomic motion in the ab plane.
Fig.3 Raman spectrum of BFO nanoparticles. The inset shows the FTIR spectrum. In the present study, the phonon modes at 127, 170 and 215 cm-1 are assigned to A1
modes and modes at 86, 312, 447 and 614 cm-1 are ascribed to E modes. Modes located at low frequencies (< 200 cm-1) are related to the different sites occupied by Bi atoms which enhance 7
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the Jahn-Teller distortion of FeO6 octahedra. Whereas, the modes above 200 cm-1 are responsible for the internal vibrations of the octahedra. The high frequency modes observed at 778 and 1238 cm-1 are the overtones of first order E4 and E9 corresponding to 2E4 and 2E9 modes. The six characteristic modes A1-1, A1-2, A1-3, A1-4, E1 and E2 at 127, 170, 217, 470, 262 and 275 cm-1 steriochemically active Bi3+ lone pair of BFO [18]. 3.3 Morphological Analysis
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respectively are considered to be responsible for the ferroelectric nature originating from the
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The morphology of the sample is investigated by FESEM and TEM. FESEM image (Fig. S1) reveals that the powder is composed of loosely aggregated extremely fine particles. It is
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obvious that the particles have narrow size distribution and homogeneous shape. TEM image and SAED pattern of BFO nanoparticles given in Fig. 4 confirm the poly crystalline nature of the sample. The average particle size estimated from TEM image is 49 nm, consistent with the XRD analysis (48 nm). The HRTEM image in the inset of Fig.4 (a) displays ordered crystalline planes with an interplanar ‘d’ spacing of 0.28 nm corresponding to the (104) plane. Concentric rings with bright spots in the SAED pattern (Fig.4 (b)) are indexed to the (104), (202) and (024) planes
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of rhombohedral BFO which are also in agreement with the observed planes in XRD.
Fig.4 (a) TEM image (HRTEM in the inset) and (b) SAED pattern of BFO nanoparticles 3.4 Thermal Analysis To evaluate the crystallization and phase transition temperatures, thermal analysis has been carried out from ambient to 1100̊ C range. The related TG curve with three stages of weight loss is evinced in Fig. 5. The first two regions (0- 250 and 250 - 350̊ C) correspond to the loss of 8
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physisorbed water and surface hydroxyl groups. The loss in the region 350-480̊ C is ascribed to the decomposition of nitrates and burning up of the remaining carbonaceous matter from the precursors and chelating agent respectively [19]. Though the TGA analysis is carried out for the calcined sample for which almost all the residual nitrates and organic matter have been burnt out,
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the curve denotes a small weight loss. There is a possibility of having distorted atoms with broken exchange bonds on their surfaces which easily induce oxidation/reduction chemical reactions or adsorb water molecules. So, the observed weight loss is entailed by these surface imperfections. No more weight loss is viewed after 480̊ C. Thus, it can be deduced that in order
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to obtain phase pure BFO, the as synthesized sample needs to be calcined at a temperature > 500̊ C. DTA curve for a heating cycle at a rate of 5̊ C/min is given in the inset of Fig.5. Two peaks,
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one at 822̊ C corresponds to α-β transition (TC-ferroelectric to paraelectric, (rhombohedral-
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orthorhombic)) and another at 980̊ C is due to β-γ transition (orthorhombic-cubic) [5].
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Fig.5 TGA curve (DTA in the inset) of BFO nanoparticles
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The obtained variations of the PPE amplitude and phase with modulation frequency are shown in Fig.6. Knowing the PPE phase and amplitude, the thermal diffusivity (α) and thermal effusivity (e) are determined and from these values, the thermal conductivity (k) and specific heat capacity (Cp) are calculated from the relations [4],
= (
!
"# = ! $(
(4) !
(5)
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where ρ is the density of the sample. The calculated values of k and Cp are10.07 W/mK and 1358 J/kgK respectively. For polycrystalline and nanocrystalline BFO, temperature dependence of these parameters in the absence and presence of an external magnetic field have been reported by
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magneto-elastic and spin-phonon couplings.
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Uma and Philip [20]. The observed anomalies in the thermal properties are explained in terms of
Fig.6 Frequency dependence of PPE amplitude and phase of BFO nanoparticles 3.5 Linear and Nonlinear Optical Properties
Fig.7 (a) illustrates the optical absorption of BFO in the UV-Vis region, derived from the
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UV-Vis DRS using Kubelka-Munk (K-M) function,
% (& =
( '( ) (
(6)
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where R is the reflectance. The graph is plotted with F(R) as a function of wavelength (nm). Considering the point group symmetry breaking from Oh to C3v prompted from the distorted
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cubic structure, BFO is expected to have six transitions between 0 and 3 eV. In the present DRS, a shoulder centered at 1.91 eV is attributed to the 6A1g – 4T2g excitation which arises due to the dd crystal field excitation of Fe3+ions. In the higher energy region, absorption at 2.38 and 3.35 eV are assigned to the charge transfer (CT) excitations driven by Fe1 3d-Fe2 3d inter-site electron transfer and interatomic O 2p-Fe 3d transition respectively [21, 22].The absorption cut-off wavelength is about 630 nm (1.97 eV) suggesting the visible light absorption by BFO. The electronic transition from the top of the valance band, which is mainly composed of O-2p hybridized with Bi-6s, to the bottom of the conduction band of Fe-3d results in this absorption. The K-M function is related to the bandgap as, 10
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(
% (& =
(
∝ =
0
! + ,'-. / )
,
(7)
Where K(λ) and s(λ) are the absorption and scattering coefficients respectively, hν is the photon
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(Fig.7(b)) to zero, the bandgap of BFO is found to be 2.05 eV.
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energy and Eg is the bandgap. By extrapolating the straight line portion of the plot (αhν)2 vs hν
Fig.7 (a) UV-Vis DRS spectrum and (b) bandgap energy plot of BFO nanoparticles Following the open aperture Z-scan technique, the third order optical nonlinearity is
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studied and the intensity dependent transmission recorded is shown in Fig.8. If the material becomes less transparent with the increase of incident laser intensity, it is called a reverse saturation absorption; otherwise saturation absorption. Thus, the open aperture Z-scan trace is supposed to have a maximum or minimum transmittance at the focus (f=0). In the current
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investigation, a smooth valley pointing out a RSA in the material is achieved. When the excited laser energy is larger than half of the bandgap of the material (hν>Eg/2, here it is 2.33>1.03), it is
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known that there is a possibility of multiple photon absorption. The strength of the nonlinear absorption is found from the numerical fitting to different transmission equations and the best fit is obtained for two photon absorption (2PA) along with the saturation absorption (SA). At 532 nm (2.33 eV), a reasonable mechanism is that the electrons excited from the lowest energy state are trapped in the intermediate state which enhances the absorption of one more photon resulting in 2PA. Z- scan experiment with relatively long (5 ns) laser pulses allow multiple absorption from the same laser pulse, which enhances the excited state absorption. In this particular case, the nonlinear absorption coefficient (α) is given by,
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(2 =
34
7
(8)
567 9 8
where αₒ is the unsaturated linear absorption coefficient, I is the laser intensity and Is is the transmitted intensity for a given input intensity is described by :;
:<
= =>
34
2? 2
7 78
56 9
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saturation intensity of the sample. The corresponding propagation equation to find out the
(9)
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By solving this equation, the obtained values of Is and 2PA coefficient (β) are 1 x 1012 W/m2 and 0.56 x 10-10 m/W respectively. Lakshmi Reddy et al. have reported a β of the same order for
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copper doped zinc aluminum ferrite for an energy of 70 µJ with 5 ns pulse duration [23]. Also β of the same order has been reported for nanostructures like Bi nanorods [24] and Te nanowires
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[25].
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Fig.8 Optical limiting behavior of BFO nanoparticles extracted from the Z-scan data (in the inset) for 150 µJ The optical limiting curve extracted from the Z- scan data (inset) is demonstrated in Fig.8. Here, the normalized transmittance is plotted against the input laser fluence given by,
%(@ = 4√ln 2 6
-EF
H 9 G !)
I(@
(10)
which has the maximum value at the focal point. Here, Ein is the input laser pulse energy and w(z) is the beam radius given by, 12
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I(@ = I(0 K1
+@!@M / N
!
(11)
with w(0) being the focal spot radius and z0 = (πw02)/λ, the Rayleigh range. At each sample
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position z, the sample discerns different laser fluence. The optical power limiting property of BFO with a limiting threshold (the fluence at which the transmission decreases to 50 % of the linear transmission) of 2.44 x 1013 W/m2 depicts a deviation from Beer’s law under strong irradiance and the output energy drops steadily. Linear and nonlinear optical properties of some
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perovskite nanoparticles are listed in Table 1 for comparison.
Table 1: Some representative optical limiting perovskite nanoparticles.
BaTiO3
2.65
BaTiO3
3.00
ErMn0.95Cr0.05O3
-
BiFeO3
2.05
Laser Parameters
β (×10-11 m/W)
Is (×1011 W/m2)
Limiting Threshold (J/cm2)
Ref.
7.18
48.9
2.33
[26]
0.17
10
-
[27]
0.9
2.9
0.19
[28]
5.6
10
2.44
Present
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Material
Band gap (eV)
532 nm, 5 ns, 70 µJ
800 nm, 2 ps, 70 µJ
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532 nm, 5 ns, 90 µJ
532 nm, 5 ns,
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150 µJ
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3.6 Dielectric Analysis
In order to evaluate the frequency dependent properties associated with grain boundaries
and intrinsic properties of the material, the dielectric properties have been studied as a function of frequency and temperature. The bulk ohmic resistance, which is the real part (Z’) of complex impedance is plotted against the imaginary part of the complex impedance (Z”) at different temperatures (Fig.9 (a)) to check the temperature dependence of the conductivity. It is known that the semicircle in the high frequency region is recounted to the grain interior resistance and the effect of blocking electrodes is described to the spike in the low frequency region. The real (ε’) and imaginary (ε”) parts of the dielectric constant are calculated using, 13
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O∗ =
QRST ∗
(12)
where j = √1,V = 2πν, ν is the frequency, C = ε0A/d, ε0 is the permittivity of free space, A is the
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effective area of the electrodes and d is the thickness of the pellet. A plot of ε’ vs log f at different temperatures specifies a low frequency dispersion indicating the dc conductivity for the sample. This type of relaxation is related to the domain wall relaxation in ABO3 systems. With the application of an electric field, the decrease in ε’ stems from the instantaneous nonoccurrence
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field leads to relaxation in ε’ (Fig.9 (b)) [29].
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of polarization as the charges possess inertia. The delay in response as a function of the given
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Fig.9 (a) Nyquist plot for 100 - 250̊ C range and (b) frequency dependent dielectric constant for BFO nanoparticles at various temperatures The frequency dependence of ε” at various temperatures and temperature dependence of ε’ at various frequencies for BFO nanoparticles are shown in Fig. 10 (a) and (b) respectively. At
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low frequencies and high temperatures, a significantly high ε” exhibited by the sample is interpreted as due to space charge polarization present in BFO. It is reported that the high
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conductivity and loss at high temperatures in bismuth ferrite based systems are initiated by the secondary phases segregated at the grain boundaries. These phases have a doping effect which transforms the dielectric into a semiconductor. In bulk ceramics, defects associated to the grain boundaries also have a contribution to high loss, resulting in a complex impedance behavior [29]. Temperature dependence of ε’ (Fig.10 (b)) shows a gradual increase with temperature. In the neighborhood of TN at 375̊ C, a dielectric anomaly indicating the ferromagnetic to paramagnetic transition is observed as predicted by the Landau – Devonshire theory of phase transition and it signifies the M-E coupling in the sample [30].
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Fig.10 (a) Frequency dependence of dielectric loss at various temperatures (b) Temperature dependence of dielectric constant at various frequencies for BFO nanoparticles Studies on the relaxation of dielectric modulus have been carried out from ambient to 500̊
W ∗ = WX
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C range in the complex modulus W∗ formalism. The complex modulus W ∗ is given by,
YW"
(13)
where WX and W" are the real and imaginary parts of electric modulus. Variations of WX and W" as a function of frequency and temperature are shown in Fig.S2. The figures clearly depict the maximum values at high frequencies and temperatures and it approaches nearly zero at low
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frequencies verifying the existence of a considerable electrode and/or ionic polarization in the said temperature range [31]. The non-exponential decay of the electric field is confirmed by the asymmetric peaks in the studied temperature range and it is inferred as a consequence of distribution of relaxation times. AC conductivity (σac) measurements (Fig.S3) have been carried given by,
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out in the frequency regime 100 Hz – 5 MHz to understand the conduction mechanism. σac is
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[\ = V] X ^_ `
M
(14)
On the application of the electric field, polarons are formed by the distortion/polarization
of the lattice by the motion of electrons. If such deformation is of the order of lattice constant, small polarons are formed and vice versa. If σac decreases with the increasing frequency, it is explained by the large polaron model and small polaron model accounts for the increase in σac with frequency [32]. In the present investigation, the conduction mechanism is largely due to the small polarons and a slight deviation is due to the mixed polaron conduction comes from the structural inhomogeneity owing to the discontinuous grain growth. 15
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The Arrhenius plot with ln σacT vs 1/T given in Fig. 11 clearly shows that σac increases linearly with temperature and is fitted to the Arrhenius equation, '-cd ef
9
(15)
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[\ = [M ab 6
where σ0 is the pre-exponent factor, Eac is the activation energy for ionic migration, KB is the Boltzmann constant and T is the temperature. From the linear fit, activation energy for ionic migration is found to be 0.581, 0.496, 0.373 and 0.242 eV respectively for 1 kHz, 10 kHz, 100
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kHz and 1 MHz, decreasing with increasing frequency. The frequency dependence of σac is given by AVn, where the coefficient A and the exponent n are dependent on temperature and intrinsic properties of the material. The obtained n value or the activation energy ensures the frequency
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dependent conduction due to small polarons [32].
Fig.11 Arrhenius plot for BFO nanoparticles at various frequencies
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3.7 M-H Loop Measurement
For bulk BFO, the magnetic hysteresis loop shows a typical antiferromagnetic (AFM)
behavior with zero coercivity. Magnetic ordering in BFO is quite ambiguous because of the Dzyaloshinskii- Moriya interaction, resulting in a canted AFM ordering provided by the transition metal cation Fe3+. This canted Fe
3+
moments induce a lattice strain which in turn
increases the free energy of the lattice. In order to decrease this free energy, a spiral spin structure is superimposed on to the AFM ordering resulting in the rotation of spin. In bulk, the order parameter of this helical ordering between two successive planes is found to be 62 nm [33 16
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(a, b)]. If the size of the particle is less than 62 nm, as in the present case, the helical order gets suppressed resulting in a weak FM in the AFM lattice as given in Fig.12. This coexistence FM and AFM orderings leads to the setting up of an exchange bias phenomenon that could give rise to a shift in unidirectional anisotropy as well as coercivity (+167 and -135 Oe) as can be viewed
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in the inset of Fig.12. Thus, the observed magnetization with Ms, Mr and Hc of 1.177, 0.188 emu/gm and 151 kOe respectively is explained as due to the high surface to volume ratio, due to the lower crystallite size (48 nm). FM behavior with a relatively high saturation magnetization of 3.5 emu/gm is reported for BFO nanoparticles annealed at 350̊ C and the enhancement is
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attributed to the presence of impurities like γ- Fe2O3 [34]. While size-dependent magnetic properties with Mr/Ms = 0.06 (here it is 0.193) for a particle size of 51 nm have been studied
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earlier for glycol assisted BFO [35].
Fig.12 M - H hysteresis curve of BFO nanoparticles at room temperature
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3.8 P-E Loop Measurements
In perovskite oxides, the ferroelectric (FE) order arises from several sources viz., empty d
shell, lone pair drives, charge ordering with geometrical or magnetic frustration and magnetically induced from broken spiral magnetic spin structure. In BFO, FE is induced by the lone pair of Asite Bi3+, a steriochemically active 6s2 lone pair, which causes Bi 6p orbital to come closer in energy to O 2p orbital. This leads to the hybridization between these orbitals which in turn drive the off-centering of the cation towards the anion resulting in FE [5]. Fig.13 exhibits the polarization vs electric field plot for BFO and the unsaturated loops with polarization of lower 17
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magnitudes indicate the low resistivity and large leakage current of the sample. Inset of the figure shows the enlarged pattern for applied fields of 1500 and 2000 V. The obtained polarizations and coercivity for a varying field from 1500 to 6500 V are Ps= 0.372 - 2.257, Pr = 0.081 – 1.129 µC/cm2 and Ec = 2.32 – 23 kV/cm. For an average crystallite size of 47 nm, Pr of
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0.17 µC/cm2 and Ec of 10 kV/cm are discussed by Hu et al. which increases to 0.82 µC/cm2 and 33 kV/cm by Gd dopant [36]. The enhancement is imputed to the structural distortion induced by the mismatch of Bi and Gd ionic radii. Also for spin coated thin films, 2Pr and 2Ec of 0.931
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µC/cm2 and 57.37 kV/cm respectively are conferred by Huang et al. [37].
Fig.13 P - E loops of BFO nanoparticles at room temperature
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3.9 Magneto-Electric Coupling
To study the M-E effect, the induced polarization with the applied magnetic field is measured with Hall probes. In the measurement setup, for ac measurements a parallel dc bias
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field of 2000 Oe at a fixed frequency of 850 Hz is applied using Helmholtz coils and for dc bias field, a 10 Oe ac magnetic field of the same frequency is applied using a set of permanent magnets. In single-phase materials, the M-E output arises due to the interaction between the ferroelectric and magnetic sub-lattices through stress or strain [38]. When a magnetic field is applied to a M-E material, the stress induced strain reorients the electric dipoles creating an ac voltage on the top and bottom surfaces of the material through M-E coupling. The M-E coefficient (α) is given by,
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=
g
hijcd
(16)
where V is the M-E voltage, t is the thickness of the sample and Hac is the applied ac magnetic field [39]. The dependence of M-E voltage with the applied ac and dc magnetic fields at room
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temperature is shown in Fig. 14. The M-E voltage shows a linear variation with the applied ac field and from the slope, the coupling coefficient (α) is calculated to be 0.011 Vcm-1Oe-1. Though the observed value of α is low, it is comparable to the reported values of rare earth (Nd,
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Sm, Gd, and Dy) doped bismuth ferric titanate [38].
Fig.14 Variation of M-E voltage with magnetic field for BFO nanoparticles at room temperature
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4. Conclusions
Single phase BFO nanoparticles have been synthesized by citric acid based sol-gel
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method. Crystal structure is confirmed from powder XRD with an average crystallite size of 48 nm. From the phonon transport studies, thermal conductivity and specific heat capacity are found to be 10.07 W/mK and 1358 J/kgK respectively. From the open aperture Z-scan measurements, BFO is found to be an efficient optical limiter for the protection of human eyes and optical sensors with a low limiting threshold. A decrease in both dielectric constant and loss with increasing frequency and a ferromagnetic to paramagnetic phase transition at 350̊ C in the vicinity of TN indicate the presence of M-E coupling. Also, frequency dependent conductivity caused by small polaron excitations is observed. An enhanced multiferroism resulting from the suppression of helical structure with Hc = 151 kOe and Ec = 23.01 KV/cm are identified. The 19
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linear response of ME voltage with a coupling coefficient of 0.011 V/cm-Oe indicates the coexistence of electric and magnetic phases. Overall, a nearly complete optical and magnetoelectric characterizations of BFO nanoparticles are reported in this work.
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Acknowledgement The authors acknowledge the help of Dr. Reji Philip (Raman Research Institute, Bangalore) for the Z-scan measurements. They also thank the Department of Science and Technology (DST-FIST), Government of India for experimental facilities.
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Research Highlights •
Single phase BiFeO3 nanoparticles by sol-gel method.
•
Reverse saturable absorption (RSA) based optical limiting behavior with low limiting
Verification of magnetic transition from dielectric analysis.
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Room temperature multiferroic magneto-electric properties.
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•
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threshold.
S0254-0584(18)30467-X
DOI:
10.1016/j.matchemphys.2018.05.062
Reference:
MAC 20677
To appear in:
Materials Chemistry and Physics
Received Date: 1 November 2016 Revised Date:
14 May 2018
Accepted Date: 23 May 2018
Please cite this article as: P.N. Francis, S. Dhanuskodi, M.S. Jayalakshmy, M. Muneeswaran, J. Philip, N.V. Giridharan, Optical limiting and magnetoelectric coupling in multiferroic BiFeO3 nanoparticles, Materials Chemistry and Physics (2018), doi: 10.1016/j.matchemphys.2018.05.062. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
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Optical Limiting and Magnetoelectric Coupling in Multiferroic BiFeO3 Nanoparticles
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P. Nisha Francis 1, S. Dhanuskodi1,∗, M. S. Jayalakshmy2, M. Muneeswaran3, J.Philip4, N. V. Giridharan3 1 Nonlinear Optical Materials Laboratory, School of Physics, Bharathidasan University, Tiruchirappalli 620024, India. 2 International and Interuniversity Centre for Nanoscience and Nanotechnology, M. G University, Kottayam 686560, India. 3 Department of Physics, National Institute of Technology, Tiruchirappalli 620015, India. 4 Amal Jyothi College of Engineering, Kanjirapally, Kottayam 686518, India *Corresponding author E-mail: [email protected]
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Abstract
Using citric acid as a chelating agent, Bismuth ferrite or BiFeO3 (BFO) nanoparticles
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have been synthesized by sol-gel method and formation of single phase rhombohedral BFO is confirmed by powder XRD. Three A1 and four E modes related to the different sites occupied by Bi atoms are found in Raman spectrum. From HRTEM analysis, the average particle size of the nanoparticles is evaluated as 49 nm. The prominent peaks in DTA at 822 and 980̊ C reveal α-β (rhombohedral-orthorhombic) and β-γ transitions (orthorhombic-cubic) respectively. The thermal conductivity and specific heat have been measured by a photopyroelectric (PPE) thermal wave
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technique. Transitions due to charge transfer excitation and d-d crystal field excitation of Fe3+ ions have been detected by the UV-Vis DRS and the energy bandgap is 2.05 eV. Nanoparticles exhibit reverse saturable absorption (RSA) in open aperture Z-scan technique (Nd: YAG, 532 nm, 5 ns, 150 µJ) with an optical limiting threshold of 2.44 x 1013 W/m2. Frequency and
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temperature dependent dielectric measurements reveal the ferromagnetic to paramagnetic phase transition at 375̊ C. Electrical conduction due to small polaron transport with activation energy of
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0.581 eV at 1 kHz is confirmed from AC conductivity (σac) measurements. Magnetization measurement indicates a weak ferromagnetism with the saturation magnetization (Ms) 1.177 emu/gm and coercive field (Hc) 151 kOe at room temperature. For applied fields 1500 – 6500 V, saturation polarization (Pr) varies from 0.082 to 1.13µC/cm2 with a corresponding increase of coercive field (Ec) from 2.32 to 23.01 kV/cm. The coexistence of ferroelectric and ferromagnetic orders is confirmed through magneto-electric (M-E) coupling measurement with a M-E coupling coefficient (α) of 0.011 Vcm-1Oe-1.
Keywords: Multiferroic, Bismuth ferrite, Optical limiting, Magneto-electric
1
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1. Introduction With the discovery of second harmonic generation in quartz crystal in 1961 by Franken et al. [1], nonlinear optics (NLO) has burgeoned into the fields of science and technology. This
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seminal work was followed by the discovery of a rich diversity of nonlinear optical effects like harmonic generation, optical parametric amplification, nonlinear absorption, four wave mixing etc. To spot the suitability of a material for nonlinear optical applications, it is necessary to determine its nonlinear absorption and refraction properties. A strong nonlinear absorption leads
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to good optical limiting whereas nonlinear refraction assists all optical switching applications. Zscan technique introduced by Bahae et al. is well known for its sensitivity and simplicity for the measurement of third order nonlinear optical properties like nonlinear refraction and absorption
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[2]. With the widespread use of optical detectors and sensors, the need for optical limiting devices for both pulsed and continuous wave regime has quickly arisen to protect the photosensitive elements. To safeguard the human eyes from the enfeebling laser effects, materials with a low threshold value are needed since the maximum permissible liability for eyes with a laser pointer is ~2.5 mW/cm2 in the visible spectral region [3]. Also, the development of lasers as the convenient and powerful sources for localized energy has led to the need for
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materials having large damage thresholds. Formation of a temperature gradient and the corresponding thermal expansion by an incident laser beam can rupture a material, which prevents it from many device applications. Materials with high values for specific heat capacity
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and thermal conductivity tend to have high damage threshold as only a smaller temperature gradient is formed in the material while absorbing thermal energy. Photopyroelectric (PPE) technique has developed as a well established method for the simultaneous determination of
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these thermal parameters such as the thermal conductivity and the specific heat capacity of crystalline samples which can be grown only to small sizes [4]. Perovskite oxides and related compounds have recently drawn much interest due to their
technological applications. Among them, bismuth ferric oxide or Bismuth ferrite (BFO) with a non-centrosymmetric structure has long been known to be an antiferromagnetic (TN = 370̊ C), ferroelectric (TC = 825̊ C) multiferroic [5]. Thus, this rhombohedrally distorted perovskite has received considerable attention as a multiferroic with room temperature magneto-electric (M-E) coupling though the weak magnetization and inhomogeneity induce a leakage current in the 2
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material making it difficult to observe the ferroelectric (FE) loops. When the particle size goes below ~ 62 nm, the periodicity of helical ordering, the modulated spin structure is suppressed and ferromagnetism (FM) is found to set in [6]. The coexistence of these ferroic orders finds applications in new multi-functional devices like data storage based on electric field control of
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magnetization and vice versa [7]. To avoid the volatilization of bismuth at high temperatures, it is necessary to follow a low temperature synthesis route. Also, as its practical applications are hampered by problems due to leakage current on account of non-stoichiometry, it is required to obtain nanosized stoichiometric single phase BFO. Nearly pure BFO nanostructures have been
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successfully synthesized via solid-state reaction route with a phase transition from rhombohedral to pseudo-cubic symmetry at 20 wt% Ba doping [8]. Multi-ferroic bismuth ferrite nanoparticles
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have been dispersed into the matrix of the ordered mesoporous silica in one pot synthetic protocol and the prepared materials have high specific surface area along with high crystallinity [9]. 3D mesoporous network of interconnected BFO nanoparticles with enhanced catalytic properties has been prepared by Papadas et al. through a facile nanoparticle templating process [10]. So far, BFO has been prepared by several physical and chemical methods like the solid state reaction [11], hydrothermal [12], sol-gel [13] etc. Though various properties of BFO have
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been reported earlier by previous workers, many of its properties such as nonlinear optical effects, M-E coupling etc. have not been reported so far. The present investigation deals with these properties as well as some associated properties of nanosized BFO synthesized via citrate sol-gel route.
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2. Materials and Methods
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2.1 Experimental Procedure
BFO nanoparticles were synthesized by metal ions complexing with citric acid (C6H8O7)
by sol-gel method. Starting materials bismuth nitrate, iron nitrate and citric acid (Merck, GR) were used without further purification.
The precursor solution was prepared by mixing
equimolar (0.015 M) ratio of Bi(NO3)3.5H2O and Fe(NO3)3.9H2O in double distilled water under constant magnetic stirring for 1 hr. To maintain the pH ~ 1, HNO3 was added. Citric acid was added as a chelating agent in the 1:2 molar ratio with metal ions and stirred at 50̊ C for another 1hr. Metal nitrate precursor solution was poured slowly to this and the colour of the solution changed from yellow to orange and turned brown upon further heating. The obtained light 3
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brownish solution was kept on a hot plate at 80̊ C till it became a viscous resin. During continuous heating, dried resin was auto-ignited and it released a good amount of ammonia gas and became a dark brownish powder which was further ground and calcined at 500̊ C for 2 hrs to
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get phase pure BFO [14]. 2.2 Characterizations
A powder X-ray diffractometer (XRD, Rigaku, D/Max Ultima III) with Cu Kα radiation was used for the structure analysis. Fourier transform infrared (FTIR) spectrum was taken using
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FTIR spectrometer (Jasco, 460 plus) for molecular confirmation. Raman spectrum was recorded at room temperature using a confocal laser Raman spectrometer (Witec, CRM200) with 488 nm
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output from an Ar+ laser. Morphology was analyzed using high resolution transmission electron microscope (HRTEM, Jeol/JEM 2100) equipped with selected area electron diffraction (SAED). In order to monitor the decomposition and pyrolysis of the powder, differential thermal analysis was carried out upto 1100̊ C in a DTA instrument (Netzsch, STA 449F3). Thermal parameters, thermal conductivity and specific heat capacity, were determined by the PPE technique, described elsewhere [4]. A He-Ne laser (632.8 nm, 20 mW), intensity modulated by a
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mechanical chopper, was used as the optical heating source. The sample was illuminated by the modulated beam of light which gave rise to periodic temperature variations by optical absorption. To enhance the optical absorption, a very thin layer of carbon black from a benzene flame was carefully coated on the sample having thickness 0.39 mm. Modulation frequency of
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light is kept above 40 Hz to ensure that the sample was thermally thick during measurements. The modulated radiations absorbed by the sample got converted into thermal waves at its surface, propagate through the sample and was detected with a pyroelectric detector
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(polyvinylidenediflouride film of thickness 28 µm, coated with Ni - Cr on both sides, with a pyroelectric coefficient P = 0.25x10-8 Vcm-1K-1). The output signals (amplitude and phase) were measured with a lock-in-amplifier (Stanford Research Systems, SR 830 DSP). Linear optical absorption was measured by recording the UV-Vis diffuse reflectance
spectrum (UV-Vis DRS) with a Shimadzu, UV – 2450 solid state laser with an integrated sphere assembly. The nonlinear optical absorption was measured by following the open aperture Z-scan technique. In this experiment, the 5 ns laser pulses from a frequency doubled Nd: YAG laser (532 nm, 10 Hz) were focused using a lens (f = 10 cm) and at each position ‘z’ along the beam 4
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axis, the transmittance was determined. The experiment was carried out at an average energy of 150 µJ for which the linear transmittance is tuned to be about 70%. By numerical fitting, the nonlinear optical parameters and the corresponding limiting threshold were calculated. The dielectric properties, dielectric constant and loss, were measured using a computer interfaced
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LCR meter (Hioki, 3535-50 LCR HiTester) in the frequency range 100 Hz - 5 MHz from ambient to 400̊ C. The magnetization M as a function of applied field H at room temperature was evaluated using a vibrating sample magnetometer (Lakeshore VSM, 7404). Room temperature ferroelectric hysteresis loops were assessed using a ferroelectric loop tracer (Radiant
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Technologies, USA). A dynamic lock-in-amplifier technique has been used for the measurement of M-E voltage. The constant dc magnetic field on ac scan and constant ac magnetic field on dc
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scan were fixed at 2000 Oe and 10 Oe respectively for the measurement. Electrical properties were studied on discs of sample made by pelletizing the ground powder, with thin layer of silver paste acting as an electrode.
3. Results and Discussion 3.1 Structural Analysis
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The XRD spectrum of BFO calcined at 500̊ C is presented in Fig. 1(a). The R3c rhombohedral structure is confirmed with the occurrence of (012), (104), (110), (202) and (024) planes and in an equivalent hexagonal representation of rhombohedral unit cell, the lattice parameters are a= 5.572 and c= 13.834 Å. The peak positions and the relative intensities are in
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accordance with JCPDS 86 - 1518 and no other crystal phases like Bi2Fe4O9 or Bi25FeO40 are observed. An average crystallite size D of ~ 48 nm is calculated from XRD data using Debye-
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Scherrer equation,
=
(1)
where K is the shape factor, λ is the wavelength of X-ray used, βhkl is the instrumental broadening
and is the Bragg angle.
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Fig.1 (a) XRD pattern of BFO nanoparticles calcined at 500̊ C (b) Magnified pattern of the characteristic peaks The magnified pattern of the characteristic peak of BFO in the vicinity of 2 = 32̊ is
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shown in Fig.1 (b) and it clearly shows the splitting of the doublet (104) and (110) confirming the rhombohedral R3c of BFO which are found to be merged together with the structural transitions from rhombohedral to either orthorhombic or triclinic while doping with rare earth or transition metals [13, 15]. It is possible to evaluate the peak broadening due to dislocation in terms of the crystallite size D and strain ε. A mathematical expression relating these two parameters,
4
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=
(2)
is proposed by Williamson and Hall (W-H). A graph is drawn with 4 sin along the x-axis and
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βcos along the y-axis (Fig.2). Using the intercept value of the linear fit (0.0027), D is derived as 51 nm, close to the value calculated from Debye-Scherrer equation (48 nm) and ε is found from
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the slope as 0.00104.
Fig.2 W - H plot for BFO nanoparticles 6
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3.2 Functional Group Analysis Formation of the perovskite structure is confirmed by the presence of metal- oxygen bands between 700 and 400 cm-1 in the FTIR spectrum. Inset of Fig. 3 displays the same in
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which two absorption peaks at 551 and 443 cm-1 are attributed to the Fe-O stretching and bending vibrations of octahedral FeO6 group respectively [16]. As both the BiO6 and FeO6 octahedral structures possess absorption bands in the same region, the observed ones are due to the overlapping of both Bi-O and Fe-O vibrations. The measured Raman spectrum, fitted and de-
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convoluted into individual Gaussian components is given in Fig.3. For the distorted rhombohedral BFO with 10 atoms in the unit cell, 18 Raman modes are summarized using the
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following irreducible representation [17], Γopt.R3c = 4A1 + 5A2+9E
(3)
where A1 and E are polar optical modes which are Raman and IR active while A2 modes are Raman inactive. In the polar optical modes, A1 symmetry phonons are longitudinal optical (LO) which is associated with the atomic motion along c axis while E symmetry phonons are
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transverse optical (TO) which are associated with the atomic motion in the ab plane.
Fig.3 Raman spectrum of BFO nanoparticles. The inset shows the FTIR spectrum. In the present study, the phonon modes at 127, 170 and 215 cm-1 are assigned to A1
modes and modes at 86, 312, 447 and 614 cm-1 are ascribed to E modes. Modes located at low frequencies (< 200 cm-1) are related to the different sites occupied by Bi atoms which enhance 7
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the Jahn-Teller distortion of FeO6 octahedra. Whereas, the modes above 200 cm-1 are responsible for the internal vibrations of the octahedra. The high frequency modes observed at 778 and 1238 cm-1 are the overtones of first order E4 and E9 corresponding to 2E4 and 2E9 modes. The six characteristic modes A1-1, A1-2, A1-3, A1-4, E1 and E2 at 127, 170, 217, 470, 262 and 275 cm-1 steriochemically active Bi3+ lone pair of BFO [18]. 3.3 Morphological Analysis
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respectively are considered to be responsible for the ferroelectric nature originating from the
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The morphology of the sample is investigated by FESEM and TEM. FESEM image (Fig. S1) reveals that the powder is composed of loosely aggregated extremely fine particles. It is
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obvious that the particles have narrow size distribution and homogeneous shape. TEM image and SAED pattern of BFO nanoparticles given in Fig. 4 confirm the poly crystalline nature of the sample. The average particle size estimated from TEM image is 49 nm, consistent with the XRD analysis (48 nm). The HRTEM image in the inset of Fig.4 (a) displays ordered crystalline planes with an interplanar ‘d’ spacing of 0.28 nm corresponding to the (104) plane. Concentric rings with bright spots in the SAED pattern (Fig.4 (b)) are indexed to the (104), (202) and (024) planes
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of rhombohedral BFO which are also in agreement with the observed planes in XRD.
Fig.4 (a) TEM image (HRTEM in the inset) and (b) SAED pattern of BFO nanoparticles 3.4 Thermal Analysis To evaluate the crystallization and phase transition temperatures, thermal analysis has been carried out from ambient to 1100̊ C range. The related TG curve with three stages of weight loss is evinced in Fig. 5. The first two regions (0- 250 and 250 - 350̊ C) correspond to the loss of 8
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physisorbed water and surface hydroxyl groups. The loss in the region 350-480̊ C is ascribed to the decomposition of nitrates and burning up of the remaining carbonaceous matter from the precursors and chelating agent respectively [19]. Though the TGA analysis is carried out for the calcined sample for which almost all the residual nitrates and organic matter have been burnt out,
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the curve denotes a small weight loss. There is a possibility of having distorted atoms with broken exchange bonds on their surfaces which easily induce oxidation/reduction chemical reactions or adsorb water molecules. So, the observed weight loss is entailed by these surface imperfections. No more weight loss is viewed after 480̊ C. Thus, it can be deduced that in order
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to obtain phase pure BFO, the as synthesized sample needs to be calcined at a temperature > 500̊ C. DTA curve for a heating cycle at a rate of 5̊ C/min is given in the inset of Fig.5. Two peaks,
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one at 822̊ C corresponds to α-β transition (TC-ferroelectric to paraelectric, (rhombohedral-
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orthorhombic)) and another at 980̊ C is due to β-γ transition (orthorhombic-cubic) [5].
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Fig.5 TGA curve (DTA in the inset) of BFO nanoparticles
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The obtained variations of the PPE amplitude and phase with modulation frequency are shown in Fig.6. Knowing the PPE phase and amplitude, the thermal diffusivity (α) and thermal effusivity (e) are determined and from these values, the thermal conductivity (k) and specific heat capacity (Cp) are calculated from the relations [4],
= (
!
"# = ! $(
(4) !
(5)
9
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where ρ is the density of the sample. The calculated values of k and Cp are10.07 W/mK and 1358 J/kgK respectively. For polycrystalline and nanocrystalline BFO, temperature dependence of these parameters in the absence and presence of an external magnetic field have been reported by
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magneto-elastic and spin-phonon couplings.
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Uma and Philip [20]. The observed anomalies in the thermal properties are explained in terms of
Fig.6 Frequency dependence of PPE amplitude and phase of BFO nanoparticles 3.5 Linear and Nonlinear Optical Properties
Fig.7 (a) illustrates the optical absorption of BFO in the UV-Vis region, derived from the
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UV-Vis DRS using Kubelka-Munk (K-M) function,
% (& =
( '( ) (
(6)
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where R is the reflectance. The graph is plotted with F(R) as a function of wavelength (nm). Considering the point group symmetry breaking from Oh to C3v prompted from the distorted
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cubic structure, BFO is expected to have six transitions between 0 and 3 eV. In the present DRS, a shoulder centered at 1.91 eV is attributed to the 6A1g – 4T2g excitation which arises due to the dd crystal field excitation of Fe3+ions. In the higher energy region, absorption at 2.38 and 3.35 eV are assigned to the charge transfer (CT) excitations driven by Fe1 3d-Fe2 3d inter-site electron transfer and interatomic O 2p-Fe 3d transition respectively [21, 22].The absorption cut-off wavelength is about 630 nm (1.97 eV) suggesting the visible light absorption by BFO. The electronic transition from the top of the valance band, which is mainly composed of O-2p hybridized with Bi-6s, to the bottom of the conduction band of Fe-3d results in this absorption. The K-M function is related to the bandgap as, 10
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(
% (& =
(
∝ =
0
! + ,'-. / )
,
(7)
Where K(λ) and s(λ) are the absorption and scattering coefficients respectively, hν is the photon
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(Fig.7(b)) to zero, the bandgap of BFO is found to be 2.05 eV.
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energy and Eg is the bandgap. By extrapolating the straight line portion of the plot (αhν)2 vs hν
Fig.7 (a) UV-Vis DRS spectrum and (b) bandgap energy plot of BFO nanoparticles Following the open aperture Z-scan technique, the third order optical nonlinearity is
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studied and the intensity dependent transmission recorded is shown in Fig.8. If the material becomes less transparent with the increase of incident laser intensity, it is called a reverse saturation absorption; otherwise saturation absorption. Thus, the open aperture Z-scan trace is supposed to have a maximum or minimum transmittance at the focus (f=0). In the current
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investigation, a smooth valley pointing out a RSA in the material is achieved. When the excited laser energy is larger than half of the bandgap of the material (hν>Eg/2, here it is 2.33>1.03), it is
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known that there is a possibility of multiple photon absorption. The strength of the nonlinear absorption is found from the numerical fitting to different transmission equations and the best fit is obtained for two photon absorption (2PA) along with the saturation absorption (SA). At 532 nm (2.33 eV), a reasonable mechanism is that the electrons excited from the lowest energy state are trapped in the intermediate state which enhances the absorption of one more photon resulting in 2PA. Z- scan experiment with relatively long (5 ns) laser pulses allow multiple absorption from the same laser pulse, which enhances the excited state absorption. In this particular case, the nonlinear absorption coefficient (α) is given by,
11
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(2 =
34
7
(8)
567 9 8
where αₒ is the unsaturated linear absorption coefficient, I is the laser intensity and Is is the transmitted intensity for a given input intensity is described by :;
:<
= =>
34
2? 2
7 78
56 9
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saturation intensity of the sample. The corresponding propagation equation to find out the
(9)
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By solving this equation, the obtained values of Is and 2PA coefficient (β) are 1 x 1012 W/m2 and 0.56 x 10-10 m/W respectively. Lakshmi Reddy et al. have reported a β of the same order for
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copper doped zinc aluminum ferrite for an energy of 70 µJ with 5 ns pulse duration [23]. Also β of the same order has been reported for nanostructures like Bi nanorods [24] and Te nanowires
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[25].
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Fig.8 Optical limiting behavior of BFO nanoparticles extracted from the Z-scan data (in the inset) for 150 µJ The optical limiting curve extracted from the Z- scan data (inset) is demonstrated in Fig.8. Here, the normalized transmittance is plotted against the input laser fluence given by,
%(@ = 4√ln 2 6
-EF
H 9 G !)
I(@
(10)
which has the maximum value at the focal point. Here, Ein is the input laser pulse energy and w(z) is the beam radius given by, 12
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I(@ = I(0 K1
+@!@M / N
!
(11)
with w(0) being the focal spot radius and z0 = (πw02)/λ, the Rayleigh range. At each sample
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position z, the sample discerns different laser fluence. The optical power limiting property of BFO with a limiting threshold (the fluence at which the transmission decreases to 50 % of the linear transmission) of 2.44 x 1013 W/m2 depicts a deviation from Beer’s law under strong irradiance and the output energy drops steadily. Linear and nonlinear optical properties of some
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perovskite nanoparticles are listed in Table 1 for comparison.
Table 1: Some representative optical limiting perovskite nanoparticles.
BaTiO3
2.65
BaTiO3
3.00
ErMn0.95Cr0.05O3
-
BiFeO3
2.05
Laser Parameters
β (×10-11 m/W)
Is (×1011 W/m2)
Limiting Threshold (J/cm2)
Ref.
7.18
48.9
2.33
[26]
0.17
10
-
[27]
0.9
2.9
0.19
[28]
5.6
10
2.44
Present
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Material
Band gap (eV)
532 nm, 5 ns, 70 µJ
800 nm, 2 ps, 70 µJ
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532 nm, 5 ns, 90 µJ
532 nm, 5 ns,
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150 µJ
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3.6 Dielectric Analysis
In order to evaluate the frequency dependent properties associated with grain boundaries
and intrinsic properties of the material, the dielectric properties have been studied as a function of frequency and temperature. The bulk ohmic resistance, which is the real part (Z’) of complex impedance is plotted against the imaginary part of the complex impedance (Z”) at different temperatures (Fig.9 (a)) to check the temperature dependence of the conductivity. It is known that the semicircle in the high frequency region is recounted to the grain interior resistance and the effect of blocking electrodes is described to the spike in the low frequency region. The real (ε’) and imaginary (ε”) parts of the dielectric constant are calculated using, 13
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O∗ =
QRST ∗
(12)
where j = √1,V = 2πν, ν is the frequency, C = ε0A/d, ε0 is the permittivity of free space, A is the
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effective area of the electrodes and d is the thickness of the pellet. A plot of ε’ vs log f at different temperatures specifies a low frequency dispersion indicating the dc conductivity for the sample. This type of relaxation is related to the domain wall relaxation in ABO3 systems. With the application of an electric field, the decrease in ε’ stems from the instantaneous nonoccurrence
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field leads to relaxation in ε’ (Fig.9 (b)) [29].
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of polarization as the charges possess inertia. The delay in response as a function of the given
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Fig.9 (a) Nyquist plot for 100 - 250̊ C range and (b) frequency dependent dielectric constant for BFO nanoparticles at various temperatures The frequency dependence of ε” at various temperatures and temperature dependence of ε’ at various frequencies for BFO nanoparticles are shown in Fig. 10 (a) and (b) respectively. At
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low frequencies and high temperatures, a significantly high ε” exhibited by the sample is interpreted as due to space charge polarization present in BFO. It is reported that the high
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conductivity and loss at high temperatures in bismuth ferrite based systems are initiated by the secondary phases segregated at the grain boundaries. These phases have a doping effect which transforms the dielectric into a semiconductor. In bulk ceramics, defects associated to the grain boundaries also have a contribution to high loss, resulting in a complex impedance behavior [29]. Temperature dependence of ε’ (Fig.10 (b)) shows a gradual increase with temperature. In the neighborhood of TN at 375̊ C, a dielectric anomaly indicating the ferromagnetic to paramagnetic transition is observed as predicted by the Landau – Devonshire theory of phase transition and it signifies the M-E coupling in the sample [30].
14
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Fig.10 (a) Frequency dependence of dielectric loss at various temperatures (b) Temperature dependence of dielectric constant at various frequencies for BFO nanoparticles Studies on the relaxation of dielectric modulus have been carried out from ambient to 500̊
W ∗ = WX
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C range in the complex modulus W∗ formalism. The complex modulus W ∗ is given by,
YW"
(13)
where WX and W" are the real and imaginary parts of electric modulus. Variations of WX and W" as a function of frequency and temperature are shown in Fig.S2. The figures clearly depict the maximum values at high frequencies and temperatures and it approaches nearly zero at low
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frequencies verifying the existence of a considerable electrode and/or ionic polarization in the said temperature range [31]. The non-exponential decay of the electric field is confirmed by the asymmetric peaks in the studied temperature range and it is inferred as a consequence of distribution of relaxation times. AC conductivity (σac) measurements (Fig.S3) have been carried given by,
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out in the frequency regime 100 Hz – 5 MHz to understand the conduction mechanism. σac is
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[\ = V] X ^_ `
M
(14)
On the application of the electric field, polarons are formed by the distortion/polarization
of the lattice by the motion of electrons. If such deformation is of the order of lattice constant, small polarons are formed and vice versa. If σac decreases with the increasing frequency, it is explained by the large polaron model and small polaron model accounts for the increase in σac with frequency [32]. In the present investigation, the conduction mechanism is largely due to the small polarons and a slight deviation is due to the mixed polaron conduction comes from the structural inhomogeneity owing to the discontinuous grain growth. 15
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The Arrhenius plot with ln σacT vs 1/T given in Fig. 11 clearly shows that σac increases linearly with temperature and is fitted to the Arrhenius equation, '-cd ef
9
(15)
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[\ = [M ab 6
where σ0 is the pre-exponent factor, Eac is the activation energy for ionic migration, KB is the Boltzmann constant and T is the temperature. From the linear fit, activation energy for ionic migration is found to be 0.581, 0.496, 0.373 and 0.242 eV respectively for 1 kHz, 10 kHz, 100
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kHz and 1 MHz, decreasing with increasing frequency. The frequency dependence of σac is given by AVn, where the coefficient A and the exponent n are dependent on temperature and intrinsic properties of the material. The obtained n value or the activation energy ensures the frequency
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dependent conduction due to small polarons [32].
Fig.11 Arrhenius plot for BFO nanoparticles at various frequencies
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3.7 M-H Loop Measurement
For bulk BFO, the magnetic hysteresis loop shows a typical antiferromagnetic (AFM)
behavior with zero coercivity. Magnetic ordering in BFO is quite ambiguous because of the Dzyaloshinskii- Moriya interaction, resulting in a canted AFM ordering provided by the transition metal cation Fe3+. This canted Fe
3+
moments induce a lattice strain which in turn
increases the free energy of the lattice. In order to decrease this free energy, a spiral spin structure is superimposed on to the AFM ordering resulting in the rotation of spin. In bulk, the order parameter of this helical ordering between two successive planes is found to be 62 nm [33 16
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(a, b)]. If the size of the particle is less than 62 nm, as in the present case, the helical order gets suppressed resulting in a weak FM in the AFM lattice as given in Fig.12. This coexistence FM and AFM orderings leads to the setting up of an exchange bias phenomenon that could give rise to a shift in unidirectional anisotropy as well as coercivity (+167 and -135 Oe) as can be viewed
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in the inset of Fig.12. Thus, the observed magnetization with Ms, Mr and Hc of 1.177, 0.188 emu/gm and 151 kOe respectively is explained as due to the high surface to volume ratio, due to the lower crystallite size (48 nm). FM behavior with a relatively high saturation magnetization of 3.5 emu/gm is reported for BFO nanoparticles annealed at 350̊ C and the enhancement is
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attributed to the presence of impurities like γ- Fe2O3 [34]. While size-dependent magnetic properties with Mr/Ms = 0.06 (here it is 0.193) for a particle size of 51 nm have been studied
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earlier for glycol assisted BFO [35].
Fig.12 M - H hysteresis curve of BFO nanoparticles at room temperature
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3.8 P-E Loop Measurements
In perovskite oxides, the ferroelectric (FE) order arises from several sources viz., empty d
shell, lone pair drives, charge ordering with geometrical or magnetic frustration and magnetically induced from broken spiral magnetic spin structure. In BFO, FE is induced by the lone pair of Asite Bi3+, a steriochemically active 6s2 lone pair, which causes Bi 6p orbital to come closer in energy to O 2p orbital. This leads to the hybridization between these orbitals which in turn drive the off-centering of the cation towards the anion resulting in FE [5]. Fig.13 exhibits the polarization vs electric field plot for BFO and the unsaturated loops with polarization of lower 17
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magnitudes indicate the low resistivity and large leakage current of the sample. Inset of the figure shows the enlarged pattern for applied fields of 1500 and 2000 V. The obtained polarizations and coercivity for a varying field from 1500 to 6500 V are Ps= 0.372 - 2.257, Pr = 0.081 – 1.129 µC/cm2 and Ec = 2.32 – 23 kV/cm. For an average crystallite size of 47 nm, Pr of
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0.17 µC/cm2 and Ec of 10 kV/cm are discussed by Hu et al. which increases to 0.82 µC/cm2 and 33 kV/cm by Gd dopant [36]. The enhancement is imputed to the structural distortion induced by the mismatch of Bi and Gd ionic radii. Also for spin coated thin films, 2Pr and 2Ec of 0.931
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µC/cm2 and 57.37 kV/cm respectively are conferred by Huang et al. [37].
Fig.13 P - E loops of BFO nanoparticles at room temperature
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3.9 Magneto-Electric Coupling
To study the M-E effect, the induced polarization with the applied magnetic field is measured with Hall probes. In the measurement setup, for ac measurements a parallel dc bias
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field of 2000 Oe at a fixed frequency of 850 Hz is applied using Helmholtz coils and for dc bias field, a 10 Oe ac magnetic field of the same frequency is applied using a set of permanent magnets. In single-phase materials, the M-E output arises due to the interaction between the ferroelectric and magnetic sub-lattices through stress or strain [38]. When a magnetic field is applied to a M-E material, the stress induced strain reorients the electric dipoles creating an ac voltage on the top and bottom surfaces of the material through M-E coupling. The M-E coefficient (α) is given by,
18
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=
g
hijcd
(16)
where V is the M-E voltage, t is the thickness of the sample and Hac is the applied ac magnetic field [39]. The dependence of M-E voltage with the applied ac and dc magnetic fields at room
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temperature is shown in Fig. 14. The M-E voltage shows a linear variation with the applied ac field and from the slope, the coupling coefficient (α) is calculated to be 0.011 Vcm-1Oe-1. Though the observed value of α is low, it is comparable to the reported values of rare earth (Nd,
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Sm, Gd, and Dy) doped bismuth ferric titanate [38].
Fig.14 Variation of M-E voltage with magnetic field for BFO nanoparticles at room temperature
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4. Conclusions
Single phase BFO nanoparticles have been synthesized by citric acid based sol-gel
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method. Crystal structure is confirmed from powder XRD with an average crystallite size of 48 nm. From the phonon transport studies, thermal conductivity and specific heat capacity are found to be 10.07 W/mK and 1358 J/kgK respectively. From the open aperture Z-scan measurements, BFO is found to be an efficient optical limiter for the protection of human eyes and optical sensors with a low limiting threshold. A decrease in both dielectric constant and loss with increasing frequency and a ferromagnetic to paramagnetic phase transition at 350̊ C in the vicinity of TN indicate the presence of M-E coupling. Also, frequency dependent conductivity caused by small polaron excitations is observed. An enhanced multiferroism resulting from the suppression of helical structure with Hc = 151 kOe and Ec = 23.01 KV/cm are identified. The 19
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linear response of ME voltage with a coupling coefficient of 0.011 V/cm-Oe indicates the coexistence of electric and magnetic phases. Overall, a nearly complete optical and magnetoelectric characterizations of BFO nanoparticles are reported in this work.
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Acknowledgement The authors acknowledge the help of Dr. Reji Philip (Raman Research Institute, Bangalore) for the Z-scan measurements. They also thank the Department of Science and Technology (DST-FIST), Government of India for experimental facilities.
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1972-1977.
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Research Highlights •
Single phase BiFeO3 nanoparticles by sol-gel method.
•
Reverse saturable absorption (RSA) based optical limiting behavior with low limiting
Verification of magnetic transition from dielectric analysis.
•
Room temperature multiferroic magneto-electric properties.
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•
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threshold.