Studies of electrical conductivity and magnetic

JOURNAL OF ADVANCED DIELECTRICS Vol. 4, No. 2 (2014) 1450011 (6 pages) © The Authors DOI: 10.1142/S2010135X14500118

Studies of electrical conductivity and magnetic properties of Bi1x Gdx FeO3 multiferroics Samita Pattanayak*, R. N. P. Choudhary and Piyush R. Das Multifunctional Material Research Laboratory, Siksha `O' Anusandhan University, Bhubaneswar 751030, India *[email protected]

J. Adv. Dielect. 2014.04. Downloaded from www.worldscientific.com by 54.210.20.124 on 09/18/15. For personal use only.

Received 31 December 2013; Revised 7 April 2014; Accepted 14 April 2014; Published 21 May 2014 The polycrystalline samples of Bi1x Gdx FeO3 (x ¼ 0, 0.1, and 0.2) multiferroic oxides have been synthesized by a solid-state reaction/mixed oxide technique. The preliminary X-ray structural analysis with room temperature diffraction data confirmed the formation of single-phase systems. Study of room temperature scanning electron micrograph (SEM) of the surface of the above samples exhibits a uniform distribution of plate- and rod-shaped grains throughout the sample surface with less porosity. The dielectric behavior of the materials was studied in a wide range of frequency (1 kHz–1 MHz) and temperature (30–400 C). The nature of temperature dependence of dc conductivity confirms the Arrhenius behavior of the materials. The frequency–temperature dependence of ac conductivity suggests that the material obeys Jonscher's universal power law. An increase in Gd-content results in the enhancement of spontaneous magnetization BiFeO3 (BFO) due to the collapse of spin cycloid structure. The magnetoelectric coupling coefficient of BFO has been enhanced on Gd-substitution. Keywords: Multiferroics; dielectric properties; electrical conductivity; magnetic properties.

1. Introduction A single-phase multiferroic material is the one that possesses two or all three of the so-called \ferroic" properties: ferroelectricity, ferromagnetism and ferroelasticity.1,2 Magnetoelectric (ME) coupling, on the other hand, may exist whatever the nature of magnetic and electrical order parameters, and can, for example, occur in paramagnetic ferroelectrics. ME coupling may arise directly between the two order parameters, or indirectly via strain. We also consider here strainmediated indirect ME coupling in materials where the magnetic and electrical order parameters arise in separate but intimately connected phases. Multiferroic materials possess extensive potential applications high-sensitivity ac magnetic field sensors and electrically tunable microwave devices such as filters, oscillators and phase shifters.3,4 Out of all the multiferroic oxides known today, BiFeO3 (BFO) has drawn a lot of attention. This compound has a distorted perovskite (ABO3 Þ structure with the rhombohedral space group R3c.5 Antiferromagnetic (AFM) properties of BFO have first been studied in 1962,6 and BFO is precisely the most intensively studied material now. Bulk BFO intrinsically shows multiferroic properties with high ferroelectric transition temperature (1100 K) and weak AFM Neel temperature (643 K).7 BFO thus has become an extremely popular multiferroic material now because it is rare or even unique material having ferroelectric and AFM ordering at room temperature. Unfortunately, there are some drawbacks of BFO which include

high leakage current, low dielectric constant and high tangent loss. These inherent problems limited the applications of the material for electronic device applications.8 Therefore, several attempts have been made in the last decade to solve these problems by substitutions of suitable elements at the Bi/Fe-sites of BFO and/or fabricating its composites.9,10 Recently, enhanced dielectric and magnetic properties of Gd-modified BFO11,12 have been reported. As there is no report on detailed investigations of electrical properties of Gd-modified BFO, this report is an attempt in this direction. Therefore, the main objective of the present work is to study the preliminary structure, but detailed electric and magnetic properties of single-phase Bi1x Gdx FeO3 (x ¼ 0, 0.1, 0.2) to get better information on suitability of the materials for some applications.

2. Experimental Procedure The polycrystalline samples of Bi1x Gdx FeO3 (BGFO) (x ¼ 0, 0.1, 0.2) were synthesized by a high-temperature solid-state reaction technique using high-purity (AR grade) ingredients Bi2O3, Fe2O3 and Gd2O3 (M/s Loba Chemie Co, India). These oxides were first thoroughly mixed mechanically using agate mortar and pestle in dry (air) medium for 1 h, and then in wet (methanol) media for 2 h. The homogeneous mixtures of the compounds were calcined at the optimized temperatures: 800 C, 820 C and 830 C for x ¼ 0, 0.1

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and 0.2, respectively, for 4 h in highly pure alumina crucibles. The fine powder was then cold pressed into disc-shaped pellets (diameter ¼ 10 mm and thickness ¼ 1–2 mm) at the pressure of 4*106N/m2 using a hydraulic press. Polyvinyl alcohol (PVA) was used as a binder material to fabricate the pellets. The pellets were sintered at an optimized temperature of 830 C, 860 C and 870 C for x ¼ 0, 0.1 and 0.2, respectively for 4 h. An X-ray diffraction (XRD) patterns/diffraction data of calcined powders were obtained at room temperature using an X-ray powder diffractometer (Rigaku Miniflex). The CuK radiation ( ¼ 1:5405 Å) was used for collecting the above pattern/data in a wide range of Bragg's angles (20 2 80 C) at a scan rate of 3 /min. The microstructures of sintered pellets were recorded by using a scanning electron microscopy (SEM-JEOL-JSM, model 6510). For electrical measurements of the compounds, air-drying silver paint was applied on both the parallel faces of the pellets, and then dried at 150 C for 8 h. The electrical parameters of the materials were measured as a function of frequency (1 kHz–1 MHz) at different temperatures (25–400 C) using a phase sensitive LCR/impedance meter (PSM 1735, N4L). The I–V characteristics of BGFO was measured as a function of voltage (1–100 V) at an interval of 25 C starting from room temperature (25 C) up to 400 C using a programmable electrometer (Keithley, model 6517B). The room temperature magnetic moment versus magnetic field (M–H) hysteresis loop of the samples was studied using the Physical Property Measurement System (PPMS; Quantum Design). The samples were further magnetically poled employing an electromagnet with bias magnetic field of 0.5 T for 1 h before the measurement of their ME coupling coefficient. The ME effect was measured using ME setup of M/S Marine India Company.

Fig. 1. Comparison of the XRD patterns of Bi1x Gdx FeO3 at room temperature.

BGFO leads to a decrease of unit cell volume. Table 1 shows the comparison of lattice parameters ða; cÞ and volume of Bi1x Gdx FeO3 (x ¼ 0:0, 0.1, and 0.2). From XRD patterns, it is also clear that Gd-substitution provides weak phase purity but stabilizes the perovskite structure. The scanning electron micrograph (SEM) of BGFO pellets is shown in Fig. 2. It is observed that the grain size of BFO is significantly higher than that of Gd-containing samples. On increasing Gd content in BGFO, the size of the grains decreases. The grains being smaller, they aggregate to form bigger clusters as observed in the micrograph. This observation is very much consistent with structural analysis. The presence of voids of irregular dimensions indicates that the pellets have certain degree of porosity.

3. Results and Discussion

3.2. Electrical properties

3.1. Structural analysis

Figures 3(a) and 3(b) show the frequency dependence of relative permittivity ("r Þ and tangent loss (tan Þ for all the samples at 300 C. Here we observed that the value of permittivity decreases with rise in frequency, but beyond certain frequency, it remains constant. At low frequencies, all different types of polarizations such as electronic, ionic, dipolar and interfacial are present. The presence of dipolar polarization of low frequency region is very much important for

The XRD patterns of Bi1x Gdx FeO3 recorded at room temperature on powder samples are compared in Fig. 1. The nature of diffraction patterns confirms the formation of single-phase compounds. All the reflection peaks were indexed, and the lattice parameters were determined in various crystal systems and cell configurations using computer software \POWDMULT".13 Based on the best agreement between observed (obs) and calculated (cal) interplanar distance d (i.e., (dobs – dcal Þ ¼ minimum), a suitable unit cell (rhombohedral system) was selected. The effect of small value of Gdsubstitution (0–20%) on the crystal structure of BGFO was analyzed. All the major peaks of the pattern were possible to index in the rhombohedral phase (R3c). It is observed that on increasing Gd concentration of BGFO, the position of peaks shifts toward the higher angle side. This is due to substitution of smaller ionic size of Gd ion (0.938 Å) as compared to that of larger Bi ion (1.03 Å). Therefore, an increase in Gd in

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Table 1. Comparison of lattice parameters a; c and volume (Å) with estimated standard deviation (in parenthesis) of Bi1x Gdx FeO3 (x ¼ 0, 0.1, and 0.2) samples. X 0 0.1 0.2

a

c

a=c

Volume (Å) 3

5.5706 (7) 5.5750 (5) 5.5647 (2)

13.8114 (7) 13.7870 (5) 13.8319 (2)

2.4793 2.4729 2.4856

372.96 371.15 370.93



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Fig. 2. (a)–(c) SEM micrograph of Bi1x Gdx FeO3 (x ¼ 0, 0.1 and 0.25) ceramics.

applications in fabricating capacitive and insulating ceramics with dielectric constant and low tangent loss. This nature is useful for finding the contribution of polarization predominately present in the compounds in a particular frequency range. The decreasing tendency of dielectric constant and loss is found to be same for all compositions of BGFO. Though similar nature of variation has been found in tangent, the stronger frequency dependence of the tangent loss could be attributed to the conductive process.14 The main

contribution to the conduction process could be associated with the space charge effect present in the samples, and probably it arises from trapped charges of the ceramic/electrode interfaces.

Fig. 3. Frequency dependence of (a) relative permittivity ("r Þ and (b) tangent loss (tan ) of Bi1x Gdx FeO3.

Fig. 4. Variation of ac conductivity (ac Þ with frequency of Bi1x Gdx FeO3 at 300 C temperature.

3.3. Ac conductivity Figure 4 shows the variation of ac conductivity (ac Þ with frequency at 250 C temperature. The ac conductivity was

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calculated using an empirical dielectric relation: ac ¼ ! "r ð"0 Þ tan ð! ¼ angular frequency, ð"0 Þ ¼ vacuum permittivity). Study of ac conductivity is useful to understand the frequency dependence of electrical transport properties of the materials. The frequency dependence of ac conductivity is explained using Jonscher's power law,15


T ð!Þ ¼ ðoÞ þ ð!Þ ¼ o þ a! n : T ð!Þ is the total ac conductivity, ðoÞ is the dc conductivity term (i.e., independent of frequency), ð!Þ is the dispersive component of ac conductivity, n is frequency independent but temperature-dependent parameter. The value of n lies between zero and one. According to Jonscher's power law, relaxation is due to mobile charge carriers. When a mobile charge carriers jump from their original position to a new site, they will be in a state between two potential energy minima. If n < 1, the motion is translational and if n > 1 the motion is localized. At higher temperatures the conductivity curves show frequency-independent plateau in the low frequency region whereas at higher frequencies ac ! n . The value of ac is found to be increasing on increasing Gd content of BGFO. On increasing Gd content in BGFO, feature of high-frequency dispersion of conductivity is retained whereas lowfrequency conductivity dispersion shows a plateau-like trend. The increasing trend of ac on rising frequency in the hightemperature region for BGFO may be attributed to the disordering of cations between neighboring sites and presence of space charge polarization.16 At high frequencies, all the curves tend to merge into one. 3.4. Dc conductivity The dc conductivity (dc Þ was calculated using the empirical relation dc ¼ J=E (where J and E are the current density (A/ cm 2 Þ and electric field (V/cm) on the samples).17 Figure 5

Fig. 5. Variation of temperature dependence of dc conductivity of Bi1x Gdx FeO3.

shows variation of dc conductivity (calculated from slope of J–E plot) with inverse of absolute temperature (103/T) of Bi1x Gdx FeO3. The nature of the plots follows the Arrhenius law, ¼ o expðEa KB TÞ,18 where o ¼ pre-exponential factor, Ea is activation energy and KB ¼ Boltzmann constant. It shows that the value of dc increases on increasing temperature. It also confirms the NTCR behavior of the sample.19 The value of calculated activation energy (Ea Þ are 0.68, 0.7 and 0.74 eV for x ¼ 0, 0.1 and 0.2, respectively of Bi1x Gdx FeO3. The conductivity increases considerably with Gd substitution, which is due to the hopping of charge carriers through available oxygen vacancies. In Gd-substituted BFO samples, the high value of activation energy at evaluated temperatures shows a possible diffusion of oxygen ion vacancy through grain boundary.20 3.5. Magnetic properties The room temperature (M–H) hysteresis loops of Bi1x Gdx FeO3 samples are described in Fig. 6. The M–H loops exhibit the linear behavior of AFM in nature. The calculation of other magnetic parameter (remnant magnetization) shows that BFO has nonzero remnant magnetization (Mr) of the value of 4:72 10 4 emu/g at a coercive magnetic field (Hc Þ of 0.424 kOe. This small value is because of the AFM nature of BFO. However, the Mr increases on increasing Gd concentration in BFO. The enhancement in the above magnetization on increasing Gd concentration are due to the radius of Gd 3þ ion being smaller than Bi 3þ ion resulting in larger distortion in lattice structure. It is because BFO systems have the spiral spin structure which can be destroyed by Gd 3þ substitution.21 The increase in magnetization and/or AFM nature of these samples can be attributed to the breakdown of the balance between the antiparallel sublattice magnetization of Fe 3þ . The collapse of space modulated spin structure on Gd substitution leads to long range canted AFM order.22 The true sign of multiferroic property of a material is the presence of piezo- and ME coupling effects. The ME coupling coefficient can be obtained by using an empirical equation: 31 ¼ 3 /"0 "r ðd33 /H1 Þ,23,24 where "0 and "r are the permittivities of the vacuum and relative dielectric constant, respectively, 3 is the mechanical stress, d33 is piezoelectric coupling coefficient and H is applied magnetic field. The magnitude of ME coupling coefficient can be obtained using the ME voltage (V3 Þ of open circuit as a function of the applied magnetic field. The value of V3 can be calculated using a relation; V3 ¼ H1 31 te where te is the thickness of the sample. Figure 7(a) shows the variation of induced voltage V3 on the application of applied ac magnetic field (Hac Þ at a 1000 Oe dc bias magnetic fields in the Bi1x Gdx FeO3 samples. This plot shows that on increasing Gd concentration in BGFO leads to the enhancement of induced voltage in the samples. Figure 7(b) shows the values of ME coefficient under variable

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Fig. 6. Variation of magnetization (M ) with magnetic field (H ) of Bi1x Gdx FeO3.

(a)

(b)

Fig. 7. (a) Variation of induced voltage (V3 Þ with applied ac magnetic field (Hac Þ of Bi1x Gdx FeO3. (b) Variation of ME coefficients (Þ under variable dc magnetic field (Hdc Þ of Bi1x Gdx FeO3.

dc bias magnetic field with an additional 50 Hz ac magnetic field. It is observed that the value of ME decreases on increasing dc magnetic field. It is also observed that on increasing Gd content of BGFO, the values of increase. A higher value of ME coupling coefficient is observed for the sample with 20% Gd content. Thus ME effect of the sample strongly depends on the magnetic bias field and material.

4. Conclusion The polycrystalline samples of Bi1x Gdx FeO3 were prepared by a standard mixed oxide method. Preliminary structural studies suggest the existence of rhombohedral phase symmetry in Bi1x Gdx FeO3 (x ¼ 0, 0.1, and 0.2). The dielectric constant is found to be increasing, and the tangent loss

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decreases on increasing Gd content of BGFO at room temperature. The frequency dependence of ac conductivity obeys the Jonscher's universal power law for all compositions. Detailed study of electrical conductivity of the materials exhibits its NTCR behavior. The value of ME voltage and coupling coefficient of BFO is enhanced by substituting Gd at its Bi site.

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