Frequency Dependent Relaxation Process and Magnetization in Bi 0.8 K0.2 FeO 3 Nanopowders

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1 Frequency Dependent Relaxation Process and Magnetization in Bi 0.8 K0.2 FeO 3 Nanopowders M Nadeem 1), S Sajjad Hussain 2), *Saira Riaz 2) Shahid Atiq 2), Farzana Majid 2) and Shahzad Naseem 2) 1), 2) Centre of Excellence in Solid State Physics, University of the Punjab, Lahore 54590, Pakistan 1) saira_cssp@yahoo.com ABSTRACT BiFeO 3 is one of the multiferroic materials which exhibit both electrical and magnetic ordering above the room temperature. Hence, it is obvious that bismuth iron oxide can be a potential candidate for the applications of transducers, spintronics and photovoltaics. Hence, it becomes very much essential to reduce the leakage current by increased dielectric constant and to improve the magnetic properties. Potassium doped bismuth iron oxide (Bi 1-x K x FeO 3 ) nanopowders were prepared using sol-gel method. Dopant concentration (x) was fixed at 0.2 (Bi 0.8 K 0.2 FeO 3 ) and these nanopowders were calcined at 400 ºC to 700 ºC with interval of 100 ºC. XRD results confirm formation of BiFeO 3 phase at calcination temperature 400 ºC and 500 ºC. Mixed BiFeO 3 and Bi 2 Fe 4 O 9 phase has been obtained at calcination temperature 600 ºC. Bi 2 Fe 4 O 9 phase has been found at temperature 700 ºC. Nanopowders calcined at ºC exhibited strong ferromagnetic behavior. Whereas, highest saturation magnetization of emu was observed for nanopowders calcined at 500 ºC. Maximum value of dielectric constant (95.6; log f = 5.0) has been observed at 500 ºC. Lowest conductivity was obtained for nanopowders at 500 ºC indicating decrease in leakage current for Bi 1- xk x FeO 3 nanopowders. Real and imaginary impedance results show presence of single relaxation process in Bi 1-x K x FeO 3 nanopowders. 1. INTRODUCTION Considerable attention has been focused on multiferroic materials that exhibit more than one ferroic (ferromagnetic, ferroelectric, ferroelastic) at the same time. Presence of more than one ferroic order in material can lead to coupling effect between magnetic and electric properties and results in magnetoelectric effect (Zhai and Wang 2017; Belik 2017; Couture et al.2017). Presence of magnetoelectric effect in the material leads to its applications in Magneto-Electric random-access memories, inductors with electrostatic tunability, zero-power magnetic sensors etc. (Couture et al.2017). However, origin of ferroelectricity and ferromagnetism are different. For ferroelectricity empty d-shell is an important requirement while, for ferromagnetism partially filled d-shell is required (Golic et al. 2016). 1) Graduate Student 2) Professor

2 Among various multiferroic materials, bismuth iron oxide (BiFeO 3 ) is a promising singlephase material that displays both magnetic and ferroelectric order simultaneously at room temperature. BiFeO 3 crystallizes in perovskite structure with rhombohedral destruction in the unit cell. It belongs to R3c space group with γ = 120 and α = 90 and lattice parameters a = Å, c = Å. BiFeO 3 exhibits spontaneous polarization, below its Curie temperature, in the direction of one of the eight pseudocubic axes (Golic et al. 2016). Transition from rhombohedrally distorted structure to orthorhombic structure is observed at Curie temperature (Tc=1103K) (Golic et al. 2016; Shah et al.2014a; Majid et al. 2015; Riaz et al. 2014a; Shah et al. 2014b). In crystal structure of BiFeO 3 each spin up Fe 3+ cation is surrounded by six nearest spin-down Fe 3+ cations. This results in G-type antiferromagnetic behavior of BiFeO 3. BiFeO 3 also exhibits weak ferromagnetic behavior with cycloidal spin structure and modulation length of 620Å (Sarkar et al. 2016; Quan et al. 2016; Xing et al. 2017; Chybczynska et al. 2017; Riaz et al. 2015; Riaz et al. 2014b). But there are some inherent drawbacks associated with BiFeO 3 that includes: 1) Presence of secondary phases due to volatile nature of bismuth oxide; 2) High leakage current due to presence of oxygen vacancies and electron hopping on iron (Fe) site; 3) Low magneto-electric coupling; 4) Antiferromagnetic/weak ferromagnetic behavior (Wavy et al.2017). For overcoming these difficulties, various Bi-site and Fe-site dopants has been studied including Ba 2+ (Ting et al. 2017), Ca 2+ (Riaz et al. 2014a; Karpinsky et al. 2017), Mn 2+ (Zhang et 2017), Ni 3+ (Khajoririt et al. 2017), Mn 2+ (Riaz et al. 2015; Karpinsky et al. 2017) etc. Among various dopants, potassium (K + ) can be a potential candidate as it can alter the Bi-site cation displacement. It can thus lead to changes in ferroelectric ordering that is related to displacement of Bi 3+ cations. It can also leads to changes in charge at Fe-site. This will result in modifications in magnetic properties as well (Dhahri et al. 2008). Synthesis of potassium doped BiFeO 3 (Bi 1-x K x FeO 3 ) nanopowders with dopant concentration x = 0.2 is reported in this research work. These nanopowders were calcined at 400 C to 700 C. Effect of calcination temperature on different properties has been studied. 2. EXPERIMENTAL DETAILS For synthesis of potassium doped bismuth iron oxide (Bi 0.8 K 0.2 FeO 3 ) nanopowders sol-gel technique has been employed. Nitrates of bismuth & iron along with potassium hydroxidewere used as precursors. Individual precursor was dissolved in organic solvent i.e. ethylene glycol. These individual solutions were mixed together and heat treated at 80 C. Whereas doped sol was heated at 100 C to obtain Bi 0.8 K 0.2 FeO 3 powders. These powders were pressed to form pallets. Pressed pallets were calcined at C. Structural analyses were obtained with help of X-ray Diffractometer (Bruker D8- Advance). Dielectric properties of Bi 0.8 K 0.2 FeO 3 powders were studied by impedance

3 analyzer (6500B).M-H curves were obtained using Vibrating Sample Magnetometer (Lakeshore model 7407). 3. RESULTS AND DISCUSSIONS Figure 1 shows XRD patterns for Bi 0.8 K 0.2 FeO 3 nanopowders calcined at C. Presence of diffraction peaks corresponding to planes (012), (104), (110), (006), (202), (211) and (116) indicated the formation of perovskite structure of BiFeO 3 (JCPDS card no ). Perovskite structure of BiFeO 3 belong to R3c space group for powders calcined at 400 C and 500 C (Fig. 1(a,b)). Peaks intensities increase as calcination temperature was increased from 400 C to 500 C. This indicates grain growth with increase in crystallinity of nanopowders. Peaks corresponding to planes (201) and (231) (marked by ^ in Fig. 1(c)) were observed for nanopowders calcined at 600 C thus, indicating inclusion of Bi 2 Fe 4 O 9 phase in BiFeO 3. For nanopowders calcined at 700 C (Fig. 1(d)) no peaks corresponding to BiFeO 3 phase was observed. Appearance of Bi 2 Fe 4 O 9 phase at calcination temperature of 600 C and elimination of BiFeO 3 phase at 700 C is attributed to bismuth loss at high temperatures due to volatile nature of Bi 2 O 3 (Ibrahim et al. 2017). Fig. 1. XRD patterns for Bi 0.8 K 0.2 FeO 3 nanopowders calcined at (a) 400 C (b) 500 C (c) 600 C (d) 700 C (*BiFeO 3 JCPDS card no ; ^Bi 2 Fe 4 O 9 JCPDS card no ). Crystallite size (D) (Cullity 1978) and dislocation density (δ) were calculated with Equations D Bcos 1 t (1) (2) Fig. 2 shows the variation in crystallite size and dislocation density with calcination temperature. Fig. 2 (a) depicts the increasing behavior in crystallite size up to 500 ºC. In

4 case of nanopowders/nanoparticles dangling bonds are present at the surface. These dangling bonds are related to oxygen, bismuth or iron defects present at the grain boundaries. These defects favor the merging of smaller grains to form larger grains upon calcination with decrease in number of defects (Kuo 2006) This results in decrease in dislocation density (Fig. 2(b)) at calcination temperature 500 C. At calcination temperature of 600 C and 700 C a corresponding decrease in crystallite size to 10.4nm and 10nm was observed. Relatively smaller value of crystallite size at 600 ºC and 700 ºC is due to presence of mixed phases and absence of BiFeO 3. Fig. 2. (a) Crystallite size (b) Dislocation density for Bi 0.8 K 0.2 FeO 3 nanopowders as a function of calcination temperature. Powder Cell Software was used to refine lattice parameters. Refined parameters are listed in table 1. Unit cell volume increases from 369.8Å 3to Å 3 with increase in calcination temperature from 400 C to 500 C. Increase in unit cell volume indicates strengthening of BiFeO 3 phase as was observed in Fig. 1(b). Relatively lower value of unit cell volume at 600 C is because of mixed phases observed in XRD patterns. Again increase in unit cell volume at 700 C arises due to complete elimination of perovskite BiFeO 3 phase and transition to orthorhombic Bi 2 Fe 4 O 9 phase. Table 1. Lattice parameter and unit cell volume for Bi 0.8 K 0.2 FeO 3 nanopowders calcined at C. Calcination temperature Lattice parameters Unit cell volume ( C) (Å 3 ) 400 a =5.565Å ; c=13.79 Å a =5.569Å ; c=13.81 Å a =5.560Å ; c=13.73å a = 7.85Å ; b = 8.40Å ; c=5.85å Magnetic properties of Bi 0.8 K 0.2 FeO 3 nanopowders were studied at room temperature. It can be seen in Fig. 3(a) that Bi 0.8 K 0.2 FeO 3 nanopowders calcined at 400 C-600 C exhibit strong ferromagnetic behavior despite the antiferromagnetic nature of bulk BiFeO 3. Basically in an antiferromagnetic material two spins sublattice is present.

5 Antiferromagnetic interaction arises between the spin of different sublattices. According to Neel, the ferromagnetic behavior of small antiferromagnetic particles arises due to uncompensated magnetic moments between the two sublattices. These uncompensated spins in antiferromagnetic material become considerable when the particle size is reduced. In small antiferromagnetic particles, long range order is interrupted at the surface of particles. As particle size decreases, surface to volume ratio increases. This results in large uncompensated spins at the particle surface. This, thus, leads to ferromagnetic behavior in otherwise antiferromagnetic systems (Riaz et. 2015; Riaz et al. 2014b; Park 2007). In addition, substitution of K + cations with Bi 3+ requires charge compensation mechanisms. Replacement of K + with Bi 3+ cations can disturb the long range ferroelectric order. This leads to reduced positive charge on Bi site. This reduced positive charge needs to be compensated. There are two charge compensation mechanisms that can take place in K + doped BiFeO 3 ; 1) Charge compensation takes place by creation of oxygen deficiency; 2) Another way the charge can be compensated is by increasing the positive charge in BiFeO 3. This effect can arise from change in valence state of iron cation i.e. Fe 3+ changes to Fe 2+ cations. Another way to increase the charge is by conversion of Bi 3+ cations to Bi 5+ cations (Dhahri et al. 2008). Among these charge compensation mechanisms, mixed valence state of iron cation i.e. Fe 3+ and Fe 4+ play a critical role in double exchange interaction and results in ferromagnetic behavior in BiFeO 3 as pointed out by Dhahri et al. (2008). Ferromagnetic behavior of nanopowders in Fig. 3(a) suggests that charge compensation in replacement of Bi 3+ cations with K + cations occurs through mix valence state of iron cation. It can be seen in Fig. 3(b) that saturation magnetization of Bi 0.8 K 0.2 FeO 3 nanopowders increases from emu to emu as calcination temperature was increased from 400 C to 500 C. Increase in magnetization arises due to strengthening of BiFeO 3 phase as observed in Fig. 1(a, b). Further increase in temperature causes decrease in saturation magnetization. Again this decrement is correlated with presence of mixed phases as observed in XRD. Transition in magnetic behavior was observed at 700 C due to phase transition from mix BiFeO 3 and Bi 2 Fe 4 O 9 phases to Bi 2 Fe 4 O 9. Fig. 3. (a) M-H curves for Bi 0.8 K 0.2 FeO 3 nanopowders calcined at C; (b) Saturation magnetization for Bi 0.8 K 0.2 FeO 3 nanopowders as a function of calcination temperature.

6 Dielectric constant (ε) and Tangent loss (tanδ) were calculated using Equations 3-4 (Barsoukov and Macdonald 2018); Cd A o 1 tan 2 f o Where, d= thickness of specimen. A =Area. ε o = Permittivity of free space, ρ = Resistivity of Bi 0.8 K 0.2 FeO 3 nanopowders It can be seen in Fig. 4(a) that dielectric constant is frequency independent in low frequency region (1.3 < log f< 6.5) and becomes frequency dependent at high frequencies (log f> 6.5) as opposed to normal dispersion behavior of dielectric materials. Such anomaly in dielectric constant arises because of the presence of lattice defects, vacancies and defects associated with grain boundaries in polycrystalline dielectric material. Space charge carriers get trapped within these defects. At high frequencies, these space charge carriers are released and thus, contribute to polarization. Thus, increased dielectric constant has been observed at high frequencies. Such anomaly in dielectric constant can also arise because of resonance effect. Resonance effect arises when jumping frequency of electrons become equal to frequency of externally applied electric field (Qian et al. 2009). Tangent loss (Fig. 4(b)), on the other hand, decreases in low frequency region and becomes almost constant at high frequencies. This behavior in tangent loss can be understood on the basis of Maxwell-Wagner Model. This model considers polycrystalline material to be composed of two layers i.e. grains and grain boundaries. At low frequencies, due to high resistance of grain boundaries high energy is required by electrons to jump from one potential well to another. This results in high tangent loss at low frequencies. As frequency increases, the energy required by electrons is less. This results in low losses at high frequencies (Jamal et al. 2011; Sharif et al. 2016) It can be seen in Fig. 4(c) that dielectric constant increases from 87.2 to 95.6 (log f = 5.0) as calcination temperature was increased from 400 C to 500 C. With increase in calcination temperature to 600 C and 700 C decrease in dielectric constant to 75 and 69 (log f = 5.0). This increase in dielectric constant is associated with increase in crystallite size that was observed in Fig. 2(a). In case of nanomaterials due to large surface to volume ratio surface bond contraction arises. This results in enhanced crystal field. Increase in crystallite size results in decrease in crystal field thus, increasing the dielectric constant (Ye et al. 2000). Another factor that strongly affects the dielectric constant is presence of oxygen vacancies. As it has been discussed earlier in magnetic properties that because of the substitution of K + ions with Bi 3+ cations mixed valence iron cations are created. Presence of oxygen vacancies would pin the domain walls thus decreasing the domain main wall mobility and results in low value of dielectric constant (Song et al. 2014) However, increase in dielectric constant and decrease in (3) (4)

7 tangent loss at 500 C (Fig. 4(c)) indicate that oxygen vacancies are suppressed in these nanopowders. This results in high dielectric constant and low tangent loss in Bi 0.8 K 0.2 FeO 3 nanopowders at 500 C. Decrease in dielectric constant at 600 C and 700 C is due to presence of Bi 2 Fe 4 O 9 phase as was observed in Fig. 1(c, d). Fig. 4. (a) Dielectric constant (b) Tangent loss for Bi 0.8 K 0.2 FeO 3 nanopowders calcined at C; (c) Dielectric constant and tangent loss for Bi 0.8 K 0.2 FeO 3 nanopowders as a function of calcination temperature. Conductivity (σ) of Bi 0.8 K 0.2 FeO 3 nanopowders was calculated using Eq. (5) (Barsoukov and Macdonald 2018); 2 o f tan (5) Conductivity of Bi 0.8 K 0.2 FeO 3 nanopowders is plotted in Fig. 5. Two regions can be observed in Fig. 5. One is the low frequency region (log f< 6.0) in which conductivity almost remains constant with frequency. This type of conductivity is referred to as d.c. conductivity. D.c. conductivity in dielectric material arises due to free charge carriers. In the second frequency region (log f> 6.0), which is frequency dependent is called a.c. conductivity. a.c. conductivity arises due to presence of bound charges in dielectric material. It can be seen in inset Fig. 5 that lowest conductivity was observed for nanopowders calcined at 500 C. This reduced conductivity is indicative of low leakage current in Bi 0.8 K 0.2 FeO 3 nanopowders. This, thus, supports decrease in tangent loss for nanopowders calcined at 500 C.

8 Fig. 5. Conductivity for Bi 0.8K 0.2FeO 3nanopowders calcined at C. Table 2. Relaxation time and relaxation frequency for Bi 0.8 K 0.2 FeO 3 nanopowders. Calcination Relaxation frequency Relaxation temperature ( C) (Hz) time (μs) CONCLUSIONS Potassium doped bismuth iron oxide (Bi 0.8 K 0.2 FeO 3 ) nanopowders were synthesized using sol-gel method. These nanopowders were calcined at 400 C-700 C (interval 100 C). XRD patterns confirm formation of BiFeO 3 phase at calcination temperature 400 C with increased crystallinity of nanopowders observed at 500 C. Mix BiFeO 3 and bismuth deficient Bi 2 Fe 4 O 9 phases were observed at calcination temperature 600 C. Only Bi 2 Fe 4 O 9 phase was observed for nanopowders calcined at 700 C. Nanopowders calcined at 400 C, 500 C and 600 C exhibit strong ferromagnetic behavior. Highest dielectric constant of 95.6 (log f = 5.0) was observed for nanopowders calcined at 500 C. Bi 0.8 K 0.2 FeO 3 exhibit contribution both from d.c. and a.c. conductivity with lowest conductivity obtained for nanopowders calcined at 500 C. REFERENCES

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