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STRUCTURAL AND DIELECTRIC PROPERTIES OF LA DOPED BIFEO3 THIN FILMS.

Byline: SMH Shah, Saira Riaz, Shahid Atiq, Asif Mahmood, Shahid M Ramay and Shahzad Naseem

ABSTRACT

Multiferroic materials like BiFeO3, YMnO3, BiMnO3, TbMnO3 have attracted worldwide attraction due their applications in data storage devices, spintronic devices, sensors and multiple stage memories. Among these materials BiFeO3 is a promising candidate as it exhibits room temperature antiferromagnetic and ferroelectric properties. However, BiFeO3 suffers from some drawbacks including weak magnetic behavior, inhomogeneity in spin structure and large leakage current. In order to overcome these problems we here report La doped Bi1-xLaxFeO3 (where, x=0.0-0.5) thin films prepared by sol-gel method. The effect of La substitution on structural and dielectric properties has been investigated. The films show pure phase rhombohedrally distorted perovskite structure of BiFeO3. XRD peak shifts to higher angles due to slight difference in ionic radii of La3+ (1.16A ) and Bi3+ (1.17A ).

The dielectric constant and tangent loss decrease as frequency increases and become constant at high frequencies showing normal dispersion behavior for all concentrations (i.e. x=0.0-0.5).

1. INTRODUCTION

Multiferroic oxides are materials that combine the properties of both ferroelectric and ferromagnetic materials in single phase. This opens new applications of multiferroic materials in data storage devices, magnetic field sensors etc. [1, 2].

Bismuth iron oxide (BiFeO3), exhibiting room temperature multiferroic properties, has drawn considerable research attention owing to its potential applications in spintronic devices. In BiFeO3 both electric and magnetic orders exist simultaneously with ordering temperatures of 643K and 1103K respectively. The coupling between magnetic and electrical properties allows synchronized control of magnetism and electric polarization [3, 4]. Bismuth iron oxide crystallizes in rhombohedrally distorted perovskite structure. It exhibits G-type antiferromagnetic behavior. The spin periodicity of BiFeO3 between two successive planes is reported to be 620A . Ferromagnetic interaction is cancelled out in BiFeO3 due to the spiral spin structure. As a result of which reduction in magnetoelectric coupling arises [5]. According to Bi2O3-Fe2O3 phase diagram, bismuth iron oxide (BiFeO3) is a linear compound [6]. Deviation from stoichiometry leads to formation of bismuth rich and/or bismuth deficient phases.

The presence of secondary phases results in high leakage current and low magnetic moment. High leakage current and low magnetic behavior are the two main limiting factors for extensive use of BiFeO3 [7, 8].

In order to overcome these difficulties, various dopants have been reported in the literature including Ca, Mn, Ni, Zr etc. Rare earth dopants are substituted for Bi site. This type of substitution leads to decrease in oxygen vacancies and thus reduce the leakage current. Doping of rare earth elements also leads to enhanced ferroelectric properties. This effect is achieved by presence of strain caused by dopant atoms.

On the other hand, transition metal atoms are substituted for Fe site. This suppresses the valence fluctuation of iron atom (3+ and 2+) and reduces leakage current. Among the various dopants lanthanum substitution is the most important as its ionic radius matches well with bismuth [7, 9]. We here report preparation, structural and dielectric properties of lanthanum doped bismuth iron oxide thin films using sol-gel method. The changes in dielectric and structural properties are correlated with variation in dopant concentration.

2. EXPERIMENTAL DETAILS

Lanthanum doped bismuth iron oxide thin films were prepared using sol-gel method. Bi(NO3)3.5H2O and Fe(NO3)3.9H2O were used as precursors. Bi(NO3)3.5H2O and Fe(NO3)3.9H2O were dissolved in ethylene glycol and stirred at room temperature for 60mins. The two solutions were mixed together and heated on hot plate at 60C to obtain bismuth iron oxide sol. The details of sol-gel synthesis were reported earlier [10, 11]. For doping purposes, La(NO3)3.6H2O was dissolved in ethylene glycol and was added to BiFeO3 sol to obtain Bi1-xLaxFeO3. The dopant concentration was varied as x=0.0-0.5. The sols were spin coated on copper substrate and annealed at 300C for 60mins. Prior to spin coating, copper substrates were etched using diluted HCl and then rinsed repeatedly using DI water. The substrates were placed in ultrasonic bath in acetone and isopropyl alcohol for 10mins and 15mins respectively [12, 13].

The films were characterized structurally using Bruker D8 Advance X-ray diffractometer (XRD) with =1.5406A . Dielectric properties were carried out using 6500B Precision Impedance Analyzer.

3. RESULTS AND DISCUSSION

Fig. 1 shows XRD patterns for Bi1-xLaxFeO3 (x=0.1, 0.3, 0.5) thin films prepared using sol-gel method and annealed at 300C. The presence of diffraction peaks corresponding to planes (024), (122) and (128) indicate the formation of phase pure bismuth iron oxide. No peaks corresponding to bismuth rich and/or bismuth deficient phases were observed. In addition, no diffraction peaks corresponding to lanthanum and/or lanthanum oxide were present. The peak positions corresponding to (024) and (122) planes slightly shift to high diffraction angles. Crystallite size (t), strain and dislocation density (d) [14] of lanthanum doped bismuth iron oxide thin films was calculated using Eqs. (1)-(3).

Equations

For studying the dielectric properties of BiFeO3 thin films, impedance analyzer was used in parallel plate configuration. The parallel capacitance and parallel resistance were measured and then using Eq. 4 and Eq. 5 the dielectric constant e and dielectric loss (tangent loss tan d) were calculated.

Equations

Where, is the wavelength (1.5406A ), B is the Full Width at Half Maximum (FWHM), d is the change in d-spacing (d) with respect to the standard data. The crystallite size, dislocation density and strain are plotted as a function of dopant concentration in Fig. 2. Crystallite size increases from 29 nm to 42 nm as the dopant concentration is increased from 0.1 to 0.3. Further increase in the dopant concentration to 0.5 resulted in decrease in crystallite size to 30nm. Crystallite size depends on 1) accumulated strain energy. With increase in crystallite size to 42 nm reductions in strain was observed as can be seen in Fig. 2(b). Increase in crystallite size to 42nm also resulted in decrease in dislocation density to 5.69A-1014 lines/m2 due to reduced number of grain boundaries. 2) Neighboring grains due to curvature of energetic grain boundaries [15].

Where, C is the capacitance, d is the thickness of the specimen, A the area of the device, eo is permittivity of free space and is the resistivity of the thin films. Dielectric constant and tangent loss decrease as the frequency of externally applied field increases. At low frequencies, the space charge carriers follow the orientation of applied field. As frequency of the field increases, space charge carriers do not get enough time to get aligned in the direction of externally applied field thus make no contribution to polarization. As the result of which, dielectric constant becomes steady at high fields.

Dielectric constant increases from 26 to 44 (at f=500kHz) with increase in dopant concentration from 0.1 to 0.3. Further increase in dopant concentration to 0.5 resulted in decrease in dielectric constant to 34. The increase in dielectric constant till dopant concentration of 0.3 is due to reduced defects in thin films as was observed in Fig. 2.

4. CONCLUSIONS

Lanthanum doped bismuth iron oxide thin films were prepared using sol-gel method. Dopant concentration was varied as 0.1, 0.3 and 0.5. XRD results indicated successful incorporation of dopant in the host lattice. Crystallite size increased from 29 nm to 42 nm as dopant concentration was increased to x=0.3 and then decreased with further increase in dopant concentration. Dielectric constant and tangent loss showed normal dispersion behavior. Dielectric constant increased as the dopant concentration was increased to x=0.3 due to reduction in crystal defects.

ACKNOWLEDGEMENT

The authors extend their sincere appreciations to the Deanship of Scientific Research at King Saud University for funding of this Profile Research group (RGP-1436-26).

REFERENCES

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Date:Aug 31, 2015
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