Influence of La doping on the structural, magnetic, and dielectric properties of sol-gel derived BiFeO₃ thin films

Multiferroic perovskite oxides, particularly bismuth iron oxide (BiFeO₃), exhibit strong coupling between their ferroelectric and antiferromagnetic order parameters, making them highly attractive for applications in spintronics, non-volatile memory devices, and multifunctional sensors. However, the practical utilization of BiFeO₃ is constrained by its high leakage currents, weak magnetization, and spin cycloid distortions, necessitating compositional modifications to enhance its functional properties. In this study, lanthanum (La)-doped BiFeO₃ (Bi₁₋ₓLaₓFeO₃; x = 0.0–0.5) thin films were synthesized using the sol-gel technique, and the influence of La substitution on the structural, dielectric, and magnetic characteristics was systematically studied. X-ray diffraction (XRD) analysis confirmed the retention of a distorted perovskite structure with peak shifting towards higher diffraction angles, attributed to the subtle ionic radius difference between La 3 ⁺ (1.16 Å) and Bi 3 ⁺ (1.17 Å). However, a progressive increase in dislocation density and lattice strain was observed at x ≥ 0.4, indicating structural modifications induced by La incorporation. Dielectric spectroscopy revealed a significant enhancement in the dielectric constant, increasing from 36.8 to 287 at x = 0.3, measured at a logarithmic frequency of 5.0 Hz. The analysis of Cole-Cole plots, fitted using Z-View software, demonstrated a direct correlation between dopant concentration and grain boundary resistance, which was found to be a critical factor contributing to the observed dielectric enhancement. Additionally, impedance spectroscopy measurements over a wide temperature range indicated an activation energy varying between 1.82 eV and 2.38 eV, suggesting that La doping effectively suppresses oxygen vacancy formation, thereby reducing leakage currents. Increasing the work function from 5.08 to 5.12 eV is another factor that contributes to and supports the enhancement of the dielectric constant and the reduction of conductivity. Magnetic characterization revealed a substantial improvement in saturation magnetization (Mₛ) from 58.5 emu/cm 3 to 136.75 emu/cm 3 at x = 0.3, indicative of an increased suppression of the spin cycloid structure and enhancement in magneto-crystalline anisotropy. The interplay between La incorporation and magnetoelectric coupling was further analyzed to elucidate the correlation between structural distortion and enhanced multiferroic behavior. According to the findings, the inclusion of La into BiFeO 3 thin films has the ability to effectively alter the magnetic and dielectric properties of these films, which in turn makes it easier for these films to be utilized in advanced spintronic and multiferroic devices. A new way to design and develop spintronic and multifunctional devices based on BiFeO 3 films has been shown in this study to demonstrate a reliable path for building defect-controlled multiferroic thin films with better magnetic performance and higher dielectric stability.