Dielectric, Magnetic, Magnetodielectric, and Magnetoimpedance Properties of Aurivillius Based Composites

Abstract

The exciting electromagnetic phenomena are realized in the material when the charge and spin of the electrons are simultaneously coupled with each other. In this regard, the magnetodielectric (MD) coupling is one such mechanism, which can be achieved by systematically studying the dielectric constant under the external magnetic field. These MD materials have taken the significant attention of researchers due to the large coupling value, which could be used in magnetoelectric (ME) devices such as spintronic devices, tunable filters, ME sensors, and spin-charge transducers. Till now, various mechanisms have been developed to find out the MD coupling in the material and understand the origin of the MD effect; both the intrinsic and extrinsic have been proposed. The intrinsic MD coupling can yield a faster switching mechanism and low device loss than the extrinsic. The intrinsic MD coupling also provides the signature of the ME coupling in the material. Usually, the seldom existence of the magnetic and electric phases and the low operating temperature of the single-phase material provide the low value of the MD coupling and hinder its room temperature (RT) applications. Therefore, various types of composite fabrication may be an alternative avenue that yields large MD coupling and high operational temperature. In this regard, the Bi5Ti3FeO15 (BTFO) is a promising candidate with an orthorhombic crystal structure belonging to the Aurivillius family. BTFO material has attracted the attention of researchers due to its piezoelectric and ferroelectric (high curie temperature ~ 730 °C) characteristics. The bulk form of BTFO shows weak magnetic and MD coupling at RT. Hence, composite fabrication may be a potential path to enhance the magnetic and MD effect. For this purpose, the magnetic-rich La0.67Sr0.33MnO3 (LSMO) and Bi2Fe4O9 (BFO) compounds are selected for the composite fabrication. Finally, the present thesis systematically focuses on the magnetic, MD, and magnetoimpedance (MI) characteristics of the parent BTFO and its composite system.
For the comparison study, the polycrystalline BTFO sample is prepared via the solid-state (BTFOSS) and sol-gel (BTFO) process. Both the sample's dielectric, magnetic, and MD characteristics are studied systematically. The XRD and Raman analysis confirm the crystalline phase formation of the samples. The BTFOSS sample exhibits a high loss value (~ 25 times more) compared to the BTFO sample. The frequency-dependent dielectric constant reveals that the extrinsic source (space charge) is prominent in the BTFOSS sample with a temperature rise from 50 K to 300 K than in the BTFO sample. The existence of the MD coupling is also ascertained from the frequency and field-dependent MD effect. The maximum value of the ML% for the BTFOSS and BTFO samples is estimated to be ~ -4.71% and ~ -0.16% at 200 K, respectively. It reveals that a nearly 30 times loss reduction is observed in the BTFO sample than in the BTFOSS. However, the extrinsic sources (space charge polarization and resistive) are dominant in BTFOSS’s MD effect compared to the BTFO sample.
Thereafter, composites (1-x) BTFO-(x) LSMO (x = 0.1, 0.2, 0.3, and 0.4) are synthesized via sol-gel modified route, and phases are verified using XRD analysis. The SEM and TEM result conveys the distinct morphology of the two phases that demonstrated the plate-like grains and quasi-spherical particles of BTFO and LSMO phases. The magnetic studies reveal that the saturation magnetization (Ms ~7.0 emu/g) and coercive field (Hc ~1660 Oe) increased by nearly three and 66 times for 40% LSMO contained composite. The switching field distribution (SFD) plot indicates that strong exchange coupling exists in the x = 0.4 composite compared to others. The field-dependent MD studies reveal that maximum MD strength (~0.34%) and minimum magnetodielectric loss (ML) (~0.02%) are observed in x = 0.2 and 0.1 composites at 100 kHz, respectively. Additionally, the maximum MD coupling strength 𝛾 (-16.87 (emu/g)2) is observed in the x = 0.2 composite. The frequency-dependent magnetoresistance measurement determines the dominant capacitive MD effect in the composites.
The composites (1-x) Bi5Ti3FeO15-(x) Bi2Fe4O9 (x = 0.1, 0.2, 0.3, and 0.4) samples are prepared by the sol-gel-modified method, and the Rietveld refinement verifies the diphasic nature of the composites. The Raman spectra demonstrate the shifting of the vibrational modes due to adding the BFO phase, which is associated with the BO6 (B = Fe/Ti) octahedral. Magnetization data reveals the highest value of 𝑀𝑠 and 𝑀𝑟 is found for 20% composite, which is caused by the interaction among the AFM/FM magnetic ordering. Field variation of MD data shows ~30 times enhancement of MD% and ~7 times reduction of the ML% for x = 0.2 sample as compared to the pure sample. The field-dependent capacitive MI and Cole-Cole plot analysis reveals the capacitive origin behind the MD effect.
Finally, composite (1-x) BTFO-(x)[(0.2) LSMO-(0.3) BFO] (x = 0.5) is successfully prepared using the sol-gel modified technique. The XRD study confirmed the existence of triple (A21am, R-3c, and Pbam) phases in the composite sample. The SEM analysis has confirmed the presence of homogeneous and heterogeneous microstructures of pure and composite samples. Adding the magnetic LSMO and BFO phases enhance the overall magnetic properties of the composite sample. The magnetic parameters Ms and Hc in the composite are enhanced by nearly three times than the parent BTFO. It is due to the inherent d-d exchange interaction between the Fe-Fe, Fe-Mn, and Mn-Mn ions. The field-induced maximum strength of the MD effect is about ~ 0.12% at 50 kHz observed in the composite sample. This MD effect can be the combined effect of strain magnetostrictive LSMO and the inverse Dzyaloshinskii-Moriya interaction generated by the non-collinear Fe ions in the AFM BFO phase. The magnetic field-dependent Cole-Cole plot and field variation of MI plot analysis also reveals the resistive origin behind the MD effect in the composite.
Hence, the MD effect in the Aurivillius BTFO compound is explored, and the strength of the magnetic and MD effect is improved via the addition of magnetic-rich compounds. The above characteristics of the BTFO and its composite system might be applicable to multifunctional devices.