Magnetic, Magnetodielectric, and Magnetoimpedance Correlation in Modified Brownmillerite KBiFe2O5

Abstract

The materials with multiple responses to external stimuli facilitate use in novel device applications. In this regard, tuning dielectric or capacitance with a magnetic field is challenging and scientifically important for technological applications. The phenomenon of coupling between dielectric and magnetic properties in materials is known as the magnetodielectric (MD) effect. It is challenging to get above RT ferroic orderings and MD coupling in single-phase materials. KBiFe2O5 (KBFO) is a new, less explored, and recently discovered novel material belonging to the brownmillerite crystal (A2B2O5) family. The magnetoelectric, magnetodielectric, and magnetoimpedance analysis is entirely new and unexplored for KBFO. The dielectric, magnetic, magnetodielectric, and magnetoimpedance (MI) properties of parent KBFO and chemical doped samples are investigated. The present thesis is carried out systematically by studying the parent KBFO, the A-site doping with rare earth Holmium (Ho), the B-site doping of transition element Cobalt (Co), and the co-doping (Co and Ho) of rare earth and transition ion in both the sites of KBFO, respectively. The solid-state reaction route is used to synthesize KBiFe2O5, Co-doped KBiFe2(1-x)Co2xO5 (B-site), Ho-doped KBi(1-y)HoyFe2O5 (A-site), and KBi(1-y)HoyFe2(1-x)Co2xO5 (co-dope) samples. The Rietveld refinement of powder X-ray diffraction (XRD) patterns of prepared samples are carried out, and the monoclinic phase with the P2/c space group is confirmed for all the samples. The absence of an impurity phase in XRD analysis confirms the phase purity of the prepared samples. Additionally, the RT Raman analysis further confirms the pure phase and the preferential site of dopants in KBFO. A detailed dielectric, magnetic, MD, and MI analysis with their correlation is established in parent KBFO over a wide temperature (10 K to 780 K) and magnetic field [-1.3 T (-13 kOe) to 1.3 (13 kOe) T]. The temperature-dependent magnetization (> 300 K) of KBFO shows an anomaly near dielectric transition (780 K), indicating the signature of MD coupling. The low-temperature magnetization (< 300 K) shows its maximum remnant magnetization MR of 0.086 emu/g and saturation magnetization MS of 0.447 emu/g at 2 K. The Zero Field Cooled (ZFC) shows a blocking temperature at 13 K. Interestingly, the magnetic field-dependent MD coupling loop (inverted butterfly-shaped) of KBFO shows its maximum value of -0.3% (MD%) at room temperature (RT) only. The RT-canted antiferromagnetic behaviour of KBFO indicates that the Inverse Dzyaloshinskii-Moriya (IDM) can be the possible origin of MD coupling. The magnetic field-dependent MI loops (butterfly-shaped) are nearly opposite to the MD loop and suggests the capacitive or the intrinsic origin behind the MD coupling. Carrying forward the systematic investigation, the B-site of KBFO is doped with transition element Co (at Fe site), and the enhancement in RT magnetic and MD properties are recorded. The maximum MR of 0.179 emu/g and MS of 0.462 emu/g are recorded for the x = 5 % Co-doped (KBFCO5) sample. Similarly, the temperature-dependent magnetization data of the Co-doped (KBFCO5) sample shows the anomaly near dielectric transition (~ 775 K), indicating the signature of MD coupling. The RT magnetic field-dependent MD coupling shows the maximum MD% value of -3% for the KBFCO5 sample. The MD value enhances up to 10 times, and the magnetoloss (ML) decreases up to 50 times the parent value in the KBFCO5 sample. The correlation between the Co-doped sample's magnetic, MD, and structural is established following the RT Raman analysis. After a successful investigation on the B-site, the A-site of the KBFO is doped with rare earth Ho ion, and the investigation on magnetic, MD, and MI are carried out. The temperature-dependent dielectric and magnetization of the Ho-doped sample reveal an anomaly near ~ 780 K, which suggests the ferroelectric transition (TC), and at ~ 545 K, the antiferromagnetic transition (TN). Similar to the samples mentioned earlier, the temperature-dependent magnetization data of all the Ho doped samples shows an anomaly near ferroelectric transition, indicating the signature of MD coupling. Interestingly, TC decreases towards RT with an increase in Ho concentration in KBFO. The magnetization of the highest y = 15% Ho-doped (KBHFO15) sample shows a maximum MR 79.88 emu/g and reduced coercivity HC value of 179 Oe at RT. The RT anomalous MD coupling with enhanced switching behaviour has a maximum MD% of -0.2%. The MI measurement quantified the role of Ho-doping toward the intrinsic contribution to MD coupling. The temperature-dependent conductivity reveals that the presence of a two-conduction process (< 475 K and > 475 K) is prominent with Ho-doping. In the conduction process, the role of large-polaron hopping induced by translation motion is addressed via Ho-doping Finally, both the A-site and B-site of KBFO are doped with rare earth Ho and transition element Co simultaneously, and the magnetic, MD, and MI properties of KBFO are investigated. The RT magnetization values of the highest co doped x = y =15% (KBHFCO15) sample, such as MR, shows 20 times, and MS shows two times enhanced value compared to parent KBFO. The coercivity field shows a drop of 90% of the original value in the KBHFCO15 sample. Similarly, the magnetic anomaly near dielectric transition (TC ~ 775 K) indicates the signature of MD coupling in co-doped samples. The RT enhanced MD and ML coupling (inverted butterfly) strength of KBHFCO15 shows maximum values of 0.65% (MD%) and 1.25% (ML%), respectively. Similarly, the RT butterfly shaped MI coupling loops of the co-doped sample suggest the capacitive or the intrinsic origin behind the MD coupling. The MD coupling coefficient (γ) of parent KBFO (4%) increases with co-doping up to 11% in the KBHFCO15. Hence, the MD coupling in KBFO is shown at RT, and the RT MD coupling strength is enhanced through chemical substitutions. This study opens a new window into the unexplored research field of KBFO and provides a base for technological applications.