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The influence of different synthesis routes and the heat treatments on the fundamental physical properties of the potassium bismuth ferrite oxide KBiFe2O5 (KBFO) is investigated. The polycrystalline KBFO is prepared using a solid-state reaction route followed by conventional heating (SSR) and the sol–gel method followed by conventional heating (SGC) and microwave heating (SGM). The monoclinic phase of the prepared sample is confirmed from the Rietveld refined X-ray diffraction analysis. XRD confirms crystallite size decrease from SSR (165 nm) to SGM (91 nm) sample. The surface morphology reveals that grain size varies from 4 µm (SSR) to 100 nm (SGM). The observed variation in Raman intensity and FWHM indicates the effect of different synthesis routes on the particle sizes of KBFO. The optical energy band gap increases from 1.59 eV (SSR) to 1.73 eV (SGM) with changes in the synthesis route and heat treatment. Temperature-dependent dielectric (25 °C to 560 °C) data at 1 MHz frequency shows a significant variation in dielectric value followed by a transition at 510 °C (TC) in all the prepared samples. The dielectric value of the SSR sample almost 50 times larger than the SGC and SGM samples of KBFO.

Composite Bi5Ti3FeO15 (BTFO) and manganite-based La0.67Sr0.33MnO3 (LSMO) is prepared by the following sol-gel precursor hybrid technique in order to study the magnetodielectric (MD) properties. The coexistence of dual phases of BTFO (orthorhombic) and LSMO (rhombohedral) is confirmed from the Rietveld refinement of the X-ray diffraction (XRD) pattern. The microstructural analysis is carried out using the field-emission scanning electron microscope (FESEM), suggesting the formation of inhomogeneous distribution of both large and small grains of size 586 nm and 261 nm. In addition to the crystallite size calculation, the formation of strain is calculated using the uniform deformation model (UDM). Temperature-dependent dielectric constant and loss responses subjected to a constant magnetic field confirm the existence of the MD effect. The maximum magnitude of the MD effect at room temperature is found to be ~0.17% at a magnetic field of 1.3 T. The behavior of MD effect arises due to the presence of magnetoresistance (MR) in the core of the manganite-based composite material.

We have investigated the structural, magnetic, dielectric, and magnetodielectric properties of brownmillerite-structured polycrystalline KBiFe2O5 (KBFO) sample synthesized using conventional solid-state reaction route. Monoclinic structure with P2/c space group is determined from Rietveld refined X-ray diffraction (XRD) analysis. Temperatures-dependent magnetic and dielectric data show a broad hump around 510 °C, indicating the existence of magnetodielectric effect in KBFO. Room-temperature M–H hysteresis measurement possesses a weak ferromagnetic order (MR = 0.006 emu/g and Hc = 1100 Oe) originating from the canted Fe3+ moments due to antiferromagnetic ordering. This canted Fe3+ moment is also reflected in magnetic field variation of magnetodielectric (MD) measurement at room temperature, in which MD does not trace the original path by reversing the field. The highest magnetodielectric response is obtained to be ~ − 1.8% at room temperature. Modulus and complex impedance spectrum analysis attributes the bulk contribution to the observed room-temperature magnetodielectric at high frequency (> 10 kHz) and extrinsic contribution (

We report significant enhancement in magnetic and magnetodielectric (MD) properties of cobalt (Co) doped polycrystalline KBiFe2O5 (KBFO). Rietveld refinement of X-ray diffraction data confirms monoclinic crystal structure with P2/c space group. SEM micrographs confirmed the polycrystalline nature of the samples with grain sizes varying from 1 µm to 5 µm. Raman spectra demonstrate the shifting of vibrational modes due to Co doping which is associated with FeO4 tetrahedral. The shift in vibrational modes due to Co doping is in line with the observed changes in FeO4 tetrahedral bond angles (O1-Fe-O1 & O1-Fe-O2) and bond lengths (Fe-O1 & Fe-O2) obtained from XRD data. Room temperature magnetization data reveal the maximum enhancement in Mr (~103 times) and Ms (~200 times) values for 5% Co-doped KBFO compared with KBFO. This enhancement in magnetization data is due to a spin-canted (Fe3+) antiferromagnetic spin structure and increased lattice strain with increased cobalt substitution. The temperature-dependent dielectric and magnetization data show an anomaly near 770 K (Tc) indicates the MD effect. Compared to KBFO, nearly ten times enhancement in MD and nearly 50 times reduction in magnetic loss (ML) is observed for KBFCO5 measured at 30 kHz frequency. Landau free energy approach confirms the biquadratic nature of MD coupling, and the strength is − 8 (emu/g)2 for 5% Co-doped KBFO and − 5(emu/g)2 for 10% Co-doped KBFO, respectively. The comparative study reveals that Co substitution in KBFO enhances maximum MD% and magnetization in 5% Co substitution. These results establish the coupling between electric and magnetic orders, which can find application in AC/DC magnetic field sensors and ME solar cells.

The phase pure polycrystalline KBiFe2O5with space group P2/c is synthesized following solid state reaction route. The temperature dependentreal and imaginary part of dielectric constant is studied over wide temperature (280 K to 850 K) and frequency range (100 Hz ≤ f ≤ 1 MHz). Two distinct anomaliesat around 390 K and 750K is observedin both real (e) an imaginary (tanδ) part of dielectric constant which can be attributed to Maxwell-Wagner effect. Our DC susceptibility measurement up to room temperature does not show any transition even upto room temperature, giving an indication that its transition temperature is above room temperature. This aspect is further confirmed from M-H hysteresis loop which shows a weak ferromagnetic hysteresis loop at room temperature. The evidence of magnetodielectric (MD) coupling at room temperature is confirmedfrom magnetic field dependent dielectric measurement in the form of inverted butterfly shaped e-H loop.The phase pure polycrystalline KBiFe2O5with space group P2/c is synthesized following solid state reaction route. The temperature dependentreal and imaginary part of dielectric constant is studied over wide temperature (280 K to 850 K) and frequency range (100 Hz ≤ f ≤ 1 MHz). Two distinct anomaliesat around 390 K and 750K is observedin both real (e) an imaginary (tanδ) part of dielectric constant which can be attributed to Maxwell-Wagner effect. Our DC susceptibility measurement up to room temperature does not show any transition even upto room temperature, giving an indication that its transition temperature is above room temperature. This aspect is further confirmed from M-H hysteresis loop which shows a weak ferromagnetic hysteresis loop at room temperature. The evidence of magnetodielectric (MD) coupling at room temperature is confirmedfrom magnetic field dependent dielectric measurement in the form of inverted butterfly shaped e-H loop.