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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.

Cobalt (Co)-modified brownmillerite KBiFe2O5 (KBFO; [KBiFe2(1-x)Co2xO5 (x = 0, 0.05)]) polycrystalline is synthesized following the solid-state reaction route. Rietveld refinement of X-ray diffraction data confirmed the phase purity of KBFO and KBiFe1.9Co0.1O5 (KBFCO). The optical bandgap energy (Eg) of KBFO decreased from 1.59 to 1.51 eV because of Co substitution. The decrease in bandgap can be attributed to the tilting of the Fe-O tetrahedral structure of KBFCO. The observed room-temperature Raman peaks of KBFCO shifted by 3 cm-1 toward a lower wavenumber than that of KBFO. The shift in Raman active modes can be attributed to the change in the bond angles and bond lengths of the Fe-O tetrahedral structure and modification in response to oxygen deficiency in KBFO because of Co doping. Compared with that of KBFO, the frequency-dependent dielectric constant and dielectric loss of KBFCO decrease at room temperature, which is a consequence of the reduction in oxygen migration and modification in response to vibrational modes present in the sample.

We have investigated the structural, magnetic, magnetodielectric, and magnetoimpedance characteristics of Aurivillius-structured Bi5Ti3FeO15 (BTFO) synthesized by a generic solid-state reaction route. Rietveld refinement of X-ray diffraction pattern at room temperature (RT) confirms orthorhombic crystal structure (space group A21am). In BTFO, octahedral distortion of the perovskite unit occurs due to antisite defects Fe/Ti in the BO6 site, which results in the formation of Fe–O clusters. Raman spectra also reveal Ti/FeO6 octahedral distortion due to the vibration of Bi ions in the perovskite layer. Magnetic field-dependent magnetization (M–H) and electric field-dependent polarization (P–E) measurement at RT indicate the existence of multiferroic behavior in BTFO. The M–H hysteresis at 5 K suggests that the non-interacting superparamagnetic state is dominant over the local short-range antiferromagnetic (AFM) ordering. The AFM interaction arises due to the random distribution of antisite defects Fe/Ti causing the distorted Fe–O octahedral unit. These canted spin interact via the Dzyaloshinskii–Moriya (DM) interaction. The superexchange interaction between the Fe–O–Fe ions is stronger than the next-nearest-neighboring Fe–O–O–O–Fe interaction. This happens due to the intermediate fluorite-like layer (Bi2O2)2+, which opposes the long-range exchange interaction. The negative magnetodielectric (MD) effect is more prominent at low frequency (~ 100 Hz) due to the extrinsic contribution. In contrast, in the high-frequency region (> 50 kHz), the intrinsic contribution dominates, which is further ascertained by magnetoimpedance (MI) measurement. The maximum magnitude of the MD effect is found to be ~ 0.32% at a magnetic field of 13 kOe at 150 K. Lastly, the ferroelectric characteristic of the sample is obtained from the P–E measurement with a polarization value of 4.35 µC/cm2 with an applied electric field of 70 kV/cm.

For this study, we synthesized Aurivillius Bi5Ti3FeO15 ceramic using the generic solid-state reaction route and then performed room-temperature X-ray diffraction to confirm that the compound had a single phase with no impurities. The surface morphology of the prepared sample was observed to contain microstructural grains approximately 0.2–2 µm in size. The dielectric properties of the sample were determined as a function of frequency in a range of approximately 100 Hz to 1 MHz at various temperatures (303 K ≤ T ≤ 773 K). Nyquist plots of the impedance data were found to exhibit a semi-circular arc in the high-temperature region, which is explained by the equivalent electrical circuit (R1C1)(R2QC2), where R1 and R2 represent the resistances associated with the grains and grain boundaries, respectively, C1 and C2 are the respective capacitances, and Q is the constant phase element (CPE), which accounts for non-Debye type of behavior. Our results indicate that both the resistance and capacitance of the grain boundaries are more prominent than those of the grains. The alternating current (ac) conductivity data were analyzed based on the Jonscher universal power law, which indicated that the conduction process is dominated by the hopping mechanism. The calculated activation energies of the relaxation and conduction processes were very similar (0.32 to 0.53 eV), from which we conclude that the same type of charge carriers are involved in both processes.

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 (

The polycrystalline Fe2TeO6 (FTO) and Fe2Te(1-x)NbxO6 [x = 0.5 (FTON5), x = 0.10 (FTON10)] is prepared via solid state reaction route. Comparative structural study is done by the Rietveld refinement of the powder X-ray diffraction data of all the prepared samples. We observe the presence of ferroelectricity in FTO via remanent polarization versus electric field (PE) hysteresis loop measurement with remanent polarization ∼4.85 nC/cm2. On application of 1.2 T external magnetic field, this observed remanent polarization changes 60% which confirms the presence of magnetoelectric (ME) coupling in the material. The magnetic field dependent PE loops also confirms the presence of ME coupling in both FTON5 and FTON10. In case of FTON10 the remanent polarization changes by 80% on application 1.2 T. The presence ME coupling is also observed through the nonlinear magnetoelectric voltage response on application of modulated DC magnetic field in both FTO and FTON5. This nonlinear voltage response changes to linear behavior for FTON10 due to Nb doping.

Multiferroic composites are promising candidates for magnetic field sensors, next-generation low power memory and spintronic devices, as they exhibit much higher magnetoelectric (ME) coupling and coupled ordering parameters compared to the single-phase multiferroics. Hence, the 3-0 type particulate multiferroic composites having general formula (1 − Φ)[PbFe0.5Nb0.5O3]-Φ[Co0.6Zn0.4Fe1.7Mn0.3O4] (Φ = 0.0, 0.05, 0.1, 0.2, 0.3, 0.4, 0.5, 1.0, (1 − Φ) PFN-ΦCZFMO) were prepared using a hybrid synthesis technique. Preliminary structural and microstructural analysis were carried out using XRD and FESEM techniques, which suggest the formation of 3-0 type particulate composite without the presence of any impurity phases. The multiferroic behaviour of the composites is studied with polarization versus electric field (P-E) and magnetization versus magnetic field (M-H) characteristics at room temperature. The nature of ME coupling was investigated elaborately by employing the Landau free energy equation along with the magneto-capacitance measurement. This investigation suggests the existence of biquadratic nature of ME coupling (P2M2). The magneto-electric coupling measurement also suggests that strain mediated domain coupling between the ferroelectric and magnetic ordering is responsible for the magneto-electric behaviour. The obtained value of direct ME coefficient 26.78 mV/cm-Oe for Φ = 0.3, found to be higher than the well-known single-phase materials and polycrystalline composites.

We report the structural and magneto-dielectric properties of 0.95 Pb(Fe0.5Nb0.5)-0.05 Co0.6 Zn0.4Fe1.7Mn0.3O4 (0.95PFN-0.05CZFMO, PC5) particulate composite prepared using a hybrid synthesis technique. Rietveld refinement on the XRD patterns of the end compounds i.e. PFN and CZFMO were performed using monoclinic crystal structure with space group Cm and cubic crystal structure with space group Fd-3m respectively. The dual phase space group Cm+Fd-3m was taken to refine the XRD pattern of PC5 sample which confirms the formation of composite. The changes in the dielectric parameters by the application of magnetic field suggest existence of magneto-electric coupling behavior in the composite at room temperature (RT). The quantitative changes of the magneto-dielectric properties i.e. MC% (magneto- capacitance), ML% (magneto-loss), MI% (magneto-impedance) and MP% (magneto-phase) were calculated and discussed their dependence with magnetic fields at RT.

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.

We have reported a systematic investigation on structural, magnetic, magnetodielectric and magnetoimpedance characteristics of Y-type Ba

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.

Nanocrystalline Fe2TeO6 (FTO) is synthesized following the sol-gel process. The nanoaspect of FTO is confirmed from transmission electron microscopy, where the particles of sizes 10–40 nm are seen. Magnetic measurement on nano-FTO confirms the antiferromagnetic nature of particles having Neel temperature TN = 167 K, which is very much lower than that of bulk (210 K). Polarization verses electric field loop measurement at room temperature (RT) gives the remanent polarization (Pr) value of 0.098 μC/cm2, confirming ferroelectricity in FTO. A nonmonotonous increase in the remanent polarization is noticed when an external magnetic field is applied on the sample. This is a clear indication of prevailing substantial magnetoelectric (ME) coupling in the sample at room temperature. The quantification of magnetoelectricity is done by directly measuring the ME voltage (V) in the presence of varying dc magnetic field (H), and the ME coefficients are obtained using a quadratic relation in H. The values so obtained for the first order (α/d), second order (β/d), and third order (γ/d) ME coefficients are ∼0.22 mV cm−1Oe−1, ∼−1.70 × 10−2 mV cm−1 Oe−2, and ∼0.72 × 10−6 mV cm−1 Oe−3, respectively. The presence of ferroelectricity and magnetoelectricity above the Neel temperature is a very unusual phenomenon in this material. The presence of short range magnetic ordering which prevails even up to RT, much higher than that of long range magnetic ordering temperature, is suspected to be responsible for this.Nanocrystalline Fe2TeO6 (FTO) is synthesized following the sol-gel process. The nanoaspect of FTO is confirmed from transmission electron microscopy, where the particles of sizes 10–40 nm are seen. Magnetic measurement on nano-FTO confirms the antiferromagnetic nature of particles having Neel temperature TN = 167 K, which is very much lower than that of bulk (210 K). Polarization verses electric field loop measurement at room temperature (RT) gives the remanent polarization (Pr) value of 0.098 μC/cm2, confirming ferroelectricity in FTO. A nonmonotonous increase in the remanent polarization is noticed when an external magnetic field is applied on the sample. This is a clear indication of prevailing substantial magnetoelectric (ME) coupling in the sample at room temperature. The quantification of magnetoelectricity is done by directly measuring the ME voltage (V) in the presence of varying dc magnetic field (H), and the ME coefficients are obtained using a quadratic relation in H. The values so obtained for...