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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.

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.

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...