M. Mahesh Kumar

Structure property relations in BiFeO3/BaTiO3 solid solutions

BiFeO3, when forming a solid solution with BaTiO3, shows structural transformations over the entire compositional range. Above 70 mole % of BiFeO3 the structure is rhombohedral and below 4 mole %, it is tetragonal. In between the structure is cubic. The ferroelectric TC decreases with increasing composition of BaTiO3 and a relatively small relaxation is observed. Impedance measurements showed a structural dependence and analysis of which has clearly shown that the capacitance observed is from the bulk of the sample. Relaxation time (τ) of BiFeO3 was estimated using the relaxation times of the intermediate compositions. Magnetization measurements showed field induced ferromagnetism. As the structure becomes cubic with increasing concentration of BaTiO3, paramagnetism sets in, as evidenced by the electron spin resonance spectra.

Spontaneous magnetic moment in BiFeO3–BaTiO3 solid solutions at low temperatures

Abstract The solid solutions with perovskite structure, x ·(BiFeO 3 )–(1− x )·BaTiO 3 have been synthesised with x values ranging from 0.9, 0.8, 0.75, 0.7 and 0.6 adopting a solid state sintering route. The samples show spontaneous magnetic moment at low temperatures. At room temperature the true nature of the materials is antiferromagnetic and as the field is increased they become ferromagnetic, indicating a field-induced transition. With increasing content of BaTiO 3 and temperature, the samples have a paramagnetic state with weak ferromagnetic ordering. With increasing field, the spins flip towards the field direction giving rise to ferromagnetism.

An experimental setup for dynamic measurement of magnetoelectric effect

An experimental setup is developed for the measurement of dynamic magnetoelectric effect (ME) in polycrystalline materials, using a time varying DC magnetic field on which an AC magnetic field is superimposed. The experimental data on ME on Bi5FeTi3O15 and a solid solution of (90%)BiFeO3-(10%)BaTiO3 are obtained using this setup. The linear and higher order ME coefficients can be evaluated from the output voltage. The temperature variation of ME data gives additional information corroborating with the data on magnetization.

Magnetoelectric measurements on and

and are members of the Aurivillius family of compounds having simultaneous electrical and magnetic ordering. Magnetoelectric measurements carried out in a linear time-varying magnetic field with an alternating-current field superimposed yielded a non-linear signal. The shift from linearity, which is not usually observed for antiferromagnetic materials, may be due the tilt in the octahedra. The variation of the magnetoelectric output with temperature for and indicated magnetic anomalies with enhanced sensitivity, corresponding to those in magnetization data.

Investigation of dielectric and magnetic nature of Bi7Fe3Ti3O21

Ceramic samples of Bi{sub 7}Fe{sub 3}Ti{sub 3}O{sub 21} were prepared by a solid-state sintering route. X-ray diffraction showed that the material is single phase and isostructural with other bismuth layer structured ferroelectrics (BLSFs). Dc conductivity measured from room temperature to 640 C showed two slope changes. Dielectric measurements carried out in the temperature range 30--850 C indicated dielectric anomalies at 645 and 820 C. A pyroelectric signal was also measured and the coefficient evaluated. Magnetization measurements indicated that the material is antiferromagnetic in the temperature range 80--300 K, with a small hysteresis at room temperature. The properties are compared with other isostructural compounds.

Dielectric and Impedance Studies on BiFeO3–BaTiO3 Solid Solutions

xBiFeO 3 -(1-x)BaTiO 3 form substitutional ferroelectromagnetic solid solutions with the rhomobohedral distortion up to 67% of BiFeO 3 . The paper reports data on phase formation, electrical conductivity anti dielectric constant as a function of temperature and frequency for the samples with x = 0.9 and 0.8 which are isostructural to BiFeO 3 . The conductivity is found to be dependent on both frequency and temperature in the range 30 to 400 °C. The temperatures of dielectric anomalies are found to be lower than that of BiFeO 3 . The complex impedance studies indicate relaxation effects.

Investigation of the magnetoelectric effect in BiFeO3-BaTiO3 solid solutions

BiFeO3 forms solid solutions with BaTiO3, over the entire compositional range, with different crystal symmetries. Magnetoelectric (ME) measurements carried out with a superimposed alternating-current magnetic field, together with a time-varying direct-current magnetic field in isothermal conditions, indicated non-linearity. The peak observed coincided with a metamagnetic transition in the magnetization data. Rather than the spin flop reported earlier, it is a gradual reorientation of spins towards the field direction that destroys the spiral spin arrangement. With increasing content of BaTiO3, a quadratic signal was observed, indicating the structural dependence of the magnetoelectric effect. The temperature variation of the ME output in the investigation carried out for x = 0.75 indicates magnetic transitions.

Synthesis and physical properties of Bi5Ti3MnO15

Bi 5 Ti 3 MnO 15 is found to be a four layered perovskite, like Bi 5 Ti 3 FeO 15 having an orthorhombic structure, with the lattice parameters a = 5.40 A, b = 5.44 A and c = 41.22 A. The compound shows interesting physical property variations when Fe is replaced with Mn. A peak in resistivity is observed at 126 K. The resistivity data in the range 77-300 K is interesting as it appears similar to that of I-M like transition. Conduction in the insulating region is governed by Motts Variable Range Hopping. The magnetic nature indicates it to be an antiferromagnet at low temperatures however without a hysteresis loop at room temperature as observed in the case of Bi 5 Ti 3 FeO 15 . The sample is a ferroelectric showing anomalies at 556 and 589 K (100 kHz).

Dielectric relaxation in Ba0.96Bi0.04Ti0.96Fe0.04O3

Samples of Ba0.96Bi0.04Ti0.96Fe0.04O3 were prepared by a solid state reaction method. Data on the dielectric constant as a function of temperature and frequency were obtained in the temperature range 35–180 °C at 10 and 100 kHz. The material shows a diffuse phase transition. With increasing frequency, the transition temperature shifts towards lower temperature, resembling that of a relaxor ferroelectric with a negative shift in Tc. Impedance analysis confirms the relaxor behavior.

Electrical and dielectric properties in double doped BaTiO3 showing relaxor behavior

BaTiO3, when doped with minute concentrations of Bi+3 and Fe+3, was found to exhibit relaxor behavior. The data on dielectric constant performed as a function of frequency and temperature indicate a reduction in the transition temperature with increase in frequency. A poled sample indicated a decrease in the transition temperature compared to an unpoled sample. It was found that the grain boundary capacitance were negligible and the relaxations occurred are only due to the bulk of the sample. The results have been discussed using impedance and modulus spectroscopy.