Structural,electrical, magnetic and magnetoelectric properties of Co-doped BaTiO3 multiferroic ceramics

In this work, BaTi_1-xCo_xO₃ (BTCO) ceramics with x?=?0, 2.5, 5, 7.5 and 10?mol% have been synthesized and their structural, electrical, magnetic and magnetoelectric have been investigated. Rietveld refinement of XRD data reveals that pure BTO has pure tetragonal phase. On the other hand, between 2.5?≤?x?≤?7.5, BTCO shows both tetragonal as well as hexagonal phases. At x?=?10?mol%, BTCO shows only hexagonal phase. The grain size of the BTCO samples is found to increase with Co doping concentration. The ferroelectric polarization and relative permittivity of BTCO samples reduce with an increase in the Co concentration. A standard magnetization equation is used for fitting the magnetic hysteresis (M-H) curve, thus deconvoluting the ferromagnetic (FM) and paramagnetic (PM) components. The saturation magnetization (M_s) gradually increases from x?=?2.5 to x?=?10?mol%, the value being 0.8 memu/g and 8.92 memu/g respectively. The origin of magnetization is due to the oxygen vacancies and their associated exchange interaction. The magnetodielectric coefficient (MD) shows a reducing trend from 1.80 to 0.18 for x?=?2.5 to x?=?10?mol% respectively. The magnetoelectric coefficient $({\alpha }_{\mathit{ME}})$ for x?=?2.5?mol% is 3.399?mV/cm. Oe, while for x?=?10?mol% it is 0.896?mV/cm. Oe.     

Multiferroic properties of Bi5.75R0.25Fe1.4Ni0.6Ti3O18 (R = Eu,Sm, Nd,Bi and La) ceramics

An improved technique was used to prepare the ceramic samples of Bi_5.75R_0.25Fe_1.4Ni_0.6Ti₃O₁₈(BRFNT,R=Eu,Sm,Nd,Bi,and La).The five-layer Aurivillius phase of the samples was confirmed by X-ray diffraction(XRD)without detectable impurities.BRFNT samples exhibit the pseudo-tetragonal except the orthorhombic BSmFNT samples.The characteristic plate-like morphology was revealed by field emission transmission electron microscopy(FETEM)images.At ambient tempe rature,all the samples present both ferroelectric and magnetic properties and BEuFNT shows the best ferroelectric behavior with its remanent polarization as high as 14.9μC/cm²under 155 kV/cm,which is three times higher than that of Bi_5.75R_0.25Fe_1.4Ni_0.6Ti₃O₁₈.Moreover,the remanent magnetization of BEuFNT is increased up to 1.20 emu/g compared to that of Bi_5.75R_0.25Fe_1.4Ni_0.6Ti₃O₁₈which is only 0.53 emu/g.With increasing radius of the introduced A-site ions,the ferroelectric phase transition temperature(T_CE)is decreased while the magnetic phase transition temperature(T_CM)fluctuates.The decrease in both T_CEand T_CM corresponds to the upward shift of the related Raman modes and vice versa,indicating that the T_CEand the T_CM depend not only on the t factor,but also on the strength of the covalent bonds.     

The influence of epitaxial strain on magnetic and electrical properties of BiFeO3 thin films

Strained epitaxial BiFeO₃ films deposited on (001) SrTiO₃ substrates by metal organic chemical vapor deposition were studied by optical second harmonic generation (SHG) and SQUID magnetometry. The observed SHG intensity vs temperature dependencies indicate that for less strained films (σ < 0.6 GPa) a strong interplay between the ferroelectric and magnetic subsystems exists, while for the films with larger σ-values strain-induced destruction of the magnetic cycloidal ordering takes place.     

Impedance spectroscopy and dielectric properties of Ce and La substituted Pb0.7Sr0.3(Fe0.012Ti0.988)O3 nanoparticles

We present the structural, microstructural, dielectric and impedance behavior of Pb_0.7Sr_0.3[(Fe_2/3Ce_1/3)_0.012Ti_0.988]O₃ (PSFCT) and Pb_0.7Sr_0.3[(Fe_2/3La_1/3)_0.012Ti_0.988]O₃ (PSFLT) nanoparticles. These nanoparticles were prepared by a chemical synthesis route using polyvinyl alcohol as surfactant. The X-ray diffraction pattern shows polycrystalline nature with coexistence of tetragonal and cubic phase in both PSFCT and PSFLT nanoparticles. The average particle size has been measured using Scherer's relation. The average particle sizes also measured by TEM are 10 and 11 nm, and by SEM 9 and 12 nm, respectively, of PSFCT and PSFLT nanoparticles. By measuring the value of relative permittivity (?′) and loss (tan δ) at lower frequency, the dielectric properties show Maxwell-Wagner type interfacial polarization. However, due to nano size effect of PSFCT and PSFLT, dispersionless dielectric response has been observed up to higher frequency of 15 MHz. The frequency dependent real (Z′) and imaginary (Z″) parts of impedance confirmed the variation which was observed in dielectric properties. The values of resistance of grain boundaries, R_gb is higher than grains, R_g indicates that the effect of grain boundaries is dominant on electrical properties when the size of nanoparticles is quite small.     

Effect of A site and B site doping on structural, thermal, and dielectric properties of BiFeO3 ceramics

Polycrystalline Bi_1−xBa_xFe_1−yM_yO₃ (M = Co and Mn; x = 0.1, y = 0.1) were synthesized by solid-state route method to study the compositional driven structural transformations in multiferroics. Room temperature X-ray diffraction patterns confirmed the formation of perovskite structure. Rietveld-refined crystal structure parameters revealed the existence of rhombohedral R3c symmetry for both the samples. The samples were found to be nearly free from any other secondary phases. Raman analysis reveals that Ba atom substitutes Bi site and Mn and Co atom substitutes Fe site into the BiFeO₃ with the shifting of phonon modes. The red shift is attributed to Co or Mn doping where as blue shift occurs from Ba doping. The differential scanning calorimetry reveals the corresponding Neel temperature 370 °C and 326 °C for Co and Mn doped samples. Ba and Co substitution with x = 0.1 and y = 0.1 has not affected the Neel temperature of the parent BiFeO₃ as well of Ba and Mn substitution. The variation of frequency dispersion in permittivity and loss pattern due to A-site and B-site substitution in BiFeO₃ was observed in the dielectric response curve.     

Phase pure synthesis of BiFeO3 nanopowders using diverse precursor via co-precipitation method

Amorphous powder of BiFeO₃ (BFO) was synthesized at low-temperature (80 °C) by co-precipitation method. Optimal synthesis conditions for phase pure BFO were obtained. Powders were calcined in the temperature range from 400 to 600 °C for 1 h. Iso-statically pressed powder was sintered at 500 °C for 2 h. Differential scanning calorimetric thermo-gram guided for phase transition, crystallization and melting temperatures. X-ray diffraction confirmed the amorphous nature of as synthesized powder and phase formation of calcined powders. Calcination at temperature ≥400 °C resulted in nano crystalline powders with perovskite structure. Average crystallite size increased with the increase in calcination temperature. Scanning electron microscopic studies revealed dense granular microstructure of the sintered samples. The sintered samples exhibited high dc resistivity at room temperature which decreased with the increase in temperature. Dielectric constant, dielectric loss tangent and ac conductivity measurements were carried out in the frequency range (10 Hz to 2 MHz). The samples responded weak electric and magnetic polarization at room temperature with unsaturated and hysteresis free loops, respectively.     

Hysteresis loops of polarization and magnetization in (BiNd0.05)(Fe0.97Mn0.03)O3/Pt/CoFe2O4 layered epitaxial thin film grown by pulsed laser deposition

(BiNd_0.05)(Fe_0.97Mn_0.03)O₃ (BNFM)/Pt/CoFe₂O₄ (CFO) layered thin film was fabricated on (100) SrTiO₃ substrate by pulsed laser deposition. BNFM, Pt, and CFO layers were epitaxially grown on the substrate. Almost no increase of leakage current due to the formation of heteroepitaxial structure was found, and well-saturated hysteresis loops in the polarization vs electric field and magnetization vs magnetic field curves coexist at room temperature. The remnant polarization and remnant magnetization values were 55 μC/cm², and 70-145 mA/m, respectively.     

Can the ferroelectric soft mode trigger an antiferromagnetic phase transition?

Type-II multiferroics, where spin interactions induce a ferroelectric polarization, are interesting for new device functionalities due to large magnetoelectric coupling. We report on a new type of multiferroicity in the quadruple-perovskite BiMn₃Cr₄O₁₂, where an antiferromagnetic phase is induced by the structural change at the ferroelectric phase transition. The displacive nature of the ferroelectric phase transition at 125 K, with a crossover to an order-disorder mechanism, is evidenced by a polar soft phonon in the THz range and a central mode. Dielectric and pyroelectric studies show that the ferroelectric critical temperature corresponds to the previously reported Néel temperature of the Cr³⁺ spins. An increase in ferroelectric polarization is observed below 48 K, coinciding with the Néel temperature of the Mn³⁺ spins. This increase in polarization is attributed to an enhanced magnetoelectric coupling, as no change in the crystal symmetry below 48 K is detected from infrared and Raman spectra.     

Structural evolution and coexistence of ferroelectricity and antiferromagnetism in Fe,Nb co-doped BaTiO3 ceramics

Multiferroics are materials that exhibit two or more primary ferroic properties within the same phase and have potential applications in sensors, spintronics and memory devices. Here, the dielectric, ferroelectric and magnetic properties of novel multiferroics derived from BaTi_1?x(Fe_0.5Nb_0.5)_xO₃ (BTFN, 0.01 ≤ x ≤ 0.10) ceramics are investigated. Multiferroism in these ceramics is manifested by the coexistence of ferroelectric long-range ordering and antiferromagnetism. With increasing x-value, there is a structural evolution from a tetragonal perovskite to a mixture of tetragonal and cubic phases, accompanied by a decrease in the temperature of maximum permittivity. At room temperature, ferroelectric behaviour is evidenced by the presence of current peaks corresponding to domain switching in the current-electric field loops, while the observation of non-linear narrow magnetic hysteresis loops suggests dilute magnetism. The results indicate that in the x = 0.07 composition the antiferromagnetic order is established through an indirect super-exchange interaction between adjacent Fe ions.