Recent development in lead-free perovskite piezoelectric bulk materials

Multiferroic bismuth ferrite-based materials for multifunctional applications: Ceramic bulks, thin films and nanostructures

Controlling magnetism by means of electric fields is a key issue for the future development of low-power spintronics1. Progress has been made in the electrical control of magnetic anisotropy2, domain structure3,4, spin polarization5,6 or critical temperatures7,8. However, the ability to turn on and o robust ferromagnetism at room temperature and above has remained elusive. Here we use ferroelectricity in BaTiO3 crystals to tune the sharp metamagnetic transition temperature of epitaxially grown FeRh films and electrically drive a transition between antiferromagnetic and ferromagnetic order with only a few volts, just above room temperature. The detailed analysis of the data in the light of first-principles calculations indicate that the phenomenon is mediated by both strain and field e ects from the BaTiO3. Our results correspond to a magnetoelectric coupling larger than previous reports by at least one order of magnitude and open new perspectives for the use of ferroelectrics in magnetic storage and spintronics.

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Electric-field control of magnetic order above room temperature

Developing high-performance film dielectrics for capacitive energy storage has been a great challenge for modern electrical devices. Despite good results obtained in lead titanate-based dielectrics, lead-free alternatives are strongly desirable due to environmental concerns. Here we demonstrate that giant energy densities of ~70 J cm−3, together with high efficiency as well as excellent cycling and thermal stability, can be achieved in lead-free bismuth ferrite-strontium titanate solid-solution films through domain engineering. It is revealed that the incorporation of strontium titanate transforms the ferroelectric micro-domains of bismuth ferrite into highly-dynamic polar nano-regions, resulting in a ferroelectric to relaxor-ferroelectric transition with concurrently improved energy density and efficiency. Additionally, the introduction of strontium titanate greatly improves the electrical insulation and breakdown strength of the films by suppressing the formation of oxygen vacancies. This work opens up a feasible and propagable route, i.e., domain engineering, to systematically develop new lead-free dielectrics for energy storage. Dielectrics with high capacitive energy storage density are essential for modern electrical devices and pulsed power systems. Here, the authors realised superior energy storage performance in lead-free bismuth ferrite-based relaxor ferroelectric films through domain engineering.

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Giant energy density and high efficiency achieved in bismuth ferrite-based film capacitors via domain engineering.

Colloidal cobalt-doped TiO2 (anatase) nanocrystals were synthesized and studied by electronic absorption, magnetic circular dichroism, transmission electron microscopy, magnetic susceptibility, cobalt K-shell X-ray absorption spectroscopy, and extended X-ray absorption fine structure measurements. The nanocrystals were paramagnetic when isolated by surface-passivating ligands, weakly ferromagnetic (Ms = 1.5 x 10-3 mB/Co2+ at 300 K) when aggregated, and strongly ferromagnetic (up to Ms = 1.9 mB/Co2+ at 300 K) when spin-coated into nanocrystalline films. X-ray absorption data reveal that cobalt is in the Co2+ oxidation state in all samples. In addition to providing strong experimental support for the existence of intrinsic ferromagnetism in cobalt-doped TiO2, these results demonstrate the possibility of using colloidal TiO2 diluted magnetic semiconductor nanocrystals as building blocks for assembly of ferromagnetic semiconductor nanostructures with potential spintronics applications.

Strong Room-Temperature Ferromagnetism in Co2+-Doped TiO2 made from Colloidal Nanocrystals

In this article, we studied the effect of synthesis route on the multifunctional properties of multiferroic BiFeO3. BiFeO3 powders were prepared by conventional solid-state-reaction and sol–gel route. X-ray diffraction (XRD) patterns for these samples were collected at different stages of synthesis to analyze the phase purity of the formation. The XRD patterns reveal that sample prepared by sol–gel route attains the low temperature phase formation as compared to the solid state route. Rietveld refinement has been performed for these samples and lattice parameters, cell volume bond length etc. have been calculated from XRD patterns. Phonon modes were studied by Fourier transform infrared spectroscopy measurements and bond length calculated from XRD shows the good agreement with the bond length calculated from IR spectra. UV–visible spectra showed that BFO nanoparticles exhibit absorption peak at wavelength ~521 nm and band gap is more for the sample prepared by sol–gel route than solid state. The room temperature (RT) magnetic hysteresis (M–H) curve shows the large value magnetization in the sample prepared by sol–gel route in comparison to the sample prepared by solid state route. Similar behaviour is seen in the P–E hysteresis curve. Room temperature dielectric properties of these samples revealed that there is dispersion in the low frequency range that shows normal dielectric characteristics.

Comparative studies of pure BiFeO 3 prepared by sol–gel versus conventional solid-state-reaction method

Structural transition, magnetic and optical properties of Pr and Ti co-doped BiFeO3 ceramics

The authors prepared (1 − x)BiFeO3 – (x)Pb(Zr052Ti048)O3 for x ≤ 030 by sol-gel method and investigated the material's structures, magnetic and electrical properties Detailed Rietveld analysis of X-ray diffraction data revealed that the system retains distorted rhombohedral R3c structure for x ≤ 010 but transforms to monoclinic (Cc) structure for x > 010 Disappearance of some Raman modes corresponding to A1 modes and the decrease in the intensities of the remaining A1 modes with increasing x in the Raman spectra, which is a clear indication of structural modification and symmetry changes brought about by PZT doping Enhanced magnetization with PZT doping content may be attributed to the gradual change and destruction in the spin cycloid structure of BiFeO3 The leakage current density at 35 kV/cm was reduced by approximately three orders of magnitude by doping PZT (x = 030), compared with BFO ceramics

Phase transformation, improved ferroelectric and magnetic properties of (1 − x) BiFeO3–xPb(Zr0.52Ti0.48)O3 solid solutions

Electrical properties of (1−x) BFO – (x) PZT multiferroics synthesized by sol-gel method: Transition from relaxor to non-relaxor

An effort has been made to synthesize solid solution of a composition with x=0.05 in the system (1−x)PbZr0.52Ti0.48O3–(x)BiFeO3 by the sol–gel method. XRD patterns of the pure PZT and BFO modified PZT samples have shown single phase formation. The effects of substitution of BFO on dielectric properties of PZT have been studied in the frequency range 10 Hz to 100 kHz and temperature range from RT to 773 K. The density was optimized by sintering the BFO modified PZT samples at different temperatures in four batches, S1, S2, S3 and S4. P–E hysteresis loop measurements for all the samples have shown almost saturated polarization. It has been observed that sample S2, sintered at 950 °C, exhibits superior dielectric properties of the four samples. The occurrence of weak ferromagnetism, observed in the M–H hysteresis loop, indicates coupling between ferroelectricity and magnetism. Impedance analysis has revealed that all the samples, sintered at different temperatures, have a different grain resistance. A large change in ac conductivity around T
 c
 has been observed in all the samples.

Subhash Sharma

Papers

Comparative studies of pure BiFeO 3 prepared by sol–gel versus conventional solid-state-reaction method

Structural transition, magnetic and optical properties of Pr and Ti co-doped BiFeO3 ceramics

Phase transformation, improved ferroelectric and magnetic properties of (1 − x) BiFeO3–xPb(Zr0.52Ti0.48)O3 solid solutions

Electrical properties of (1−x) BFO – (x) PZT multiferroics synthesized by sol-gel method: Transition from relaxor to non-relaxor

Effect of processing on dielectric properties of (0.95)PbZr 0.52 Ti 0.48 O 3 –(0.05)BiFeO 3