Ni（Co,Mn）Fe2O4—PZT（B）系磁电陶瓷的研究

作者利用XRD和SEM分析和观察了Ni(Co、Mn)Fe_2O_4-PZT(B)系陶瓷的相结构,研究了混合比及烧成与极化条件对磁电性能的影响,获得了磁电转换系数(dE/dH)_max达250mV×4π/10A的磁电陶瓷材料。     

Spin reorientation behavior and enhanced multiferroic properties of co-doped YFeO3 towards a monophasic multiferroic ceramic Co0.05Y0.95Fe0.95Ti0.05O3

Monophasic nanocrystalline lead-free relaxor ferroelectric samples of YFeO₃ and Co_0.05Y_0.95Fe_0.95Ti_0.05O₃ were prepared via the standard sol-gel synthesis method. The influence of Co and Ti co-doping on the structural, dielectric, ferroelectric, and magnetic properties were investigated using X-ray, Neutron Diffraction, Electric Polarisation, and magnetization measurement techniques. The structural analysis by the Rietveld method revealed an orthorhombic (Pnma) distorted perovskite structure. The analysis of the neutron diffraction data showed a spin reorientation from IR Γ4 to Γ2 in the doped sample. The universal dielectric response (UDR) phenomenon was used to explain the dielectric behavior. The temperature-dependent dielectric constant measurement shows a shift in the ferroelectric to paraelectric phase in Co_0.05Y_0.95Fe_0.95Ti_0.05O_3. The observed P-E loops indicate lossy ferroelectric nature for YFO and CYFTO. Enhancement in the value of saturation magnetization at room temperature was observed on co-doping. Furthermore, the presence of magnetodielectric coupling was established for the co-doped sample. Co-doping with Co and Ti in YFeO₃ enhanced the multiferroic properties of the pristine sample which may be helpful for future device applicability.     

Effects of mechanochemical activation on the structural and electrical properties of orthorhombic LuFeO3 ceramics

Orthorhombic LuFeO₃ is an important member of rare-earth orthoferrites ReFeO₃ whose appealing physical features have drawn much attention due to its various potential applications. In this work, pure phase LuFeO₃ ceramics have been prepared by a mechanochemical activation-assisted solid-state reaction (MAS) method and conventional solid-state reaction (CSS) method for comparison. X-ray Diffraction (XRD) studies demonstrated that the mechanochemical activation process can lower the starting reaction temperature and substantially shorten the time to complete the reaction of LuFeO₃ ceramics. Besides, more homogeneous and highly denser LuFeO₃ ceramics can be obtained using the MAS method revealed by the density measurement and Scanning Electron Microscopy. It was revealed by XRD and Raman measurements that there is a little increase in lattice constant in the MAS sample compared to the CSS sample. The complex impedance spectra illustrated that the MAS sample has a much higher resistance than the CSS sample, which is caused by the higher density and fewer oxygen vacancies verified by X-Ray Photoelectron Spectroscopy. We believe that this study will contribute to solving the common leakage problems in ReFeO₃ ceramics and the investigation of its multiferroic properties.     

Effect of Cr doping on ferroelectric and magnetic properties of Bi0.8Ba0.2FeO3

Multiferroic ceramics Bi_0.8Ba_0.2Fe_1−xCr_xO₃ (x=0, 0.05 and 0.1) were synthesized by using the conventional solid state reaction method. The pure phase with rhombohedral structure was confirmed by the X-ray diffraction measurements for all samples. It was shown that the magnetic and the ferroelectric properties were simultaneously improved, and the maximum values of the remnant magnetization (2Mr) and the remnant polarization (2Pr) at room temperature for all samples are around 1 emu/g and 1.9 μC/cm², respectively. Furthermore, the leakage current density, the low frequency dispersion in the dielectric constant and the dielectric loss decreased with increasing the Cr concentration from x=0 to 0.1. A remarkable change in the P–E loop was observed whether a bias dc magnetic field was applied or not, approving the existence of the magnetoelectric coupling indirectly therein.     

Giant multiferroic effects in topological GeTe-Sb2Te3 superlattices

Multiferroics, materials in which both magnetic and electric fields can induce each other, resulting in a magnetoelectric response, have been attracting increasing attention, although the induced magnetic susceptibility and dielectric constant are usually small and have typically been reported for low temperatures. The magnetoelectric response usually depends on d-electrons of transition metals. Here we report that in [(GeTe)₂(Sb₂Te₃)_l]_m superlattice films (where l and m are integers) with topological phase transition, strong magnetoelectric response may be induced at temperatures above room temperature when the external fields are applied normal to the film surface. By ab initio computer simulations, it is revealed that the multiferroic properties are induced due to the breaking of spatial inversion symmetry when the p-electrons of Ge atoms change their bonding geometry from octahedral to tetrahedral. Finally, we demonstrate the existence in such structures of spin memory, which paves the way for a future hybrid device combining nonvolatile phase-change memory and magnetic spin memory.     

Variation of crystal structure,magnetization, and dielectric properties of Nd and Ba co-doped BiFeO3 multiferroics

Multiferroics having composition Bi_0.80Nd_0.20-xBa_xFeO₃ were prepared to investigate the effect of doping on crystal structure, magnetic, and dielectric properties. The Rietveld refinement deduces the formation of mixed structural symmetry. With larger content of Nd, crystal structure consisting of major rhombohedral R3c and minor orthorhombic Pnma has been accomplished. The fraction of rhombohedral phase has been found to increase with doping of Ba up to x = 0.10. At composition x = 0.15, the orthorhombic phase Pnma disappears, and there is evolution of triclinic phase P1 in place of it. The mixed structure now accomplished contains ≈61% rhombohedral R3c and rest 39% triclinic P1. In solely Ba-doped sample (ie, at x = 0.20), the fraction of rhombohedral R3c phase again rises and attains ≈92% fraction of the structure along with rest triclinic P1 phase. The M-H loops depict enormous enhancement in magnetic properties with increasing doping of Ba. Dielectric constant (ε′) and dielectric loss (tan δ) both were found to increase with doping of Ba. The anomalies present in the dielectric constant and dielectric loss with temperature may be regarded to the hopping conduction of e⁻ between Fe³⁺ and Fe²⁺ and their interaction with oxygen vacancies.     

Processing and properties of Bi0.98R0.02FeO3 (R = La,Sm, Y) ceramics flash sintered at ~650°C in <5 s

We show that flash sintering produces single-phase, nanograin-sized polycrystals of isovalent-substituted multiferroic ceramics of complex compositions. Single-phase polycrystals of Bi_0.98R_0.02FeO₃ (R = La, Sm, Y) were produced at a furnace temperature of ~650°C in a few seconds by the application of an electric field of 50 V cm⁻¹, with the current limit set to 40 mA mm⁻². The dielectric and insulating properties compared favorably with expected values. Impedance spectroscopy suggests electrically homogenous microstructure, except for the sample Bi_0.98La_0.02FeO₃ that shows a small grain boundary contribution to the impedance. These results reinforce the enabling nature of flash sintering for ceramics which pose difficulties in conventional sintering because they contain low melting constituents or develop secondary phases during the sintering protocol.     

Room-temperature multiferroicity in NiFe2O4 and its magnetoelectric coupling intensified through defect engineering

The well-known ferromagnetic oxide, NiFe₂O₄, was studied as a potential candidate for room-temperature Type II magnetoelectrics. A spin canting as one of the essential requirements for Type II multiferroics was induced by breaking the stoichiometry, that is, intentionally subtracting Fe ions. We observed that Fe ions were first subtracted exclusively from the tetrahedral sites, leading to an increase in the magnetoelectric coupling owing to an increasing degree of spin canting. The enhancement in the magnetoelectric coupling culminated beyond the subtraction level of ~30 at.%, at which Fe ions started to be removed from the octahedral sites. Alongside, we observed that the subtraction of Fe ions gives rise to a ferroelectricity due to the formation of defect complexes that establish an internal bias field.