Haigang Yang

Bio: Haigang Yang is an academic researcher from Henan Normal University. The author has contributed to research in topics: Dielectric & Crystal structure. The author has an hindex of 3, co-authored 6 publications receiving 74 citations.

Modified crystal structure, dielectric properties, and magnetic phase transition temperature of Ca doped BiFeO3 ceramic

Abstract: This paper deals with the preparation of multiferroic Bi1−x Ca x FeO3(x = 0–0.2) ceramics with the sol–gel method and a study on the influence of Ca2+ doping on the structure, dielectric, and ferromagnetic properties of BiFeO3 ceramics. The result shows that the XRD analysis reveals a phase transition in Ca-doped BiFeO3 from rhombohedral to orthorhombic when x is greater than 0.1. The dielectric constant (e r) of Bi0.9Ca0.1FeO3 measured at 1 kHz is about seven times greater than that of BiFeO3, and Bi0.8Ca0.2FeO3 is less than one-tenth of BiFeO3. It might be understood in terms of the dipole polarization, oxygen vacancy and lattice phase transition. Magnetic measurements show that the M-H of Bi1 − x Ca x FeO3 samples exhibit unsaturated and symmetric magnetic hysteresis loops with the increase of Ca2+, indicating the weakly ferromagnetic behavior. It indicates that there is coexistence of Fe2+ and Fe3+ in Bi1 − x Ca x FeO3 samples according to the XPS spectrum. The ratio of Fe2+/Fe3+ increases with doping Ca2+ content and the magnetic properties of BiFeO3 are enhanced. It is evident that the ferromagnetic phase transition of BiFeO3 samples occurs at 878 K by measuring the M–T and DSC curves. The T N of BiFeO3 will be reduced from 644 to 638 K and the T M does not change slightly at 878 K with increasing Ca2+ content. T N and T M of Bi1 − x Ca x FeO3 change depends mainly on the magnetic structure of relative stability and Fe–O–Fe super-exchange strength.

Modified crystal structure, dielectric and magnetic properties of Cr doped SmFeO3 ceramic

Abstract: The effects of Cr3+ doping on the crystal structure, dielectric and magnetic phase transition of SmFeO3 have been investigated. The XRD pattern analysis shows that all the peaks for SmFe1-xCrxO3 samples can be indexed according to the crystal structure of pure SmFeO3 and has a fine crystal structure. The dielectric constants of SmFe1-xCrxO3 (x = 0.1–0.3) ceramics (er = 2532, 1959, 1172) are only 0.86, 0.67 and 0.41 times that of SmFeO3 (er = 2914) at 1 kHz, respectively. The change of dielectric constant (er) with x is mainly the result of the internal barrier layer capacitor (IBLC) mechanism and the dipole oriented polarization mechanism. It shows that the M-H of SmFeO3 at room temperature exhibits a butterfly shape disappears with the increase of Cr3+ and the magnetic properties is also gradually weakened, indicating that Cr3+ doping can effectively regulate the M-H of SmFeO3. On the M-T curve, it is found that the TSR and TN of SFO decrease from 462 K to 687 K–428 K and 536 K with increasing Cr3+, respectively. It is mainly because Cr3+ doping deforms the lattice structure of SmFeO3, destroys the antiferromagnetic order of Fe3+-O2--Fe3+ and forms the exchange interaction of Fe3+-O2--Cr3+. As a result of the three actions, the stability of the G-type antiferromagnetic structure of SmFeO3 is weakened, and the magnetic transition temperatures TSR and TN are decreased.

High temperature dielectric properties and magnetic behavior of K1–xCaxNbO3 ceramic

Abstract: K1-xCaxNbO3 (x = 0–0.2) ceramic samples were prepared by self-sacrifice method to study the effects of Ca2+ doping on the crystal structure, dielectric properties and magnetic phase transition of KNbO3. XRD analysis showed that all samples belong to a single-phase orthogonal crystal system with a spatial group of Amm2. The valence states of K, Nb and O elements, the atomic proportion in K1-xCaxNbO3 (x = 0–0.2) samples, and the relationship between the binding energies of K, Nb and O and x were studied through XPS. SEM showed that Ca2+ doping had a significant regulation effect on the micromorphology and grain size of KNO samples. Two transitions were observed in er -T of KNbO3, the first transition was at 213 °C and the second transition was at 424 °C, corresponding to the ferroelectric phase transition from the orthogonal phase to the tetragonal phase and from the tetragonal phase to the cubic phase, respectively. The TC decreased from 424 °C to 415 °C with the increase of x. The variation relationship of M-T of K1-xCaxNbO3 (x = 0–0.2) samples within the range of 5-350k was measured. It is found from M-T curve that the M of KNO sample is positive when T Td. This conclusion was confirmed by testing the M-H variation relationship, providing experimental basis for the diamagnetism of KNO at room temperature.

Giant magnetodielectric and enhanced multiferroic properties of Sm doped bismuth ferrite nanoparticles

Abstract: Improvements in magnetodielectric and multiferroic properties are essential for visualizing the real application of multiferroics, precisely, BiFeO3 (BFO). An enhancement of multiferroic and magnetodielectric properties has been achieved for chemically prepared nanocrystalline BFO by virtue of Sm doping. The X-ray diffraction study confirms the growth of single phase nanocrystalline BFO which corroborates TEM observation. The magnetic study delineates the ferromagnetic behavior of nanocrystalline Sm-doped BFO samples even at room temperature, which is absent in pristine samples. Surprisingly, a few orders of magnitude increase in resistivity is observed in Sm doped samples. Room temperature ferroelectric measurement showed that Sm doping improves the polarization significantly. In addition, we have achieved a giant change in the magnetodielectric properties of Sm doped samples which has not been reported so far. Large lattice strain arising due to the mismatch of ionic radii and the decrease of oxygen vacancies combined could play an important role in the enhancement of multiferroic properties of nanocrystalline Sm-doped BFO which is a promising multiferroic material.

Multiferroic properties of Tb-doped BiFeO 3 nanowires

Abstract: Nanoscale, multifunctional, multiferroic materials possess strong magnetoelectric coupling (ME), open exciting multitudinous ways for designing future nanoelectronic and spintronic device applications. Bulk nanowires (100 nm), pure, and Tb-doped BiFeO3 multiferroic nanowires (20 nm) have been synthesized by colloidal dispersion template-assisted technique. The effects of Tb-doping and size of synthesized nanowires on structural, electrical, magnetic, dielectric, and magnetodielectric properties have been investigated. X-ray diffraction study reveals that doping of Tb in BiFeO3 nanowires leads to structural transformation from rhombohedral to orthorhombic. X-ray photoemission analysis confirms the +3 oxidation state of Fe and high purity of samples. Bulk nanowires exhibit antiferromagnetic characteristics, whereas the Tb-doped BiFeO3 nanowires show ferromagnetic character. Moreover, with increase in Tb concentration, the saturation magnetization increases. Temperature-dependent magnetization study suggests their size-dependent ferro and ferri-magnetic behavior. Polarization versus electric field (P–E) study reveals that pure BiFeO3 nanowires possess elliptical loop; however, doping of Tb results in rectangular loop— portentous good ferroelectric properties. All synthesized samples exhibit frequency-dependent dielectric constant which decreases with increase in frequency and remains fairly constant at higher frequencies. Leakage current density decreases with increase in Tb concentration, and has been found to be three orders of magnitude less than those of bulk BiFeO3 nanowires. The ME coupling in synthesized nanowires was estimated by measuring magnetodielectric. A very high value of ME, 7.2 %, has been found for 15 % Tb-doped BiFeO3 nanowires. In this communication, we, for the first time, report new cue on size-dependent Tb-doped BiFeO3 nanowires, which may be further used to explore its technological device applications.