Guanghui Rao

Enhanced dielectric and magnetic properties in Ru-substituted Bi0.9La0.1FeO3 ceramics

Multiferroic ceramics Bi0.9La0.1Fe1-xRuxO3 (x = 0%, 1% and 3%) were prepared using a solid-state reaction process. The structural transformation from rhombohedral to pseudocubic was confirmed in the Ru-doped samples via x-ray diffraction investigation, and the improved dielectric properties are ascribed to the morphotropic phase boundary and decreased oxygen defects existing in the Ru-substituted samples. The enhancement of magnetization is interpreted by the increased Fe3+ ions and the formation of a local ferrimagnetic structure in the Ru-doped samples.

The abnormal electrical and optical properties in Na and Ni codoped BiFeO3 nanoparticles

Bi0.97Na0.03Fe1−xNixO3 (xu2009=u20090, 0.005, 0.01, 0.015) nanoparticles are prepared via a sol-gel method. Weak ferromagnetism and exchange bias phenomenon without field cooling are observed in the samples. The oxygen vacancy concentration and leakage current density are increased with increasing the Ni content. However, with the increase of Ni content, the band gap of Bi0.97Na0.03Fe1−xNixO3 nanoparticles first decreases and then increases. To explain the abnormal phenomenon, the interplay of oxygen vacancy donor and hole acceptor is analyzed and a phenomenological qualitative model based on the electronic energy band is proposed. Additionally, the threshold switching behavior appears in Bi0.97Na0.03Fe1−xNixO3 samples with xu2009=u20090.01, 0.015 and the effect is qualitatively explained by introducing a conducting channel model based on the high-density mobile charges.

Unusual magnetic behaviors and electrical properties of Nd-doped BiFeO3 nanoparticles calcined at different temperatures

Bi1−xNdxFeO3 (xxa0=xa00, 0.05, 0.1, 0.15, 0.2) nanoparticles (about 20–50xa0nm) calcined at 500 and 600xa0°C, respectively, were prepared by an ethylene glycol-based sol–gel. The XRD analysis reveals that the BiFeO3 samples are in single phase, and their crystal structure is varied with the Nd content. Due to the small particle size, the uncompensated spin moments on the surface and the suppression of spin helical ordering structure result in a ferromagnetic phase of the BiFeO3 nanoparticles. The magnetization of the Nd-doped samples calcined at 600xa0°C is improved with the increase of Nd content, but for the Nd-doped samples calcined at 500xa0°C, it shows an opposite trend, which is ascribed to the interplay of size effect and the ratio of Fe2+:Fe3+ of samples calcined at different temperatures via XPS analysis. The dielectric properties were measured and analyzed for the samples calcined at 500 and 600xa0°C. Moreover, the leakage current value of the Bi1−xNdxFeO3 samples can be modulated by the Nd doping, and it reaches a minimum at the Nd content around 0.1.

The effect of Fe–O–Fe bond angle on modulating multiferroic properties of Ba–K-codoped BiFeO3 nanoparticles

Bi0.9−xBa0.1KxFeO3 (xxa0=xa00, 0.01, 0.02) nanoparticles have been successfully prepared using sol–gel method. X-ray diffraction data revealed rhombohedral distorted perovskite structure for all samples. The magnetization and the leakage current density both increased with K doping. Space charge limited conduction was found to be the dominant mechanism in whole electric field for all the samples investigated by plotting log J versus log E. In addition, the band gaps of all nanoparticles were investigated and the values showed no obvious change with the increase of K content due to the interplay of oxygen vacancies and bond angle of Fe–O–Fe. The dielectric properties including the dielectric constant and loss were also investigated.

Switchable Ferroelectric Diode Effect and Piezoelectric Properties of Bi0.9La0.1FeO3 Ceramics

Multiferroic ceramics Bi0.9La0.1FeO3 are synthesized by solid-state reactions and sintered at various temperatures. The rhombohedral structure with the space group R3c is confirmed by means of x-ray diffraction, and their multiferroic properties are investigated. Bi0.9La0.1FeO3 ceramics sintered at 880 degrees C are found to have the lowest leakage current density and the largest saturated polarization among all the samples. A diode-like current-voltage hysteresis that could be switched by an external voltage is observed in the Bi0.9La0.1FeO3 ceramics. A typical butterfly shaped strain-versus-voltage curve is shown with a maximum strain of 0.09% at 7 kV. Room-temperature magnetization exhibits a hysteresis loop, indicating that the modulation of the spin structure of BiFeO3 has been suppressed.

Abnormal variation of band gap in Zn doped Bi0.9La0.1FeO3 nanoparticles: Role of Fe-O-Fe bond angle and Fe-O bond anisotropy

Bi0.9La0.1FeO3 (BLFO) and Bi0.9La0.1Fe0.99Zn0.01O3 (BLFZO) nanoparticles were prepared via a sol-gel method. The oxygen vacancies and holes increase with Zn doping analyzed through X-ray photoelectron spectroscopy, which could contribute to the increase of leakage current density. However, with the increase of the defects (oxygen vacancies and holes), the band gap of BLFZO also is increased. To explain the abnormal phenomenon, the bandwidth of occupied and unoccupied bands was analyzed based on the structural symmetry driven by the Fe-O-Fe bond angle and Fe-O bond anisotropy.

Room temperature threshold switching behaviors of Bi0.9Nd0.1Fe1−xCoxO3 nanoparticles

Many investigations have reported threshold switching (TS) effects in amorphous semiconductors, but it is rarely observed in BiFeO3 nanoparticles. Here, the effects of cobalt doping on the electrical conductivity of Bi0.9Nd0.1Fe1−xCoxO3 (x = 0, 0.01, 0.03, 0.05) nanoparticles were investigated. Leakage current density (J) versus applied voltage (V) curves were measured and it was found that the leakage current increases with increases of the cobalt dopant concentration. With the increase of the maximal applied voltage, the samples with cobalt doping exhibit nearly symmetric TS behaviors at room temperature. With the increase of cobalt dopant concentration, the optical band gap is decreased, and the smallest optical band gap value (∼2.01 eV) is observed for the 3% Co doped BiFeO3 sample. X-ray photoelectron spectroscopy of the samples shows that the cobalt dopant induces an enhancement of the oxygen vacancy concentration. Possible mechanisms for the threshold switching effects are discussed on the basis of a conductive filament model. The mobile charge carriers align to form conducting filaments when the threshold voltage (VTH) is achieved, corresponding to the transition from a high-resistance state to a low-resistance state, and the rupture of the conductive path takes place when the applied voltage is decreased. The leakage current decreases dramatically and eventually returns to the initial high-resistance state.

Novel optical and magnetic properties of Li-doped quasi-2D manganate Ca3Mn2O7 particles

Ruddlesden–Popper (RP) structural Li2xCa3−2xMn2O7 (x = 0–0.05) particles were successfully synthesized via a citrate sol–gel method. At higher temperatures quasi-two-dimensional antiferromagnetic fluctuation effects were observed and antiferromagnetic order developed at ∼120 K, but with weak ferromagnetism. Li doping has a positive influence on enhancing the magnetism of the system. Oxygen vacancies in crystals have an important impact on the electronic properties. Interestingly, a small amount of Li substitution markedly decreases oxygen vacancies, which hence introduces disorder and leads to widening of the band gap. The obtained value of the band gap from UV-vis measurements is ∼2.88 eV which is much larger than the previous theory calculation. To appropriately correct the band gap, a more reliable method of the hybrid functional was used to investigate the electronic properties and structure of Ca3Mn2O7, which gave a more approximate value.

Antiferromagnetic ordering in spin-chain multiferroic Gd2BaNiO5 studied by electronic spin resonance

High-field electron spin resonance (ESR) has been employed to study the antiferromagnetic (AFM) ordering state (Tu2009<u2009TNu2009=u200955u2009K) of spin-chain multiferroic Gd2BaNiO5. The spin reorientation at TSRu2009=u200924u2009K is well characterized by the temperature-dependent ESR spectra. The magnetization data evidence a field-induced spin-flop transition at 2u2009K. The frequency-field relationship of the ESR data can be explained by conventional AFM resonance theory with uniaxial anisotropy, in good agreement with magnetization data. Related discussion on zero-field spin gap is presented.

Magnetization, ESR, and giant magnetocaloric effects in nanocrystals of Haldane-chain compound Gd2BaNiO5

The magnetization and electron spin resonance (ESR) in nanocrystals of Haldane-chain antiferromagnet Gd2BaNiO5 have been investigated. It is revealed that a reduction in crystal size results in an enhancement of magnetization due to a large number of paramagnetic Gd3+ and Ni2+ ions forming on the surfaces of nanocrystals. The smallest nanoparticles with an average size of 45u2009nm behave like a paramagnet, as evidenced by our ESR data. Upon application of an external magnetic field, the weakly coupled spins can be well aligned along the direction of the magnetic field, giving rise to a giant entropy change of −ΔSm = 36u2009J⋅kg−1⋅K−1 at 2u2009K in a field range of 0–7u2009T. This value is larger than those of most rare-earth-based compounds reported. The large value of −ΔSm, together with the absence of thermal and field hysteresis, makes Gd2BaNiO5 nanocrystals very promising candidates for low-temperature magnetic refrigeration.