Search

Memory effect has been studied in the system using magnetic nanoparticles with Ni nanocore encapsulated by non-magnetic and oxidation-resistant Ni2P nanoshell acquired through surface-phosphatizing Ni nanoparticles. The self-assembled array with interparticle spacing of about 6 nm shows memory effect up to 200 K below its average blocking temperature of 260 K. And reducing the interparticle spacing of the self-assembled array via annealing can further enlarge the temperature range of memory effect up to room-temperature. The memory effect can be understood based on the thermal relaxation theory of single-domain magnetic nanoparticles. Furthermore, the read-write magnetic coding is realized based on the temperature changes, using the memory effect up to room-temperature, which may be useful for future memory devices.

Discrete and uniform microporous CoO nanoparticles with open nanochannels around 1 nm were one-pot synthesized by the self gas-leaching method via the thermal decomposition of a Co–oleylamine complex. CoO particle-sizes can be tuned from 50 to 13 nm by controlling the concentration of the cobalt precursor, accompanying a change of the long and winding nanochannels to short and straight nanochannels. It was shown that exposing the particle interiors to external active reactants via the shorter and straighter microporous nanochannels in smaller CoO nanoparticles can greatly enhance their photocatalytic efficiency. Most importantly, all the as-synthesized microporous CoO nanoparticles showed a very highly reversible capacity and cycling stability for lithium storage. The discharge and charge capacities of the microporous CoO sample with short straight nanochannels and the smallest particle size (1432.8 and 1200 mA h g−1, respectively) are up to two times higher than those of the commercial CoO powder (673.7 and 539 mA h g−1, respectively) and that of the theoretical value of CoO (715 mA h g−1) owing to the enlarged surface area, very small particle size for increased electrode and electrolyte contact and the heightened diffusion efficiency in short nanochannels for electrolyte and Li ions. The presence of microporous voids could effectively buffer the stress induced during lithium insertion–deinsertion alleviating the pulverization of electrode material, thereby giving extraordinary cycling stability.

Single phase Bi1/2Na1/2Cu3Ti4O12 (BNCTO) ceramics with different grain sizes (1.4–4.3 μm) are prepared by a modified Pechini method to investigate their giant dielectric and nonlinear electrical behaviors. The results show that the giant dielectric and nonlinear electrical behaviors are strongly dependent on grain size. With the increment of grain size, the dielectric constant increases monotonically from 14110 (for 1.4 μm sample) to 36183 (for 4.3 μm sample) at 1 kHz, in accompaniment with the breakdown voltage reducing from 112.5 to 43.2 V/mm and the nonlinear coefficient reducing from 4.9 to 3.4. On the basis of the internal barrier layer capacitor (IBLC) model and the IBLC model of Schottky-type potential barrier, an interpretation of the grain size effect on the giant dielectric and nonlinear electrical behaviors is presented.

Bi1/2−xNa1/2−xBa2xCu3Ti4O12 (x = 0, 0.025, 0.05, 0.075) polycrystalline ceramics are prepared by a modified Pechini method. Effect of Ba doping on the dielectric and nonlinear electrical behaviors are investigated in detail. All the samples exhibit giant dielectric effect and nonlinear electrical behavior. Compared with Bi1/2Na1/2Cu3Ti4O12 ceramics, the permittivity drops down and the nonlinear electrical behavior shifts to low voltage area by Ba doping. The permittivity reduces from 20,004 to 7,006 at 1 kHz. The breakdown voltage and nonlinear coefficient values decrease to 10.8 V/mm and 1.5, respectively. Additionally, the electrostatic barrier height drops down to 0.66 eV. Giant dielectric response and nonlinear electrical behavior can be interpreted in terms of internal barrier layer capacitors model. These results imply that Ba substitution can adjust the dielectric and nonlinear electrical behaviors of Bi1/2Na1/2Cu3Ti4O12 ceramics, which is a promising candidate for low voltage varistor materials.

A series of multiferroic (1− x )BiFeO 3 − x (Bi 0.5 Na 0.5 )TiO 3 (BF–BNT) ( x = 0 − 0.6) solid solution ceramics were prepared by a sol–gel method. The XRD results show that increasing BNT content induce a gradual phase transformation from rhombohedral to pseudocubic structure near x = 0.4. Compared with pure BiFeO 3 , superior multiferroic properties are obtained for x = 0.3 with remnant polarization P r = 1.49 μC/cm 2 and saturated magnetization M s = 0.51 emu/g. Importantly, the paramagnetic (PM) to ferromagnetic (FM) transition is observed for the solutions, and the Curie temperature ( T C ) can be tuned by varying the content of BNT. This observed FM ordering is discussed in terms of the possible existence of the long-range superexchange interaction of Fe 3+ –O–Ti–O–Fe 3+ in the chemically ordered regions.

The structural, magnetic and ferroelectric properties of the (1-x)BiFeO3-xSrTiO3 (0 ≤ x ≤ 1.0) solid solutions synthesized by a modified Pechini method are investigated. Structure phase transition from rhombohedral to cubic phase was found with x increasing from the X-ray diffraction pattern. Compared with antiferromagnetic BiFeO3 compound, BFST-x (0.1 ≤ x ≤ 0.4) ceramics show ferromagnetism at room temperature, with their remnant magnetization (Mr) and coercive field (HC) strongly dependent on the content of SrTiO3. Moreover, appropriate SrTiO3 solution with BiFeO3 significantly reduces electric leakage and enhances electrical polarization. Among all the samples, the x = 0.2 samples shows the optimal ferromagnetism with Mr ～ 0.10 emu/g and the x = 0.3 samples the optimal ferroelectricity with Pr ～ 7.3 μC/cm2 at room temperature.

The structure, ferroelectric and magnetic properties of (1 − x )BiFeO 3 – x Bi 0.5 Na 0.5 TiO 3 ( x = 0.37) solid solution fabricated by a sol–gel method have been investigated. X-ray diffraction and Raman spectroscopy measurements show a single-phase perovskite structure with no impurities identified. Compared with pure BiFeO 3 , the coexistence of ferroelectricity and ferrimagnetism have been observed at room temperature for the solution with remnant polarization P r = 1.41 μC/cm 2 and remnant magnetization M r = 0.054 emu/g. Importantly, a magnetic transition from ferrimagnetic (FM) ordering to paramagnetic (PM) state is observed, with Curie temperature T C ∼ 330 K, being explained in terms of the suppression of cycloid spin configuration by the structural distortion.

A granular system composed of ferromagnetic NiFe 2 O 4 nanoparticles embedded in the Cu diluted NiO matrix has been synthesized by the chemical concentration precipitation method. Phase composition analysis shows that Cu ions have been successfully incorporated into NiO matrix substituting for Ni ions. The magnetic properties of all the bulk samples were investigated by measuring their magnetization as a function of temperature and magnetic field. The field-cooling hysteresis loop shift was observed after cooling the nanocomposites. With the increase of the Cu dilution ratio, the saturation magnetization first increases, and then decreases after reaching a maximum. The exchange bias field gradually decreases with the Cu dilution ratio, reaching a plateau when x ≥ 0.05. These results are tentatively interpretated by the misalignment of magnetic moments between the ferromagnetic and the antiferromagnetic spins. The decrease of the coupling area between the ferromagnetic phase and the antiferromagnetic phase at the interface is another possible reason.

La 0.4 Sr 0.6 Ti 1− x Ni x O 3−δ powders with x =0 and 2% were synthesized by the sol–gel method and characterized by a variety of techniques. We mainly investigate the effect of annealing conditions on the structural and magnetic properties of La 0.4 Sr 0.6 Ti 1− x Ni x O 3−δ powders. X-ray diffraction and transmission electron microscopy studies indicate all samples annealed at high temperature ( T >873 K) are single phase and no secondary magnetic phase in this study. By the commercial physical properties measurements system (PPMS), the sample with x =0 exhibits diamagnetic behavior, whereas all the samples with x =2% are found to be ferromagnetic even at room temperature. With the increase of annealing temperature, the ferromagnetism of the samples increases initially, and then it decreases after reaching the maximum value. The structural defects such as oxygen vacancies mediated mechanism is reasonable explanation of the magnetic properties in the present results.

Organic solution-phase reaction under oxygen atmosphere for synthesis of single crystalline and uniform sub-3 nm NiO nanoparticles has been developed. Larger NiO nanoparticles with diameter about 26 nm can be also prepared by seed-mediated growth method. Optical property of the NiO nanoparticles was conducted by means of UV–vis spectrum, exhibiting obviously blue shift compared with the bulk counterpart due to the quantum confinement effect. The photocatalytic activity in degradation of rhodamine B was also investigated. The results showed that the sub-3 nm NiO nanoparticles exhibit much higher activity for UV-photodegradation than the as-synthesized larger NiO particles and commercial 30 nm nanopowder.

The Fe-doped CuO nanopowders (Cu0.95Fe0.05O) were prepared by the sol–gel and the co-precipitation methods. The effects of synthesis methods on the structure, magnetic property and exchange bias effect of Cu0.95Fe0.05O nanopowders were investigated. The microstructure and magnetic properties of the two types of nanopowders were characterized by X-ray diffraction and physics property measuring systems, respectively. It was found that the sol–gel method tended to lead the formation of CuFe2O4 phase, which dominated the magnetic behaviors of the sample; the co-precipitation tended to lead larger exchange bias field at low temperature in samples, comparing with sol–gel method.

Monodisperse NiO nanocrystals with an average particle size of 3 ± 0.4 nm are successfully synthesized by the thermal decomposition of Ni-oleylamine complex in an organic solvent under a continuous O2 flux. The crystalline structure and the morphology of the product are investigated by X-ray diffraction, X-ray photoelectron spectroscopy, and transmission electron microscopy. Magnetization and alternating-current (ac) susceptibility measurements indicate that the structure of the particles can be considered as consisting of an antiferromagnetically ordered core and a spin-glass-like surface shell. In addition, both the exchange bias field and the vertical magnetization shift can be observed in this system at 10 K after field cooling. This observed exchange bias effect is explained in terms of the exchange interaction between the antiferromagnetic core and the spin-glass-like shell.

The training effect has been systematically studied in exchange coupled NiFe2O4/NiO nanogranular system. Both exchange bias field (HEB) and vertical magnetization shifts (MShift) can be observed after the system field cooled from 350 K to low temperatures, which decrease monotonically through consecutive loop cycles. During this procedure, linear dependence between HEB and MShift is found for this system, revealing the critical role of the pinned uncompensated spins. With the increase of cooling field, the relative change of HEB becomes more pronounced, which shows that the rapid reduction of the pinned uncompensated spins for high cooling field. Moreover, the reduction of HEB becomes weakened with decreasing measured temperatures, which indicated the spin configuration at low temperatures possesses higher dynamic stability. The cooling field and temperature dependence on training effect is discussed in terms of the evolution of the metastable spin configurations at the interfaces and fitted by a recent t...

A nanogranular system of multiferroic CoCr2O4 nanoparticles embedded in an antiferromagnetic Cr2O3 matrix has been synthesized through a high-temperature phase segregation route from a Co-doped Cr2O3 matrix. Magnetic studies show that exchange bias fields (HEB) accompanying vertical magnetization shifts (MShift) are observed at low temperatures after field cooled from 350 K. The corresponding exchange bias field can be as large as 1420 Oe, and the vertical magnetization shift reaches 0.116 emu/g at 10 K. The exchange bias field decreases with temperature increasing and disappears at T ≈ 70 K, while the coercive field (HC) initially increases with the temperature up to 40 K, and thereafter, it decreases to zero at 100 K. This exchange bias behavior is discussed in terms of the existence of exchange coupling between the ferrimagnetic CoCr2O4 core and spin glass-like phase at the interfaces.

Structural, magnetic, and ferroelectric properties have been investigated for Bi2Fe4(1−x)Ti4xO9 (0≤x≤0.2) bulk ceramics, which were synthesized by a modified Pechini method. X-ray diffraction reveals that all samples are single phase with no impurities detected. Compared with antiferromagnetic Bi2Fe4O9 compound, doping with Ti ions induces the appearance of weak ferromagnetism at room temperature, which is discussed in terms of the collapse of the frustrated antiferromagnetic spin structure. Moreover, appropriate Ti doping also significantly reduces electric leakage and leads to the enhancement of electrical polarization. Among all samples, the optimal multiferroics with Mr∼0.0188 emu/g and Pr∼0.262 μC/cm2 at room temperature is found for x=0.15 ceramics. It is thus shown that Ti-doped Bi2Fe4O9 is a promising candidate for preparing multiferroic materials.

A series of Cu1−xFexO (x=0.10, 0.15, 0.20, and 0.30) powder samples were synthesized by a coprecipitation method. The exchange bias field (HEB) accompanying vertical magnetization shift is observed in the system at low temperatures, after the sample is cooled from 300 to 10 K under 10 kOe magnetic field. The exchange bias effect has been investigated for Cu1−xFexO with different doping concentration. Although the magnetic properties increases with the increasing doping concentration, the HEB and vertical magnetization shift vary nonmonotonously. The significant difference is indicated the exchange bias effect can be controlled by tuning the doping concentration for altering coupling interaction at interface layers. Furthermore, the exchange bias field shows a linear dependence on the vertical shift. The exchange coupling at the interface between the ferromagnetic phase and the spin-glass-like phase (or antiferromagnetic) can explain these phenomenon.

Effect of particle size on exchange bias in Fe-doped CuO nanoparticles is investigated, which are sintered at different temperatures from 350 to 650 °C, respectively. The structure and magnetic properties for different particle size samples were probed. It is found that the system shows magnetic properties transition from paramagnetic to ferromagnetic with increasing grain size, and exhibits the variations in exchange bias field (HEB) and coercivity (HC) at low temperature after field-cooled from 300 K. With the increase in the particles size, HEB decreases monotonously. Furthermore, vertical magnetization shift was also observed for the small particles. Exchange bias is attributed to the exchange coupling interactions between ferromagnetic and spin-glass-like (or antiferromagnetic) phase interface layers.

Exchange bias nanocomposites were obtained by the chemical concentration precipitation method, in which the ferrimagnetic MFe2O4 (M=Cu,Ni) particles were embedded in the antiferromagnetic (AFM) CuO matrix. The dependence of magnetization on temperature measurements show that the exchange bias effect in these composites is ascribed to the exchange coupling at the interface between the ferrimagnetic particles and spin-glass-like phase. With continuous introduction of magnetic Ni ions, the existence of domain state structure and the formation of soft magnetic phase in AFM matrix are responsible for the different behaviors of the exchange bias field and coercivity in these nanocomposites.