Chaohong Wang

Exchange bias effect in a granular system of NiFe2O4 nanoparticles embedded in an antiferromagnetic NiO matrix

A granular system composed of ferrimagnetic NiFe2O4 nanoparticles, about 8 nm in size, embedded in an antiferromagnetic NiO matrix has been synthesized by a high-temperature phase precipitation method from Fe-doped NiO matrix. Both the exchange bias field and vertical magnetization shift can be observed in this system below 250 K after field cooling, above which the exchange bias disappears. Furthermore, the exchange bias field shows a linear dependence on the magnetization shift. This observed exchange bias effect is explained in terms of the exchange interaction between the ferrimagnetic phase and the spin-glass-like phase at the interface.

Suppression of charge order and exchange bias effect in Nd0.5Ca0.5MnO3 nanocrystalline

An Nd0.5Ca0.5MnO3 (NCMO) sample (average diameter ~45?nm) is synthesized by the sol?gel method. The temperature dependence of magnetization indicates that the charge order state is suppressed and a ferromagnetic (FM) transition occurs at ~100?K. In addition, the magnetic hysteresis loop at 10?K under a cooling field of 10?kOe shifts to both the horizontal and the vertical directions when the measure field is 10?kOe. With an increase in the measure field, both the horizontal and the vertical shifts decrease. When the measure field is 50?kOe, the vertical shift vanishes but the horizontal shift still exists. The observed exchange bias effect is attributed to the exchange coupling between the antiferromagnetic core and the FM shell which embodies spin glass-like surface layers.

Exchange bias and the origin of room-temperature ferromagnetism in Fe-doped NiO bulk samples

A series of Ni(1−x)FexO (x=0, 0.015, 0.03, 0.05, and 0.1) bulk samples was synthesized by the chemical concentration-precipitation method. Phase composition analysis was carried out, which showed that trace amounts of ferromagnetic phase NiFe2O4 could not be detected by x-ray diffraction in these bulk samples with x≤0.03. When x>0.03, NiFe2O4 ferrite is detected easily. The magnetic properties of all the bulk samples were investigated by measuring their magnetization as a function of temperature and magnetic field. The results indicated that all the bulk samples sintered in air exhibited large room-temperature ferromagnetic behavior ascribed to a ferromagnetic impurity phase. Simultaneously, an exchange bias and training effect were also observed in all the bulk samples, suggesting the possibility of the existence of a strong ferromagnetic/antiferromagnetic exchange coupling in this kind of compound. Specifically, the exchange bias field could be tuned by changing the concentration of the Fe dopant.

Synthesis and exchange bias effect of CuFe2O4/NiO nanocomposites

Nanocomposites composed of ferrimagnetic (FM) CuFe2O4 nanoparticles embedded in an antiferromagnetic (AFM) NiO matrix have been synthesized by a new chemical liquid phase route. Microstructural studies indicate that two phases are coexistent in the composites. Magnetic measurements show that the hysteresis loop displays a negative exchange bias behavior with exchange bias field (HEB) about 110xa0Oe at 10xa0K, after the sample was cooled from 300xa0K with a magnetic field of 10xa0kOe. Furthermore, the exchange bias field decreases with increasing temperature and disappears at T≈200xa0K. This exchange bias behavior is interpreted in terms of the magnetic exchange coupling between CuFe2O4 and NiO phases at the FM/AFM interfaces.

Multiple Interfacial Fe3O4@BaTiO3/P(VDF-HFP) Core-Shell-Matrix Films with Internal Barrier Layer Capacitor (IBLC) Effects and High Energy Storage Density

Flexible nanocomposites composed of high dielectric constant fillers and polymer matrix have shown great potential for electrostatic capacitors and energy storage applications. To obtain the composited material with high dielectric constant and high breakdown strength, multi-interfacial composited particles, which composed of conductive cores and insulating shells and possessed the internal barrier layer capacitor (IBLC) effect, were adopted as fillers. Thus, Fe3O4@BaTiO3 core-shell particles were prepared and loaded into the poly(vinylidene fluoride-co-hexafluoropropylene) (P(VDF-HFP)) polymer matrix. As the mass fraction of core-shell fillers increased from 2.5 wt % to 30 wt %, the dielectric constant of the films increased, while the loss tangent remained at a low level (<0.05 at 1 kHz). Both high electric displacement and high electric breakdown strength were achieved in the films with 10 wt % core-shell fillers loaded. The maximum energy storage density of 7.018 J/cm3 was measured at 2350 kV/cm, which shows significant enhancement than those of the pure P(VDF-HFP) films and analogous composited films with converse insulating-conductive core-shell fillers. A Maxwell-Wagner capacitor model was also adopted to interpret the efficiency of IBLC effects on the suppressed loss tangent and the superior breakdown strength. This work explored an effective approach to prepare dielectric nanocomposites for energy storage applications experimentally and theoretically.

Self-assembled core-shell CoFe2O4@BaTiO3 particles loaded P(VDF-HFP) flexible films with excellent magneto-electric effects

Flexible composited multiferroic films with excellent magnetic-electric effects were prepared. The films were composed of self-assembled CoFe2O4@BaTiO3 core-shell nanoparticle fillers and a poly(vinylidene fluoride-co-hexafluoropropene) matrix. The CoFe2O4@BaTiO3 core-shell particles were prepared using a hydrolysis-hydrothermal method. The surface modification of CoFe2O4 with the cationic surfactant cetyltrimethyl ammonium bromide promoted the BaTiO3 nanocrystals self-assembly precipitation. Compared with the films loaded by bare CoFe2O4, the films filled with core-shell particles exhibited reduced leakage current density, enhanced dielectric properties, ferroelectric (FE) polarization, and magneto-electric (ME) effects. An excellent ME voltage coefficient of 1835u2009mV/cmu2009Oe was measured at a DC bias field of −3500u2009Oe and an AC magnetic field of 3.5u2009Oe with a frequency of 45u2009kHz. This enhancement of ME effects was attributed to the large FE-ferromagnetic (FM) interface to volume and the increment of induc...

Enhancement of dielectric and magnetic properties in phase pure, dense BiFeO3 nanoceramics synthesized by spark plasma sintering techniques

Phase pure, dense BiFeO3 (BFO) ceramics with average grain sizes of ~u2009110xa0nm, ~u2009450xa0nm, and ~u20091.15xa0µm were fabricated by spark plasma sintering method. BFO ceramics exhibited grain-size-dependent magnetic properties, which ascribed to the antiferromagnetism–ferromagnetism (AFM–FM) transition. For BFO nanoceramics (~u2009110xa0nm), such transition was much significant, and contributed to a large exchange bias field of HEBu2009=u2009500xa0Oe at 5xa0K. In addition, BFO nanoceramics (~u2009110xa0nm) exhibited lower leakage current and higher resistivity compared to the larger-grained BFO ceramics (~u2009450xa0nm and ~u20091.15xa0µm). The calculated activation energies (Ea) and X-ray photoelectron spectroscopy analyses revealed the existence of different types of defects in BFO ceramics with different grain sizes.