Tingzhen Zhang

The study of transition on NiFe2O4 nanoparticles prepared by co-precipitation/calcination

In this study, the NiFe2O4 nanoparticles have been prepared by co-precipitation and calcination process. Using a vibrating sample magnetometer (VSM), transmission electron microscopy (TEM), X-ray diffraction (XRD), energy dispersive spectrometer of X-ray (EDX), and X-ray photoelectron spectroscopy (XPS), the samples obtained by co-precipitation and then by further calcination have been analyzed. The experimental results show that the precursor synthesized by co-precipitation is the composite of both amorphous FeOOH and Ni(OH)2, but has no amorphous NiFe2O4. The results of both EDX and XPS revealed that the FeOOH species is wrapped up by Ni(OH)2 species. In the calcination process, the amorphous composite is dehydrated and transformed gradually into crystalline NiFe2O4 nanoparticles, with the metal ions diffusing. The reaction is different from the one used to prepare other ferrite (e.g., CoFe2O4, MnFe2O4, Fe3O4, etc.) nanoparticles directly by co-precipitation. With increasing calcination temperature, the NiFe2O4 grains grow and the magnetization is enhanced.

Saturation Magnetization and Law of Approach to Saturation for Self-formed Ionic Ferrofluids Based on MnFe2O4 Nanoparticles

The magnetization curves of MnFe2O4 nanoparticles and self-formed ferrofluids based on these particles have been measured at room temperature. The median size of the particles is 13.67 nm. The specific saturation magnetization is less than the theoretical value for the ferrofluids. In the high field range from 5 kOe to 10 kOe, the higher the particle volume fraction is, the steeper the slope of the magnetization curves is when it approaches saturation. The behavior of the saturation magnetization and the law of approach to saturation are due to the presence of self-assembled aggregates of ring-like micelle structures which form in the absence of the magnetic field and field-induced aggregates, respectively. The field-induced aggregates have a dissipative structure, so that at high field, the law of approach to saturation magnetization is different from the one described using Langevin paramagnetism theory. The large particles in the ferrofluids result in apparent hysteresis.

Study of magnetisation behaviours for binary ionic ferrofluids

The magnetisation curves of ionic fluids of three types (CoFe2O4 ferrofluids, p-NiFe2O4 paramagnetic fluids and CoFe2O4–p-NiFe2O4 ferrofluids prepared by Massart method) are measured at room temperature and their magnetisation behaviours are studied. Comparison of the experimental data of CoFe2O4 ferrofluids with the Langevin theory curve demonstrates a considerable difference between them, but the curve fitted by model of a gas-like compression agrees with experimental data very well. The experimental results show that the magnetisation of the CoFe2O4–p-NiFe2O4 binary ferrofluids is larger than the sum of the magnetisation of the two single ferrofluids in high field. The magnetisation behaviour of the binary ferrofluids is explained by self-assembled ring-like aggregates of CoFe2O4 ferrofluids particles breaking.

Magnetisation behaviour of mixtures of ferrofluids and paramagnetic fluids with same particle volume fractions

In this study, γ-Fe2O3 ferrimagnetic nanoparticles and paramagnetic nanoparticles of p-MgFe2O4 (a hydroxide precursor for the preparation of magnesium ferrite materials) are produced by chemical precipitation technology. The γ-Fe2O3 ferrofluids and p-MgFe2O4 paramagnetic fluids are synthesised by Massarts method. The binary ferrofluids are obtained by mixing the ferrofluids and the paramagnetic fluids. There is insufficient magnetic interaction to aggregate the γ-Fe2O3 ferrimagnetic system and the p-MgFe2O4 paramagnetic system, so the magnetisation behaviour of the binary ferrofluids can be explored with reference to those of the single fluids. The magnetisation behaviour of single γ-Fe2O3 ferrofluids may be described by a model of gas-like compression. In the absence of a magnetic field, some particles can self-assemble into aggregates with a closed ring-like structure which make no contribution to the magnetisation of the γ-Fe2O3 ferrofluids. These ring-like aggregates result in the measured saturation magnetisation of the γ-Fe2O3 ferrofluids being smaller than the theoretical value calculated from the particles. During the magnetisation process, the polarised p-MgFe2O4 particles gas can orient the rings towards the direction of the field, so that the rings may fragment. Therefore, the measured saturation magnetisation of the γ-Fe2O3 ferrofluid component of the binary ferrofluids strengthens and the magnetisation process becomes easier than for pure γ-Fe2O3 ferrofluids.

Ultra-low voltage resistive switching of HfO2 buffered (001) epitaxial NiO films deposited on metal seed layers

A set of (001) epitaxial NiO films were prepared on highly textured (001) Pt seed layers using magnetron sputtering, and their resistive switching performance was measured. Cube-to-cube epitaxial relationships of NiO(001)//Pt(001) and NiO[001]//Pt[001] were demonstrated. Current-voltage measurements revealed that the Ag/(001)NiO/(001)Pt capacitor structures exhibited stable bipolar switching behavior with an ON/OFF ratio of 20 and an endurance of over 5 × 103 cycles. Furthermore, inserting a HfO2 buffer layer between the NiO film and the Ag top electrode increased the ON/OFF ratio to more than 103 and reduced the SET/RESET voltage to below ±0.2 V. These enhancements are attributed to the differing filament growth mechanisms that occur in the NiO and HfO2 layers. The present work suggests that Ag/HfO2/(001)NiO/(001)Pt capacitor structures are a promising technology for next-generation, ultra-low voltage resistive switching memory.