Yueqiang Lin

Field modulation of light transmission through ferrofluid film

The intensity of light transmitted through a thin ferrofluid film could be modulated by an applied magnetic field. After the magnet was switched on, the light transmission coefficient decreased first to a minimum value, then raised up to a stable level, forming a valley in the time section. The stronger the applied magnetic field is, or the higher the particle volume fraction is, the lower the valley value of light transmission coefficient. After the magnetic field was switched off, the final stable value of light transmission coefficient was affected by the remanence of magnet. The relaxation process of light transmission through the ferrofluid film could be repeated by continuously switching on and off the magnet, but the highest transmission values and the lowest transmission values can be stable without drift only after many cycles. The behavior of light transmission coefficient should be related with the particle chains’ forming, lengthening, moving, and breaking in the ferrofluid film.

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.

Preparation of Magnetic Nanoparticles via a Chemically Induced Transition: Presence/Absence of Magnetic Transition on the Treatment Solution Used

The dependence of magnetic transition on the treatment solution used in the preparation of magnetic nanoparticles was investigated using as-prepared products from paramagnetic FeOOH/Mg(OH)2 via a chemically induced transition. Treatment using FeCl3 and CuCl solutions led to a product that showed no magnetic transition, whereas the product after treatment with FeSO4 or FeCl2 solutions showed ferromagnetism. Experiments revealed that the magnetism was caused by the ferrimagnetic γ-Fe2O3 phase in the nanoparticles, which had a coating of ferric compound. This observation suggests that Fe2

The increase of the light transparency induced by a magnetic field for the colloid film based on α–FeOOH nanoparticles

α–FeOOH nanoparticles are spherical and weakly magnetic. The size of the particles is about 8 nm, so they are regarded as Rayleigh scatterers. Aqueous colloids based on these particles exhibit magnetically enhanced transmission of light; the relative transmission coefficient reaches almost 1.3 when H = 500 Oe. Since the magnetic interaction between the particles is too weak to form chain-like aggregates, the enhancing effect is mainly attributed to the variation of the absorption cross-sections of the colloidal system in relation to the coupling of magnetic and dielectric properties of the particles. Along the direction of the external magnetic field, the absorption cross-section of the colloid decreases so that the transmitted light parallel to the field direction is enhanced and increases with the field. The results of this investigation indicate that there could be potential applications for weakly magnetic colloids based on non-cubical nanocrystals.

Preparation of magnetic nanoparticles via chemically induced transition: Dependence of components and magnetization on the concentration of treating solution used

Using an FeOOH/Mg(OH)2 precursor, maghemite-based magnetic nanoparticles can be prepared by a chemically induced transition in an Iron(II) chloride (FeCl2) treating solution. FeCl2 solutions of various concentrations were used to investigate the dependence of sample components and magnetization on the treating solution. The bulk chemical species, crystal structures, surface chemical components, morphologies, and specific magnetizations of the samples were characterized. When the concentration of FeCl2 solution was in a moderate range of 0.060–0.250 M, maghemite nanoparticles coated by hydromolysite, that is, maghemite/hydromolysite nanoparticles, could be prepared. At lower concentrations, below 0.030 M, the samples contained maghemite/hydromolysite and magnesium oxide nanoparticles, and at higher concentrations, up to 1.000 M, the samples contained maghemite/hydromolysite and hydromolysite nanoparticles. The molar and mass percentages of each phase were estimated for each sample. The apparent magnetization behavior of the samples, which exhibited a non-monotonic variation with increasing concentration of FeCl2 solution, is explained from the variation of mass percentage of the maghemite phase with concentration.

Investigation into loss in ferrofluid magnetization

Ferrofluids containing γ-Fe2O3/Ni2O3 nanoparticles (not chemically treated) were synthesized using water and mixed water–glycerol as carrier liquid and the ferrofluid viscosity was modified by varying the glycerol content in the carrier liquid. The apparent magnetization of the ferrofluids decreased with increasing glycerol content. The loss in magnetization is described by the ratio of effective magnetic volume fraction to physical volume fraction of nanoparticles in the ferrofluids as a characteristic parameter. We ascribe the loss to the formation of “dead aggregates” having a ring-like structure of closed magnetic flux rather than to any chemical reaction. Such dead aggregates exist in zero magnetic field and do not contribute to the magnetization in the low or high field regime, so that the effective magnetic volume fraction in the ferrofluids decrease. An increase in carrier liquid viscosity is similar to a weakening of the thermal effect, so the number of dead aggregates increases and the magnetiza...

Preparation of Magnetic Nanoparticles via a Chemically Induced Transition: Role of Treating Solution’s Temperature

Using FeOOH/Mg(OH)2 as precursor and FeCl2 as the treating solution, we prepared γ-Fe2O3 based nanoparticles. The FeCl2 treating solution catalyzes the chemical reactions, dismutation and oxygenation, leading to the formation of products FeCl3 and Fe2O3, respectively. The treating solution (FeCl2) accelerates dehydration of the FeOOH compound in the precursor and transforms it into the initial seed crystallite γ-Fe2O3. Fe2O3 grows epitaxially on the initial seed crystallite γ-Fe2O3. The epitaxial layer has a magnetically silent surface, which does not have any magnetization contribution toward the breaking of crystal symmetry. FeCl3 would be absorbed to form the FeCl3·6H2O surface layer outside the particles to form γ-Fe2O3/FeCl3·6H2O nanoparticles. When the treating solution’s temperature is below 70 °C, the dehydration reaction of FeOOH is incomplete and the as-prepared samples are a mixture of both FeOOH and γ-Fe2O3/FeCl3·6H2O nanoparticles. As the treating solution’s temperature increases from 70 to 90 °C, the contents of both FeCl3·6H2O and the epitaxial Fe2O3 increased in totality.

Preparation of Composite Nanoparticles of Fe-Zn Bioxide Using Surface Modification and Their Subsequent Characterization

During the liquid-phase synthesis of γ-Fe₂O₃ nanoparticles using a chemically induced transition in FeCl₂ solution (400 mL, 0.25 M), the surface modification of the particles was undertaken by adding ZnCl₂ solution, in an attempt to prepare composite nanoparticles. The magnetization, morphology, chemical composition, and crystal structure of the product were characterized using a vibrating sample magnetometer, transmission electron microscopy, X-ray diffraction, energy-dispersive X-ray spectroscopy, and X-ray photoelectron spectroscopy. The experimental results showed that when the concentration of ZnCl₂ solution did not exceed 2 M (50 mL), the γ-Fe₂O₃-ZnFe₂O₄ bioxide composite nanoparticles coated by FeCl₃ · 6H₂O could be prepared. These particles were nearly spherical and their average size was about 11 nm; specific saturation magnetization was about 48 Am²/kg. The molar, mass, and volume percentages of every composition phase, as well as the average density of the composite nanoparticles, were estimated from the characterization results. A particle model with a core that is γ-Fe₂O₃ covered with ZnFe₂O₄ is proposed.

The magneto-optical behaviors modulated by unaggregated system for γ-Fe2O3–ZnFe2O4 binary ferrofluids

Under an external magnetic field, when circularly polarized light was transmitted through binary ferrofluids based on strongly magnetic γ-Fe2O3 and weakly magnetic ZnFe2O4 nanoparticles, the birefringence Δn and dichroism Δk arising from the chains of γ-Fe2O3 particles system were modulated and decreased by the unchained ZnFe2O4 particles. In our experiments, we used two types of ZnFe2O4 nanoparticles: one consisted of ZnFe2O4(1) particles with higher moments, and the other consisted of ZnFe2O4(2) particles with lower moments. Comparing the birefringence and dichroism of the γ-Fe2O3–ZnFe2O4(1) and γ-Fe2O3–ZnFe2O4(2) binary ferrofluids, it was found that the modulating action of the ZnFe2O4(2) particle system with lower moments was larger than that of the ZnFe2O4(1) particle system with higher moments. Using a model for a bi-dispersed system based on chained and unchained particles, the behavior of the modulating action was explained by an additional effective relative magnetic permeability, which depends ...