A macroscopic magneto-optical response resulting from local effects in ferronematic liquid crystals.

This study investigates the magneto-optical response of liquid crystals (LCs) with planar anchoring in the presence of γ-Fe2O3 magnetic nanoparticles (MNPs). This research demonstrates the formation of novel magnetic composite chains of LCs wrapped around γ-Fe2O3 MNP chains within the LC matrix under an applied magnetic field. These composite chains exhibit a distinct magneto-optical response, characterized by changes in birefringence and dichroism as the magnetic field direction is altered. Based on experimental findings, a two-subsystem model and an effective volume fraction of composite chains are proposed to describe the magneto-optical behavior of the γ-Fe2O3 MNP-doped LCs. The first subsystem comprises the LC matrix, which retains its inherent anisotropic optical properties and does not respond to the applied magnetic field. The second subsystem consists of the magnetic composite chains, which exhibit a distinct magneto-optical response due to their rotational alignment with the magnetic field. The difference in absorbance, 2αdd, which corresponds to dichroism, decreases with increasing magnetic field angle Θ, indicating a corresponding change in dichroism. This interplay between the two subsystems leads to the macroscopic magneto-optical response observed in the γ-Fe2O3 MNP-doped LCs. Due to the stability of the composite chains, the magneto-optical response is stable and can be reversed.

Study of optical rotation generated by the twisted nematic liquid crystal film: based on circular birefringence effect.

The optical behavior of twisted nematic liquid crystals (TNLCs) is revealed through an angular scanning technique. Experimental results show that the optical rotation and degree of polarization of transmitted light are dependent on the polarization direction of incident light. The optical rotation is reciprocal, i.e., the polarization direction of incident and transmitted light can reciprocate when optical rotation is π/2. In some cases, the optical rotation is zero. The orientation of alignment layers in the TN cell can be determined from the behavior of optical rotation, which agrees with the measurement by an atomic force microscope. The experimental results are explained with the model of circularly polarized light based on the circular birefringence effect. Linearly polarized incident light is the superposition of right- and left-handed circularly polarized light. The propagation velocity of circularly polarized light in the LC is relevant to the polarization direction of incident light, so that the refractive indices of left- and right-handed circularly polarized light, n- and n+, or circular birefringence Δn(=n--n+) are not constants. As a result, when a linearly polarized light with the wavelength λ propagates through a TN cell with the cell gap l, the polarization direction of transmitted light is rotated to an angle Δθ. The optical rotation Δθ(=π(n--n+)l/λ) is dependent on the polarization direction of incident light, whereas the averaged refractive index ⟨n⟩(=(n-+n+)/2) can be independent of that. The incident light is partially linearly polarized light in our experiments, so that the degree of polarization of transmitted light varies with the polarization direction of incident light because the optical rotatory rates for the primary and secondary light beams are different.

The extrinsic hysteresis behavior of dilute binary ferrofluids.

We report on the magnetization behavior of dilute binary ferrofluids based on γ-Fe(2)O(3)/Ni(2)O(3) composite nanoparticles (A particles), with diameter about 11 nm, and ferrihydrite (Fe(5)O(7)(OH) ・4H2O) nanoparticles (B particles), with diameter about 6 nm. The results show that for the binary ferrofluids with A-particle volume fraction φ(A) = 0.2% and B-particle volume fractions φ(B) = 0.1% and φ(B) = 0.6%, the magnetization curves exhibit quasi-magnetic hysteresis behavior. The demagnetizing curves coincide with the magnetizing curves at high fields. However, for single γ-Fe(2)O(3)/Ni(2)O(3) ferrofluids with φ(A) = 0.2% and binary ferrofluids with φ(A) = 0.2% and φ(B) = 1.0%, the magnetization curves do not behave in this way. Additionally, at high field (750 kA/m), the binary ferrofluid with φ(B) = 1.0% has the smallest magnetization. From the model-of-chain theory, the extrinsic hysteresis behavior of these samples is attributed to the field-induced effects of pre-existing A particle chains, which involve both Brownian rotation of the chains'moments and a Néel rotation of the particles' moments in the chains. The loss of magnetization for the ferrofluids with φ(B) = 1.0% is attributed to pre-existing ring-like A-particle aggregates. These magnetization behaviors of the dilute binary ferrofluids not only depend on features of the strongly magnetic A-particle system, but also modifications of the weaker magnetic B-particle system.

Nanotechnology for purifying nematic liquid crystals based on magnetic separation accompanied by phase transition.

Free ions are generally unfavorable in liquid crystal (LC) displays, and LC purification technologies are critically important. The colloidal γ-Fe2O3 magnetic nanoparticles coated with oleic acid (γ-Fe2O3@OA MNPs) have a high ratio of surface to volume, which may adsorb more free ions and are uniform in the LC at room temperature. In this work, the precipitation and separation of the doped colloidal γ-Fe2O3@OA MNPs resulting from the magnetic field accompanied by an isotropic-nematic phase transition are more efficient than in the single case of the phase transition or the magnetic field. The residual ion concentrations have decreased distinctly using the low gradient magnetic field (∇ B âˆ¼ 2 T/m) with the phase transition. In addition, when the doped colloidal γ-Fe2O3@OA MNPs are 0.4 % and 0.2 % by weight, the former concentrations of the residual ions and γ-Fe2O3@OA MNPs are lower than the latter. As a result, the commercial nematic LC can be purified by this approach based on nanotechnology in our study.

Oscillatory-like relaxation behavior of light transmitted through ferrofluids.

An oscillatory-like relaxation process in which there are two valleys in the T-t curve is observed when light is transmitted through binary ferrofluids composed of both ferrimagnetic CoFe(2)O(4) nanoparticles and paramagnetic p-MgFe(2)O(4) nanoparticles in the presence of a high magnetic field and through pure (single) CoFe(2)O(4) ferrofluids in a low magnetic field. This relaxation behavior is explained using a model of a bidispersed system based on both chained and unchained particles. In such a bidispersed system, the variation of the transmitted light results mainly from the motion of the chains, with the polarized unchained particles' gas producing the modulation effect. The oscillatory-like relaxation phenomenon depends on the features of both the chained and unchained particle systems. If either the particle volume fraction of chained particles or of unchained particles is very low, or the degree of polarization of the unchained particles gas is very weak, a simple nonlinear relaxation process, giving only a valley in the T-t curve, will appear for the transmitted light. For pure CoFe(2)O(4) ferrofluids, the number of chained and unchained particles does not remain constant under different values of the magnetic field. According to the analysis of the relaxation behavior of transmitted light, it is known that binary ferrofluids based on strong magnetic CoFe(2)O(4) particles and weak magnetic p-MgFe(2)O(4) particles can be much closer to the theoretical bidispersed system than single ferrofluids containing only strong magnetic particles.

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 γ-Fe2O3/ZnFe2O4 nanoparticles by enhancement of surface modification with NaOH.

BACKGROUND: During liquid-phase synthesis of γ-Fe2O3 nanoparticles by chemically induced transition in FeCl2 solution, enhancement of surface modification by adding ZnCl2 was attempted by using NaOH. By using transmission electron microscopy, X-ray diffraction, X-ray photoelectron spectroscopy, energy-dispersive X-ray spectrometry, and vibrating sample magnetometry, the dependence of the synthesis on the amount of additional NaOH was studied. RESULTS: The experimental results show that the surface of the γ-Fe2O3 nanoparticles could be modified by adding ZnCl2 to form composite nanoparticles with γ-Fe2O3/ZnFe2O4 ferrite core coated with Zn(OH)2 and adsorbed FeCl3, and that modification could be enhanced by adding NaOH. CONCLUSIONS: In the experimental conditions, when the concentration of additional NaOH was below 0.70 M, the amounts of ZnFe2O4 and Zn(OH)2 phases increased slightly and that of adsorbed FeCl3 was unchanged. When the concentration of NaOH exceeded 0.70 M, the amount of FeCl3, ZnFe2O4, and Zn(OH)2 increased.

The dependence of Ni-Fe bioxide composites nanoparticles on the FeCl2 solution used.

BACKGROUND: Ni2O3- γ-Fe2O3 composite nanoparticles coated with a layer of 2FeCl3·5H2O can be prepared by co-precipitation and processing in FeCl2 solution. Using vibrating sample magnetometer (VSM), X-ray diffraction (XRD), transmission electron microscopy (TEM) and X-ray photoelectron spectroscopy (XPS) diffraction techniques, the dependence of the preparation on the concentration of the FeCl2 treatment solution is revealed. RESULTS: The magnetization of the as-prepared products varied non-monotonically as the FeCl2 concentration increased from 0.020 M to 1.000 M. The Experimental results show that for the composite nanoparticles, the size of the γ-Fe2O3 phase is constant at about 8 nm, the Ni2O3 phase decreased and the 2FeCl3·5H2O phase increased with increasing concentration of FeCl2 solution. The magnetization of the as-prepared products mainly results from the γ-Fe2O3 core, and the competition between the reduction of the Ni2O3 phase with the increase of the 2FeCl3·5H2O phase resulted in the apparent magnetization varying non-monotonically. CONCLUSIONS: When the concentration of FeCl2 treatment solution did not exceed 0.100 M, the products are spherical nanoparticles of size about 11 nm; their magnetization increased monotonically with increasing the concentration of FeCl2 solution due to the decreasing proportion of Ni2O3 phase.