Physical properties of aqueous suspensions of goethite ($\alpha$-FeOOH) nanorods

At volume fractions larger than 8.5%, aqueous suspensions of lath-like goethite (alpha-FeOOH) nanorods form a lyotropic nematic phase. In this article, we first discuss the nematic ordering within statistical-physics models of the isotropic/nematic phase transition. We then describe the influence of a magnetic field on the nematic phase. Because the nanorods bear permanent magnetic moments, the nematic suspensions have dipolar order and very low Frederiks thresholds. Moreover, the nematic phase aligns parallel to a small magnetic field but realigns perpendicular to a high field because of a competition between the permanent moments of the nanorods and their negative anisotropy of magnetic susceptibility. This magneto-optical study of the nematic phase is completely consistent with that of the isotropic phase of the same suspensions published in Part I (this issue, p. 291). Besides, we demonstrate the field-induced biaxiality of a nematic single domain aligned perpendicular to the field. We also describe here preliminary experiments where an a.c. electric field is applied to the nematic phase. Both field amplitude and frequency were found to control the alignment direction and homeotropic-to-planar alignment transitions were observed. From this data, simple models were used to estimate some physical constants of the nematic phase.     

On the theory of rheological properties of magnetic suspensions

The paper deals with a theoretical study of influence of magnetic field on effective viscosity of suspension of non-Brownian magnetizable particles. It is supposed that experimentally observed magnetorheological effects are provided by chain-like aggregates, consisting of the particles. Unlike previous works on this subject, we take into account that the chains cannot be identical and estimate their size distribution. The following power law (η-η₀)/η₀∼Mn^-Δ, detected in many experiments, is obtained theoretically (η and η₀ are the suspension effective viscosity and the carrier liquid viscosity, respectively, Mn is the so-called Mason number, proportional to the shear rate and inversely proportional to the square of magnetic field). The calculated magnitude of the exponent Δ increases with the applied magnetic field from approximately 0.66 to 0.8-0.9 and slowly increases with the volume concentration ? of the particles. These results are in agreement with known experiments.     

Facile synthesis and magnetic properties of manganese doped ZnS nanorods

The synthesis of both Mn(II) doped and undoped ZnS nanorods were carried out using a simple soft-chemical route using mercaptoethanol as capping agent. Their morphological, structural and magnetic properties are presented. The crystal structures of the as obtained products were investigated through X-ray diffraction study reveals the formation of hexagonal wurtzite structure. The growth of the nanorods is achieved by careful control over the precursor addition, temperature and time duration. The nanorods are single crystalline and the diameter of the rods was found to vary in the range of 20-50 nm. Vibrating sample magnetometer measurements at room temperature show paramagnetic behavior for the doped nanorods.     

The influence of surface-active substances on the crystallization process and magnetic properties of goethite nanoparticles

To prevent the process of aggregation and growth of ??-FeOOH nanoparticles, during chemical syntheses various surface-active substances (SASs) with a concentration of 3 g/dm³ were added into the solution. The applied SASs were: cetylpyridinium chloride (CPC), sodium dodecyl sulphate (SDS), and complexone EDTA. Using various methods it was found that SAS molecules have a dual effect on the obtained nanoparticles: on one hand SAS application increases the number of small ??-FeOOH nanoparticles with sizes of 2?C5nm. On the other hand, SAS molecules react with surface atoms of the nanoparticles and form additional compounds.     

Kinetics aggregation of magnetic suspensions

We present results of theoretical and computer studies of the kinetics of chain-like aggregate formation in suspensions of non-Brownian magnetizable particles. An analytical model for calculation of the time-dependent function of distribution over chain size is suggested. This model describes the evolution of the chain structure due to the chain-chain aggregation. In order to verify this model we have compared it with the results of computer simulations of a two-dimensional model of this suspension. Results of computer simulations and of the analytical model are in reasonable agreement up to 5% of the surface concentration of the particles.     

Electron magnetic resonance of synthetic goethite in the range of the magnetic transition

Physics of the Solid State - The X-band electron magnetic resonance spectra of synthetic goethite have been investigated in the range of the magnetic phase transition. In addition to relatively...     

Size effect on the structure and magnetic properties of SmMn2O5 nanorods

Three sizes of SmMn₂O₅ nanorods that are labeled with (<L_C>) × axial lengths of 58(17) nm × 25(6) nm, 92(21) nm × 32(8) nm, and 126(25) nm × 52(13) nm were fabricated by the hydrothermal method. All the samples exhibited an antiferroicmagnetic (AFM) peak at approximately 6 K, which was associated with Sm magnetic ordering and no size independence. Another AFM magnetic ordering that belongs to the Mn ion was found with <L_C> = 58 nm, 92 nm, and 126 nm at 26 K, 28 K, and 30 K, respectively. The spin-orbit interaction increases with size in the magnetic susceptibility experiment. All the samples displayed a hysteresis loop at 2 K. The coercivity decreases as the size increases. The effects of the size on the crystal structure were elucidated from the Raman spectra of the <L_C> = 92 and 126 nm samples at various temperatures. The 126 nm sample displayed a red-shift for the Ag mode with warming, revealing that the Mn–O bonds are more sensitive to temperature in larger SmMn₂O₅ nanorods. These results demonstrate that the size effect importantly affects the structure and magnetic properties in SmMn₂O₅ multiferroic nanorods.     

Structure and transport properties of charged sphere suspensions in (local) electric fields

Motivated by both still open fundamental theoretical questions as well as novel applications, the electro-kinetics of highly charged model colloids have attracted considerable interest in the last few years. The present paper reviews corresponding new developments and trends emerging mainly from novel instrumentations for both strongly interacting and extremely dilute systems. We also highlight recent uses of local electric fields and electro-kinetic currents to realize complex micro-swimmers or prescribe crystal micro-structures.     

Doping dependent room-temperature ferromagnetism and structural properties of dilute magnetic semiconductor ZnO:Cu nanorods

Copper doped ZnO nanoparticles were synthesized by the chemical technique based on the hydrothermal method. The crystallite structure, morphology and size were determined by X-ray diffraction (XRD), high resolution transmission electron microscopy (HRTEM) and scanning electron microscopy (SEM) for different doping percentages of Cu²⁺ (1-10%). TEM/SEM images showed formation of uniform nanorods, the aspect ratio of which varied with doping percentage of Cu²⁺. The wurtzite structure of ZnO gradually degrades with the increasing Cu²⁺ doping concentration and an additional CuO associated diffraction peak was observed above 8% of Cu²⁺ doping. The change in magnetic behavior of the nanoparticles of ZnO with varying Cu²⁺ doping concentrations was investigated using a vibrating sample magnetometer (VSM). Initially these nanoparticles showed strong room-temperature ferromagnetic behavior, however at higher doping percentage of copper the ferromagnetic behavior was suppressed and paramagnetic nature was enhanced.     

Ultrasonic properties of nanoparticles-liquid suspensions

A polymer colloidal solution having dispersed nanoparticles of Cu and Au metals have been developed using a novel chemical method. Average size of the nanoparticles could be varied in the 4-10 nm range by conducting the reaction at an elevated temperature of 50-70 degrees C. Colloidal solutions of representative concentrations of 0.1-2.0 wt% Cu/Au contents in the primary solutions are used to study the modified ultrasonic attenuation and ultrasonic velocity in PVA polymer molecules on incorporating the Cu/Au particles. A characteristic behaviour of the ultrasonic velocity and the attenuation are observed at the particular temperature/particle concentration. The results demonstrate that the primary reaction during the nanoparticles-PVA colloidal formation occurs in divided groups in small micelles. The results are analyzed predicting the enhanced thermal conductivity of the samples.     

Optical properties of nanodiamond suspensions

The optical properties of nanodiamond suspensions have been calculated. The main supposition is that carbon dimers, which in many aspects are analogous to Pandey chains (2 × 1) on the surface of bulk diamond, are formed on the surface of nanodiamonds due to the surface reconstruction. All experimentally observed features of the absorption of nanodiamond suspensions have been explained on the basis of these ideas. Whereas the diamond nucleus does not absorb light in the visible spectral range, dimers on the surface of the diamond core absorb light in the entire range of optical wavelengths. In addition, there are two features at energies close to 1.5 and 5 eV in their absorption spectrum.     

Luminescence and magnetic properties of europium(III) carboxylatodibenzoylmethanates

This paper reports on the results of an investigation of the luminescence and magnetic properties of europium(III) carboxylates, which are determined by the structure of the Stark and Zeeman sublevels. It has been established that the energy splittings λ_lum between the ⁷ F ₀ ground level and the ⁷ F ₁ term in the luminescence spectra of these compounds well correlate with the energy splittings λ_magn obtained from magnetochemical measurements.     

Viscosity and dispersion state of magnetic suspensions

We investigate the viscosity behavior of a magnetic suspension in which magnetic particles are dispersed in a mixture of polyacrylic liquids. The size of magnetite particles is nearly 300 nm and the volume fraction of the magnetic particles is in the range of 0.003-0.03. The particle concentration dependence of the suspension viscosity yields the intrinsic viscosity [η], which varies from 25.6 at 5 s⁻¹ to 5.1 at 400 s⁻¹. The yield stress and the infinite shear viscosity of the suspension increase non-linearly as the particle concentration ? increases. We examine the effect of process conditions such as milling time and amount of dispersant on the viscosity behavior of the suspension. As milling time elapses, yield stress and low shear viscosity decrease and then reach constant values while the infinite shear viscosity remains constant. When oleic acid is added as a dispersant, the yield stress and low shear viscosity of the suspension show minimum values as the amount of oleic acid increases. These results agree with experimental results of sedimentation tests, which enable us to estimate the aggregate size of magnetic suspension. The yield stress and the low shear viscosity of the magnetic suspension are found to be useful in evaluating the dispersion state of the magnetic suspension.     

Influence of a surfactant on hyperfine magnetic interactions in goethite nanoparticles

Structural and magnetic characteristics of nanosized goethite (α-FeOOH) samples prepared by chemical precipitation of an iron (III) salt and alkali in aqueous solutions with different contents of a surfactant (ethyl alcohol) have been investigated. Changes in the morphology, structure, and sizes of antiferromagnetic nanoparticles, caused by the presence of the surfactant in the deposition solution, have been established by Mössbauer spectroscopy, magnetic measurements, X-ray diffraction, and electron microscopy. It is determined that an increase in the surfactant concentration leads to the bimodal size distribution of nanoparticles; in this case, the fraction of small (<10 nm) isolated particles in the sample increases and the degree of ordering of larger particles (50–100 nm) increases with a change in their shape from spherical to needle-like.     

Studies on structural and magnetic properties of Co-doped pyramidal ZnO nanorods synthesized by solution growth technique

Large-area arrays of highly oriented Co-doped ZnO nanorods with pyramidal hexagonal structure are grown on silica substrates by wet chemical decomposition of zinc–amino complex in an aqueous medium. In case of undoped ZnO with an equi-molar ratio of Zn²⁺/hexamethylenetetramine (HMT), highly crystalline nanorods were obtained, whereas for Co-doped ZnO, good quality nanorods were formed at a higher Zn²⁺/HMT molar ratio of 4:1. Scanning electron microscope (SEM) studies show the growth of hexagonal-shaped nanorods in a direction nearly perpendicular to the substrate surface with a tip size of ～50 nm and aspect ratio around 10. The XRD studies show the formation of hexagonal phase pure ZnO with c-axis preferred orientation. The doping of Co ions in ZnO nanorods was confirmed by observation of absorption bands at 658, 617 and 566 nm in the UV–vis spectra of the samples. The optical studies also suggest Co ions to be present both in +2 and +3 oxidation states. From the photoluminescence studies, a defect-related emission is observed in an undoped sample of ZnO at 567 nm. This emission is significantly quenched in Co-doped ZnO samples. Further, the Co-doped nanorods have been found to show ferromagnetic behavior at room temperature from vibrating sample magnetometer (VSM) studies.     

Site occupancy and magnetic properties of pyrochlore-structured AgOs(2)O(6)

AgOs(2)O(6) prepared from ion-exchanged superconducting β-pyrochlore KOs(2)O(6) has been shown to be non-superconducting. Synchrotron x-ray structure refinement suggests that AgOs(2)O(6) has the Ag ion mostly occupying the low-symmetry 32e site in the [Formula: see text] space group of proper occupancy, which is different from the original major occupancy at the high-symmetry 8b site for KOs(2)O(6), and similar to non-superconducting Na(1.4)Os(2)O(6)?H(2)O. Magnetic susceptibility measurements found no magnetic ordering down to ～1.7?K. The trace amount of isolated spins suggests that the Ag could be neutral and lead to a pure Os(6+) valence state of zero spin in the newly prepared AgOs(2)O(6).     

Optical properties of a nematic twist cell

We show that many of the complicated static and dynamic optical transmission characteristics of a Schadt-Helfrich twist cell, including the “bounce” in transmission that is observed at negative oblique angles of incidence after switching it off, can be predicted quite well from a very simple dynamic model. The model ignores flow and has only a viscous resistance to angular reorientation, in addition to the Ossen-Frank elastic properties. The bounce observed in some cases with normally incident light cannot be predicted by the model. As expected, the model yields equilibrium configurations at any applied voltage that give the observed transmission very accurately. Dynamic observations in the upper part of the nematic temperature range, where the observed bounce at normal incidence is small, are well reproduced by the model. At lower temperatures the discrepancies are larger. When interpreted with the model, both dynamic and static measurements concur in indicating that the directors of the liquid crystal adjacent to the surfaces assumed a more or less permanent tilt of a few degrees in our cell. Alignment was obtained by rubbing the conducting electrodes in one direction with linen cloth.     

Acentric magnetic and optical properties of chalcopyrite (CuFeS2)

The absence of spatial inversion symmetry at both local (point group 4) and global (crystal class (4)2m) levels greatly influences the electronic properties of chalcopyrite (CuFeS(2)). The predicted dichroic signals (natural circular, non-reciprocal and magneto-chiral) and resonant, parity-odd Bragg diffraction patterns at space-group forbidden reflections portray the uncommon, acentric symmetry. Despite extensive experimental investigations over several decades, by mineralogists, chemists and physicists, there is no consensus view about the electrical and magnetic properties of chalcopyrite. New spectroscopic and diffraction data, gathered at various temperatures in the vicinity of the copper and iron L(2,3) edges, provide necessary confidence in the magnetic motif used in our analytic simulations of x-ray scattering. With the sample held at 10 and 65 K, our data establish beyond reasonable doubt that there is no valence transition, and ordering of the copper moments as the origin of the low-temperature phase (T(c) ≈ 53 K) is ruled out.