Magnetic Birefringence Measurements Research Articles

Study of the birefringence noise in high-finesse ULE cavity

High-finesse ultra-low expansion (ULE) cavities are important components in many precision optical experiments, and the cavity length noise in ULE cavities has been intensively studied. However, some experiments, such as vacuum magnetic birefringence measurements, have focused on the phase difference between orthogonal polarizations. To determine the effect of high-reflectivity mirrors on birefringence noise, we investigated the birefringence noise of a ULE cavity with the finesse of 531000(1000) and obtained the noise spectrum of the intrinsic birefringence of the cavity. The single-pass phase difference due to the birefringence of the cavity mirror is (21.3±0.2)μrad and the noise for the birefringent cavity is 81.3prad/Hz at 0.1Hz and 2.6prad/Hz at 40Hz. These results may draw attention when considering noise in cavity-based optical experiments.

Magnetic birefringence of montmorillonite dispersion

Compared with unadulterated polymers, filler/polymer composites can exhibit superior properties. Modulating the orientation of montmorillonite (MMT) filler may maximize this filler’s properties in shape-anisotropic applications. We measured the magnetic birefringence of MMT dispersions in water with custom magnetic birefringence equipment and estimated the inherent birefringence and anisotropic magnetic susceptibility of MMT. We classified polydisperse MMT to prepare MMT dispersions with a narrow size distribution. From the relationship between the retardation obtained by magnetic birefringence measurements and the reciprocal of the square of the magnetic field, we estimated the inherent birefringence of MMT to be −0.060. Considering the size distribution, and by setting the anisotropic magnetic susceptibility to −1.5 × 10−5, we reproduced the magnetic flux density dependence of the magnetic birefringence of MMT with a particle size of ≤300 nm.

A novel pulsed magnet for magnetic linear birefringence measurements

In this paper, we describe a novel pulsed magnet, called foil coil, which can deliver a field transverse to the light propagation of more than 10 T over about 0.8 m operating without cryogenic equipment. It has been designed for linear magnetic birefringence measurements. We report on testing the coil and also show some physics data taken in vacuum during its commissioning in the framework of the Biréfringence Magnétique du Vide (BMV) apparatus, with special attention to noise induced by the pulse itself. Finally, we compare the preliminary results obtained here with data from the previous BMV coil.

Two-state model for nematic liquid crystals made of bent-core molecules.

Nematic (N) liquid crystals made of bent-core molecules exhibit unusual physical properties such as an intermediate phase between the N and isotropic (I) phases, a very weak NI transition as inferred from magnetic birefringence measurements in a low field, which is apparently incompatible with a large shift in the NI transition temperature (T_{ni}) measured under a high field. Using our conformational studies on the aromatic cores, we propose that only conformers which are more straightened than those in the ground state (GS) form clusters with a few layers, which persist even in the isotropic phase, as inferred from x-ray and rheological experiments. We present a Landau-de Gennes theory of the medium, including an orientational coupling between the clusters and the GS molecules, which accounts for all the unusual properties. The intermediate phase to isotropic transition is predicted to exhibit critical behavior at a very low magnetic field of <1kG.

Negative Diamagnetic Anisotropy and Birefringence of Cellulose Nanocrystals

We report magnetic birefringence measurements up to high fields (17.5 T) of dilute aqueous suspensions of rod-like cellulose nanocrystals with well characterized distributions of lengths, widths and thicknesses. We compare these data with three models, one with colinear (1), one with perpendicular cylindrically symmetric tensors for diamagnetic susceptibility and refractive index (2) and one with biaxial diamagnetic anisotropy (3). We find that taking into account polydispersities of length, width, and thickness is essential for accurate fitting and that model 1 is the most appropriate, presumably because of the twisting of the suspended nanocrystal along their long axis. The best-fitted susceptibility anisotropy was Δχz(xy) = χzz–(χxx+χyy)/2 = −2.44 × 10–6 when considering only the crystalline core of nanocrystals and, more appropriately, Δχz(xy) = −0.95 × 10–6 when including crystalline core and skin. The latter value is slightly higher than Δχz(xy) = −0.68(5) × 10–6 deduced from estimations using Pasca...

Virtual isotropic‐nematic transitions in alkyl cyanobiphenyls

Pretransitional fluctuations in the isotropic phase of liquid crystalline and non‐liquid crystalline alkyl cyanobiphenyls have been investigated using light scattering and magnetic birefringence measurements. We find evidence for a virtual isotropic‐nematic phase transition in short‐chain alkyl cyanobiphenyls with no observable nematic phase. The measured temperature dependence of fluctuations is well‐described by mean‐field theory. Virtual phase transition temperatures extrapolated from separate light scattering and magnetic birefringence experiments are in good agreement. Landau–de Gennes model parameters for the compounds investigated are calculated from the experimental results.

Birefringence of magnetite-based magnetic fluids: the effect of the surface-coating layer

Water-based and hydrocarbon-based magnetic fluids (MFs) containing Fe/sub 3/O/sub 4/ nanoparticles are investigated using static magnetic birefringence measurements. A strong dependence of the birefringence signal upon the magnetic permeability is found. In addition to the field dependence of the magnetic permeability, inclusion of dimers in the description of the birefringence signal yields excellent agreement between parameters obtained from birefringence and high-resolution microscopy data.

Static magneto optical birefringence of new Electric Double Layered Magnetic Fluids

Magnetic birefringence measurements are performed under a static field on Electric Double Layered Magnetic Fluids based on copper and zinc ferrite nanostructures. The optical birefringence is related to a single-particle effect and well described by a Langevin model which includes a log- normal distribution of particles. By the field-induced birefringence level, these new magnetic fluids are comparable to usual ones, a result which could offer a new way for biological applications.

Magnetic Orientation of Crystalline Polymers in View of Liquid Crystallinity during Induction Period of Crystallization

Magnetic orientation of poly(ethylene-2,6-naphthalate) (PEN) and isotactic polystyrene (iPS) has been reported. The orientation starts to occur during the induction period of isothermal melt crystallization process, i.e. during the initial stage of crystallization process where no crystal growth is yet observed with respect to the wide angle X-ray diffraction. In the case of PEN, the crystallites obtained at later stages of crystallization were aligned with its c-axis parallel to the magnetic field, while in the case of iPS, the crystallites obtained were aligned with its c-axis perpendicular to the magnetic field. X-ray, infrared, and magnetic birefringence measurements indicated that mesophases appearing during the induction period could be responsible for the magnetic orientation of these polymers.

Mean-field theory for the phase transitions of nematic polymers in a magnetic field: application to magnetic birefringence

With the help of a theory which is based on functional integrals, we calculate the evolution, with temperature and degree of polymerization, of the orientational order parameter of thermotropic nematic polymers under a magnetic field. We compare our results with those obtained experimentally from magnetic birefringence measurements on polyesters. The validity of the application of the mean-field theory in the case of semiflexible liquid crystal polymers which consist of sequences of mesogenic units and flexible spacers is also discussed.

Magnetic orientation of poly(ethylene-2,6-naphthalate)

A new finding is reported of the magnetic orientation of poly(ethylene-2,6-naphthalate) (PEN) near its melting point, evidenced by X-ray analyses and magnetic birefringence measurements. Since PEN does not exhibit features characteristic of liquid-crystalline polymers under a polarization microscope, the magnetic orientation was unexpected. The orientation is probably due to some ordered structure susceptible to a magnetic field, but the detail of the structure is not fully understood at present.

Linear magnetic birefringence measurements of Faraday materials.

Measured values of the linear magnetic birefringence (Cotton-Mouton) coefficients of several Faraday-active materials (${\mathrm{Tb}}^{3+}$, ${\mathrm{Ce}}^{3+}$, and ${\mathrm{Pr}}^{3+}$ rare-earth composites) are presented. These coefficients are compared with the same coefficients measured for diamagnetic materials: ZnSe, and the liquid solvents benzene, chloroform, acetone, and water. Relatively large Cotton-Mouton coefficients, which have a strong dependence on the paramagnetic susceptibility and concentration of rare-earth ions present in these materials, have been observed for the paramagnetic Faraday materials. Difficulties introduced into the measurement of small Cotton-Mouton effects by misalignment and/or an inherent linear birefringence of a crystalline sample are discussed.

Reanalysis of the electrostatic component of the persistence length of poly(styrene sulphonate)

The electrostatic component of the persistence length ( L el ) for three molecular-weight preparations of poly(styrene sulphonate) was determined from magnetic birefringence measurements (m.b.m.) by Weill and Maret in 1982. The ratio L el / L a el plotted as a function of the Debye-Huckel screening length (λ DH ) exhibited significant deviations from unity over the range24 ⩽ λ DH (a˚) ⩽ 108, where L a el is the analytical expression for the electrostatic persistence length for finite chain lengths given by Odijk. Weill, Maret and Odijk attempted to explain the discrepancy between theory and experiment by invoking ‘semidilute’ solution conditions for the long-chain results concomitant with a different degree of counterion condensation than used for the short-chain results. It is proposed in the present paper that a ‘three-region’ model for the counterion distribution about the polyion accurately represents the m.b.m. data without having to invoke differential counterion condensation properties of short and long chains, or having to postulate semidilute solution conditions. The three regions are: (1) Manning-type condensation on the surface of the polyion; (2) a counterion sheath in the vicinity of the polyion; and (3) the remaining solution.

Light scattering and magnetic birefringence measurements at the isotropic-nematic and nematic-reentrant-isotropic transitions in a lyotropic nematic liquid crystal.

The pretransitional behavior in the isotropic and reentrant-isotropic phases near a uniaxial nematic phase in the potassium laurate--decanol--${\mathrm{D}}_{2}$O system has been studied by light scattering and magnetically induced birefringence. The light-scattering measurements show mean-field behavior at both isotropic transitions and strong angular dependences from which a bare correlation length of 22 \AA{} has been determined for both isotropic phases. The magnetic-field effects, however, depart from mean-field behavior, and, in conjunction with the light-scattering data, reveal a temperature-dependent magnetic anisotropy and furnish evidence for a change in micellar shape.

Effects of counterion identity on a micellar liquid crystal

Magnetic birefringence measurements are reported for mixtures of cesium- and rubidium-perfluorooctanoate in water above the nematic-isotropic phase transition temperature. Despite significant differences in the phase diagrams between the two materials, it was found that both the supercooling temperature T ∗ of the isotropic phase, as well as the micelle aggregation number, are quite insensitive to relative surfactant concentration.

Mixed micellar liquid crystals: magnetic birefringence measurements of surfactants with differing tail lengths near the isotropic-nematic phase transition

Mixed micellar liquid crystals: magnetic birefringence measurements of surfactants with differing tail lengths near the isotropic-nematic phase transition

Packaging of DNA in cauliflower mosaic virus and bacteriophage S D studied with magnetic birefringence

Magnetic birefringence measurements are reported on solutions of two double-stranded DNA containing viruses: icosahedral cauliflower mosaic virus (CaMV) and SD, an isometric bacteriophage with a short tail. Assuming the signal comes predominantly from the DNA, constraints can be put on its packing geometry. The anisotropy of CaMV is consistent with the DNA having a near orthogonally wound spool-like structure. The DNA anisotropy is smaller in S D but is compatible with a spool-like structure if the windings are tilted about the spool axis or if a significant proportion of the DNA is effectively isotropically packed. Intermediate structures are also possible.

Concentration-dependent magnetic birefringence measurements in the isotropic phase of a lyotropic liquid crystal

The concentration and temperature dependences of the Cotton-Mouton coefficient C for the binary mixture cesium perfluoro-octanoate and water close to a nearly-second-order nematic-isotropic transition are reported. It was found that (T-${T}^{\mathrm{*}}$)C is a weakly decreasing function of concentration, indicating a significant concentration dependence of the micelle structure.

Magnetic-susceptibility measurements below a nearly-second-order nematic-isotropic phase transition in a lyotropic liquid crystal.

Magnetic-birefringence measurements were performed in the nematic phase of the binary lyotropic liquid crystal cesium perfluoro-octanoate and water within 75 mK of the superheating limit. Several results were obtained: (1) The induced birefringence was linear in H for small H, but showed significant deviations from linearity for H&gt;5 T; (2) the elastic constant was found to be very small near the transition, indicating that director fluctuations may contribute a significant multiplicative factor to the average order parameter; and (3) the order-parameter exponent \ensuremath{\beta}=0.34\ifmmode\pm\else\textpm\fi{}0.06. This last result is examined in the context of a mean-field theory with higher-order terms in the free-energy expansion and in the context of the d=3, N=5 N-vector model.

Local order in polyelectrolytic solutions detected by magnetic birefringence measurements on the (Tyr‐Glu)n copolymer

AbstractThe local electrostatic order in polyelectrolytic solutions has been studied by magnetic birefringence measurements on short segments (n ∼ 34) of the (Tyr‐Glu)n polypeptide in high magnetic fields up to 13 T. We give evidence that only above a critical polymer concentration do small domains exist within which the polymers show a parallel ordering. This structural order disappears suddenly at ionic strengths above about 5 × 10−2 M and above a melting temperature of about 50°C.