Anisotropy Of The Magnetic Susceptibility Research Articles

Late Cretaceous and Recent tectonic stress orientations recorded by basalt dykes at Capo Passero (southeastern Sicily)

At Capo Passero, on the southeasternmost tip of Sicily (Hyblean Plateau) a basaltic sequence is intercalated in Upper Cretaceous limestones. This rock succession is transected by a swarm of feeder dykes (84-71 Ma) trending roughly NE-SW. The basaltic dykes show the same orientation as the direction of maximum horizontal compression which has been inferred for the Late Cretaceous by the analysis of micro- and mesostructures on the coeval deep-marine carbonates (Amerillo Fm.) on the western margin of the Hyblean Plateau. Deformations within the basaltic dykes during their intrusion have been recognized by the analysis of petrofabrics. Microstructures consist of sinistral strike-slip faults trending N80°, which probably formed during the late stage of cooling of the magma. The analysis of the anisotropy of the magnetic susceptibility (AMS) highlights the primary nature of the magmatic fabric. The magnetic foliation is in fact subparallel to the walls of the dykes and corresponds to the magma flow. Locally, the Upper Cretaceous dykes are displaced up to 8 m along dextral strike-slip faults trending N110–120° which originated under a tectonic stress regime that is still active in this region, as confirmed by in situ stress measurements carried out in the neighbouring areas. The analysis of microstructures within the dykes which can be ascribed to this new deformational process shows a predominant pattern of fractures trending N120° and N140° infilled with calcite exhibiting different phases of growth. These fractures cut and displace pre-existing structures. A deeper analysis of the directions of the minimum susceptibility k 3 shows that the k 3 component is parallel to the recent shear planes cutting the dykes and fits their sense of movement. This could imply a tectonic overprinting of the magnetic fabric by the intervening recent stress field.

Paleomagnetism of glacial varves from the last glaciation maximum in Patagonia at Lago Blanco

Paleomagnetic measurements have been carried out from a sequence of 178 glacial varves from Lago Blanco, west Patagonia, Argentina. The deposition of the glaciolacustrine sediments has been determined as associated with the last glaciation maximum, occurring either at around 20 000–19 000 or at around 14 500–14 000 years BP. The magnetic mineral composition was determined by the Simplex method. Both haematite (0.16%) and magnetite (0.1%) were identified. The intensities have in general high values and vary in agreement with the susceptibility. After demagnetization at 20 mT, the general trend of declination shifts from about 15°W at the bottom to 5°E at the top, and swings markedly to the east between varves 59 and 93. The general trend of inclination shifts from about −30° at the bottom to about −20° at the top. Extremely low inclinations are recorded at around varve 70. Virtual geomagnetic pole (VGP) positions are generally located in northern Canada with a short and rapid swing down to central Africa. Neither sedimentological changes nor anomalies in the anisotropy of the magnetic susceptibility that might be responsible for this swing have been observed.

ChemInform Abstract: Small Cyclophanes: The Bent Benzene Business

Short bridges in (nlmeta- and (nlparacyclophanes with n c 8 bend the benzene ring into a boat shape. Naturally, this influences the properties of the aromatic system, but two seemingly contradictory trends are observed. On the one hand, the chemical reactivity of such small cyclophanes is strongly increased and quite unusual; sometimes, it is remindful of that of a localized cyclohcxatriene-like system. On the other hand, aromatic delocalization appears to be hardly impaired as judged from aromatic carbon-carbon bond lengths and from ring current effccts such as proton chemical shifts and anisotropy of the magnetic susceptibility. In line with recent theoretical analyses implying that dclccalization in benzene is due to the a-system and counteracted by the x-system, it is proposed that bent benzene rings are full-flcdged aromatics, and that their sometimes olcfin-like reactivity is predominantly a consequence of their high strain energy being dramatically rclieved in the initial stages of such reactions.

Cotton-mouton constants and the magnetic anisotropy of pyridine, picolines, and lutidines

The anisotropy of the magnetic susceptibility of pyridine, in contrast to its optical anisotropy, hardly depends on solvation effects. The mean magnetic susceptibilities of picolines and the anisotropic magnetic characteristics of the Car-CH3 group depend on the position of the methyl group on the ring, while the corresponding electric characteristics are not sensitive to this factor.

Small cyclophanes: the bent benzene business

Abstract

Anistropic Cu Knight shift and magnetic susceptibility in the normal state of YBa2Cu3O7.

Precise measurements of the Knight shift at Cu(1) (chain) and Cu(2) (plane) sites have been made in the normal state of a Y${\mathrm{Ba}}_{2}$${\mathrm{Cu}}_{3}$${\mathrm{O}}_{7}$ powder sample, for which the $c$ axis of each grain has been aligned in a magnetic field. By combining these data with the Knight shift data in the superconducting state and the anisotropic magnetic susceptibility, it is shown that (1) the anisotropy of the magnetic susceptibility is mainly due to the orbital moment of the $3d$ electrons, (2) the magnitude of the spin contribution to the Knight shift is similar at both sites, and (3) the isotropic part of the Cu hyperfine field is large and positive (112 kOe/${\mathrm{\ensuremath{\mu}}}_{\mathit{B}}$).

Thermal, Electrical and Magnetic Properties of the Ferromagnetic Dense Kondo System CeSix

Single crystals of the ferromagnetic dense Kondo system CeSi x were grown, and the specific heat, the electrical resistivity and the magnetic susceptibility were measured. Electrical resistivity measurements clearly show the typical Kondo behavior, which unambiguously tells that CeSi x is a ferromagnetic dense Kondo system. The appreciable anisotropy of the magnetic susceptibility is analyzed with the crystalline field effect and a conjecture is made that the ground doublet is a \(\varGamma_{7}\)-like doublet.

Studies of the hyperfine enhanced nuclear antiferromagnet Cs2NaHoCl6

A neutron scattering study of the hyperfine enhanced nuclear spin system Cs2NaHoCl6 has been carried out following adiabatic demagnetization cooling. The nuclear spin system was cooled to about 2 mK, belowT N =4.8 mK. We did not observe any antiferromagnetic diffraction peaks in the (1 $$1\bar 10$$ ) reciprocal lattice plane. The anisotropy of the magnetic susceptibility above and below the Neel temperature was also measured. The magnetic structure is discussed.

Properties of the liquid crystals of some biopolymers.

Nematic liquid-crystalline solutions of polypeptides such as PBLG and PELG orient in the direction of a very low static electric field as if the polypeptide molecules were a collection of rod-like molecular clusters having a huge, permanent electric dipole moment. This dipole moment originates from fluctuation of the distribution of polymer helices in both directions of the rod-like molecular cluster with roughly equal likelihood. The molecular cluster of PBLG is estimated to be composed of some 5 x 10(5) molecules and has dimensions of a few to several micrometers x 0.1 microns phi in methylene bromide when the degree of polymerization (mean) is 650. The nematic solutions also orient in the direction of a static magnetic field. The peptide groups of the polypeptide form planes which are parallel to the polymer helix and the magnetic-field orientation is due to the demagnetizing field produced in the peptide groups perpendicular to their planes by the ring current of the pi electrons. The formation of rod-like molecular clusters is absolutely necessary to invite the magnetic-field orientation as in the electric-field orientation. The anisotropy of the magnetic susceptibility has been estimated to be on the order of 10(-8) emu/g, and the twist elastic constant of the cholesteric structure on the order of 10(-7) dyn/cm. Derivatives of PBLG with long alkyl chains as pendant groups assume thermotropic liquid crystals, in which melted alkyl chains behave as a solvent in a similar manner as in lyotropic liquid crystals of the parent PBLG. An analogy of thermotropic liquid crystals of the modified polypeptides with those of discotic liquid crystals in the columnar phase has been suggested. Concentrated solutions of a double-stranded helix of poly(A).poly(U) and a triple-stranded helix of poly(A).2poly(U) form liquid crystals both of the nematic and the cholesteric types. The nematic solutions of these polyribonucleotide complexes orient perpendicular to the direction of a static magnetic field owing to the diamagnetic dipole moments induced in the purine or pyrimidine bases of the polyribonucleotides perpendicular to their planes, which are roughly perpendicular to the polymer helices. Any rod-like polymer has a good chance to form liquid crystals in which the occurrence of forces acting among the molecules is not necessary. The PLGA-Na in a neutral aqueous solution undergoes a "coil-to-alpha helix transition" when the polymer concentration is increased.(ABSTRACT TRUNCATED AT 400 WORDS)

Studies of high-Tc oxide superconductors

Recent results of experimental studies on high-Tc oxide superconductors are presented. Anisotropy and temperature dependence of the magnetic susceptibility χ of La2CuO4 do not seem to be easily explained by a simple antiferromagnetic spin ordering with the spin direction within the CuO layers. Moreover with increasing magnetic field perpendicular to the layers, χ exhibits a sudden increase below TN. The tunneling study revealed that the energy gap value of YBa2Cu3Oy is consistent with BCS theory and any additional structure has not been observed in the present tunneling spectra.

The Blake magnetic polarity episode in cores from the Mediterranean Sea

Palaeomagnetic and oxygen isotope studies on samples taken from five cores in the Mediterranean area are described. Directions of the stable remanent magnetisation show the presence of an anomalous (reversed) zone during the transition between the oxygen isotope substages 5e and 5d. This position corresponds to the age of about 117 kyr B.P. and thus to the Blake polarity episode. The duration of this episode is estimated to be about 6000 years. The study of the anisotropy of the magnetic susceptibility shows no indication of sedimentological disturbances in recorded sections. The Blake episode therefore represents a stratigraphic marker which may be used to link the continental and marine palaeoclimatic and sedimentary records.

Spin Structure of the Hyperfine Enhanced Nuclear Antiferromagnet Cs2NaHoCl6

The hyperfine enhanced nuclear spin of 165Ho in Cs2NaHoCl6 shows antiferromagnetic ordering below about 4.8mK. A neutron diffraction experiment has been carried out following adiabatic demagnetization cooling. We did not observe any antiferromagnetic diffraction peaks in the (11̄0) reciprocal lattice plane. The anisotropy of the magnetic susceptibility above and below TN was also measured. The magnetic structure is discussed.

Carbon-13 spectral line broadening in paramagnetic solids

Sensitivity enhancement by cross-polarization and resolution enhancement by dipolar decoupling and magic-angle sample spinning has become an increasingly important method for structural and dynamic characterization of solid materials (I). Paramagnetic compounds, with several notable exceptions (24, have resisted characterization using line-narrowing methods. A common assumption about the failure of this experiment is that an efficient nuclear electron dipolar relaxation mechanism, modulated by the rapid electron relaxation times, r,, and Th, can render the relaxation times of the observed carbon nucleus, fi and T$ , sufficiently short that an unusually broad carbon spectrum results. Our experiments on several compounds demonstrate that this assumption is incorrect, at least among the paramagnetic lanthanide systems studied. Short carbon relaxation times as a source of the line broadening may be tested by measuring the carbon relaxation times in question, fi and E. We have previously reported the spin-lattice relaxation times for the carbon atoms of praseodymium acetate monohydrate and find that the carboxyl and methyl carbons have T, values of 125155 and 600 ms, respectively (3). These times predict a linewidth less than 3 Hz, far narrower than those observed in the MASS spectra of this compound, about 280 Hz for the carbonyl. A convenient and efficient method for estimating the transverse relaxation times for the carbon atoms is to take the Fourier transform of the echo train (5, 6) accumulated on the paramagnetic compound. The resulting spectrum consists of a series of sharp lines the intensity of which maps the powder pattern and the width of which correspond to the linewidth implied by the decay time of the whole echo train. The width of these lines is typically about 25 Hz for the carbonyl lines, far narrower than the MASS lines or the single crystal lines. Thus, the width of the carbon lines in these paramagnetic solids is not determined by the spin-lattice or spin-spin relaxation times at the carbon atoms. The limiting linewidths of diamagnetic organic solids have been considered by VanderHart, Earl, and Garroway (7) and by Alla and Lippmaa (8). The broadening in both the static and MASS spectra may be attributed to the anisotropy of the diamagnetic susceptibility in the aromatic compounds. The situation is aggravated by paramagnetic materials where the anisotropy of the magnetic susceptibility may be far larger (9-11). Following Alla and Lippmaa (8) the bulk effects are demonstrated in Fig. 1, which shows the spectrum of adamantane and adamantane in samples where praseodymium

Carbon-13 NMR of liquid crystal solutions with magic-angle spinning. Hindered rotation in 6-(dialkylamino)fulvenes

With magic-angle spinning, the carbon-13 NMR spectra of nematic liquid crystals having positive anisotropy of the magnetic susceptibility show sharp peaks. Solutes which have carbon-13 peaks in the ‘‘window’’ regions of the solvent spectra can be conveniently studied. The hindered rotation of the dialkylamino group in 6-(dimethylamino)fulvene, 6-(diethylamino)fulvene, and 6-(N-piperidyl)fulvene in a liquid crystal solvent (E8) has been investigated by carbon-13 NMR with magic-angle spinning. The results are compared with those obtained in isotropic solutions of the same compounds in CDCl3 and CDCl3+E8. The free energy of activation (ΔG≠) of each compound in the three solvents is the same within experimental error.

Anisotropy of the magnetic susceptibility of C-H, C-Cl, AND C=C bonds in a series of ethylene derivatives according to data on the Cotton-Mouton effect and proton magnetic resonance

The anisotropies of the magnetic susceptibility of Col-Cl and Col-H bonds are negative. The magnetic-susceptibility ellipsoid of the C=C bond lacks axial symmetry, is diamagnetic with respect to kL and kV, and is paramagnetic with respect to kT.

Magneto- and electro-optic constants of some nitrobenzenes and anisotropy of the magnetic susceptibility of the nitro group

1. The magnetic-susceptibility ellipsoids of a nitro group attached to an aromatic ring and of a nitrate anion have been determined by measuring the Cotton-Mouton effect. The axis of maximal diamagnetism of the ellipsoids obtained coincides with the normal to the plane in which the bonds are located. 2. The significant acoplanarity of the trinitrotoluene molecule in solution has been established by measuring the magnetic and electric birefringences. 3. The molecule of p-dinitrobenzene in solution has an anomalously high atomic polarization and a small constant dipole moment.

Anisotropy of the magnetic susceptibility of the carbonyl group and molar cotton-mouton constants for aromatic aldehydes and ketones

1. From the molar Cotton-Mouton constants for acetone and cyclohexanone, we determined the magnetic susceptibility tensor for the C=O bond, which is paramagnetic in the plane of the carbonyl and diamagnetic in the perpendicular direction. 2. Within the framework of the tensor additive scheme, we calculated the theoretical molar Cotton-Mouton constants (which agree with experimental data) for aryl-containing aldehydes and ketones. This confirms the additive nature of the molecular magnetic susceptibility tensor for the studied compounds.

Anisotropy of the magnetic susceptibility of the C-O bond from cotton-mouton effect data for anisoles and 1,4-dioxane

1. From the molar Cotton-Mouton constants of anisoles and 1,4-dioxane, we calculated the anisotropy of the magnetic susceptibility for the Csp2−0 and Csp3−0 bonds. 2. We have determined the influence of conjugation and hybridization effects on the diamagnetic susceptibility tensor for the C-O bond.

Direct Determination of the Anisotropy of the Magnetic Susceptibility in Smectic Liquid Crystals

A sample of an aligned smectic liquid crystal, suspended in a magnetic field, can be excited to oscillations. Measurement of the oscillation period allows a direct determination of the anisotropy of the magnetic susceptibility. Liquid crystals exhibiting the phase sequence isotropicnematic- smectic A can be aligned very well and precise y.A values can be determined. A direct transformation from the isotropic to the smectic phase or phase transitions between smectic phases can cause an incomplete alignment of the liquid crystal and worse results.

The de Haas-van Alphen Effect and Anisotropy of the Magnetic Susceptibility in Zn–Ag and Zn–Ga Dilute Alloys

We report the results of measurements of the de Haas-van Alphen effect and the anisotropy of the magnetic susceptibility associated with the needle Fermi surface in the 3rd-band of pure zinc, dilute Zn–Ag and Zn–Ga alloys. The cross-sectional area of the needles increases for silver impurity, and decreases for gallium impurity. A large maximum in the temperature dependence of the anisotropy of the magnetic susceptibility is observed in pure zinc, Zn–Ag and Zn–Ga alloys. The temperature indicating its maximum increases in Zn–Ag and decreases in Zn–Ga alloys. These results of the magnetic susceptibility and the changes in the cross-sectional area of the needles with alloying can be reasonably accounted by temperature and impurity dependence of axial ratios.