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1.
Mössbauer spectra of biotite at 4 K are reported. The biotite crystals were oriented with the c-axis parallel to the γ-ray direction and some spectra were recorded with external magnetic fields of 40 kOe applied at right angles to the c-axis. Decrease of the magnetic-hyperfine field of both Fe2+ and Fe3+ ions on application of the external field shows that both Fe3+-Fe3+ pairs and Fe2+-Fe2+ pairs are coupled ferromagnetically.  相似文献   

2.
The new approach to the modeling of quiescent solar prominences is proposed. We solve the inverse magnetohydrostatic problem, when the pressure, density and temperature of plasma in the filament are calculated from the equilibrium equations using the given magnetic structure (magnetic flux function is proposed to be known). The new exact nonlinear solutions for dense (n ≈ (2?3) × 1011 cm?3) and cold (T ≈ (5?10) × 103 K) filaments, embedded in the plan, vertically stratified atmosphere (hot solar corona) free of magnetic field, are derived. The filaments are stretched along the horizontal axisy(the translational symmetry is assumed: ?/?y = 0) and located parallel to and above a photospheric, magnetic polarity reversal line. The magnetic field lines have a structure of magnetic flux rope with helical field lines in three-dimensional space; the strength of magnetic field falls rapidly with distance from a rope axis. No external longitudinal magnetic field is needed to equilibrate the prominence. The net electric current along the filament is equal to zero. The model of magnetic arcade with the deflection (sag) on the top, proposed by Pikelner (1971) as a basic form of normal prominence, is calculated also using the method proposed. It is shown that such magnetic arcade, having the magnetic field strength of few gauss only, can effectively maintain the equilibrium of cool dense filament at the heights about 50–60 Mm.  相似文献   

3.
It is shown that composite radio spectra of the hot spots of the radio galaxy Cygnus A can be fully explained by assuming a nonuniform distribution of the magnetic fields inside the hot spots, without invoking any physical mechanisms other than synchrotron radiation. The magnetic fields are strong (B ?? 10?2?10?1 G) at the center of the hot spots, and decreases at the hot-spot edges to the level of the magnetic field of the radio lobes in which the hot spots are embedded (B ?? 10?4?10?5 G). The difference in the magnetic field between the hot-spot center and edge decreases during the evolution, while the average magnetic-field intensity increases.  相似文献   

4.
Magnetization and neutron diffraction measurements have been made on grunerite, Fe7Si8O22(OH)2, a monoclinic double-chain silicate with Fe2+ octahedral bands. The mineral orders antiferromagnetically at 47K into a collinear structure with a second transition at 8K to a canted arrangement. The magnetic susceptibility follows a Curie-Weiss Law above 120K, with a paramagnetic Curie temeprature ?p=67K. Magnetization measurements below 47K indicate a spin-flop or metamagnetic transition in an applied field of about 12KOe. Powder neutron diffraction measurements between 8–45K reveal that all the Fe2+ spins within an octahedral band are ferromagnetically coupled parallel to the b axis, with each band antiferromagnetically coupled to neighboring bands. Below 8K Fe2+ spins at the M1 and M4 sites are canted away from the b axis, whereas those at the M2 and M3 sites are not significantly affected. The ordered Fe2+ moment on the M4 site is substantially lower than those on the other sites, most likely indicating strong covalency effects, i.e. considerable spin transfer to neighboring oxygen atoms.  相似文献   

5.
The electronic and magnetic structure of the chain silicate orthoferrosilite Fe 2 2+ Si2O6 has been investigated by electronic structure calculations in the local spin density approximation. All calculations are based on experimentally determined geometrical data at room temperature. The calculated spin-allowed d–d excitation energies and hyperfine parameters are in quantitative agreement with the respective experimental data from optical absorption and Mössbauer spectroscopy. Inside one ribbon that is parallel to the crystallographic c axis and contains two non-equivalent M1 and M2 sites, all iron spins are ferromagnetically coupled with coupling constants of about +16 cm?1. Between these ribbons within the (b, c)-plane a weak ferromagnetic coupling of about +2 cm?1 is obtained. Neighboured (b, c)-planes are coupled antiferromagnetically via chains of Si B -tetrahedra but ferromagnetically via chains of Si A -tetrahedra. Such a theoretically determined "double-plane antiferromagnetic" spin structure is at variance with an experimentally derived magnetic structure. This discrepancy is attributed to differences between the geometry at room temperature and at temperatures below the Néel temperature currently not available.  相似文献   

6.
Mössbauer spectra of a synthetic, pure almandine (Alm100) and a natural almandine-rich garnet (Alm86) were taken at temperatures between 295 and 4.2 K. Different widths and depths of the lines observed in the paramagnetic state require the spectra to be fitted with at least two components of slightly different quadrupole splittings and isomer shifts. Alm100 shows a gradual onset of magnetic order between 10.4 and 9.0 K, with about 50 percent of the sample magnetically ordered at 9.6 K. At 4.2 K both samples have complex spectra that can be fitted by two eight-line magnetic hyperfine patterns of equal intensities. These result from the existence of two different polar angles between the z-axis of the electric field gradient and the direction of the magnetic hyperfine field, or from two distinctly different electric field gradients. The former interpretation, in which the two angles are attributed to a modulated, non-collinear spin arrangement, is considered more likely. Both magnetic components have practically identical quadrupole splittings, 1/2eQV zz (1+η2/3)1/2, that average ?3.70 mm·s?1, and a small asymmetry parameter of η=0.07. Alm100 has a magnetic hyperfine field of 25.7 T at 4.2 K, whereas Alm86 has a hyperfine field of only 23.3 T; the other features of the spectra of both samples are similar.  相似文献   

7.
Natural barbosalite Fe2+Fe3+ 2 (PO4)2(OH)2 from Bull Moose Mine, South Dakota, U.S.A., having ideal composition, was investigated with single crystal X-ray diffraction techniques, Mössbauer spectroscopy and SQUID magnetometry to redetermine crystal structure, valence state of iron and evolution of 57Fe Mössbauer parameter and to propose the magnetic structure at low temperatures. At 298?K the title compound is monoclinic, space group P21/n, a o ?= 7.3294(16)?Å, b o ?=?7.4921(17)?Å, c o ?=?7.4148 (18)?Å, β?=?118.43(3)°, Z?=?2. No crystallographic phase transition was observed between 298?K and 110?K. Slight discontinuities in the temperature dependence of lattice parameters and bond angles in the range between 150?K and 180?K are ascribed to the magnetic phase transition of the title compound. At 298?K the Mössbauer spectrum of the barbosalite shows two paramagnetic components, typical for Fe2+ and Fe3+ in octahedral coordination; the area ratio Fe3+/Fe2+ is exactly two, corresponding to the ideal value. Both the Fe2+ and the Fe3+ sublattice order magnetically below 173?K and exhibit a fully developed magnetic pattern at 160?K. The electric field gradient at the Fe2+ site is distorted from axial symmetry with the direction of the magnetic field nearly perpendicular to Vzz, the main component of the electric field gradient. The temperature dependent magnetic susceptibility exhibits strong antiferromagnetic ordering within the corner-sharing Fe3+-chains parallel to [101], whereas ferromagnetic coupling is assumed within the face-sharing [1?1?0] and [?1?1?0] Fe3+-Fe2+-Fe3+ trimer, connecting the Fe3+-chains to each other.  相似文献   

8.
Three iron-rich 1:1 clay minerals, greenalite [Si2]{Fe 3 2+ }O5(OH)4, berthiérine [Si, Al]2{Fe2, Mg, Fe3+, Al}3 O5(OH)4 and cronstedtite [Si, Fe3+]2{Fe2+, Fe3+}3O5(OH)4 have been studied by Mössbauer spectroscopy, magnetization measurements and neutron diffraction to determine their magneticproperties. The predominant magnetic coupling is ferromagnetic for pairs of ferrous ions in the octahedral sheet, but antiferromagnetic for ferric pairs. The crystal field at Fe2+ sites in greenalite and berthiérine is effectively trigonal with an orbital singlet l z=0 as ground state. These mainly ferrous minerals order magnetically at 17K and 9K respectively. The magnetic structure of greenalite consists of ferromagnetic octahedral sheets, with the moments lying in the plane, coupled antiferromagnetically by much weaker interplane interactions. The ratio of intraplane to interplane coupling is of order 50, so the silicate has a two-dimensional aspect, both structurally and magnetically. Although the overall magnetic order is established as antiferromagnetic by neutron diffraction, the magnetization curves resemble those of a ferromagnet because of the very weak interplane coupling. Cronstedtite orders antiferromagnetically around 10K. Moments within the planes are antiferromagnetically coupled. The magnetism has no particular two-dimensional character because exchange paths between the layers are provided by the ferric cations present in the tetrahedral sheets.  相似文献   

9.
The physical conditions in the radio sources CTA 21 and OF+247 are studied assuming that the low-frequency spectral turnovers are due to synchrotron self-absorption. The physical parameters of the radio sources are estimated using a technique based on a nonuniform synchrotron source model. It is shown that the magnetic-field distributions in the dominant compact components of these radio sources are strongly inhomogeneous. The magnetic fields at the center of the sources are B ～ 10?1 G, and the fields are two to three orders of magnitude weaker at the periphery. The magnetic field averaged over the compact component is B ～ 10?3 G, and the density of relativistic electrons is n e ～ 10?3 cm?3. Assuming that there is equipartition of the energies of the magnetic field and relativistic particles, averaged over the source, 〈E H 〉 = 〈E e 〉 ～ 10?7–10?6 erg cm?3. The energy density of the magnetic field exceeds that of the relativistic electrons at the centers of the radio sources. The derived parameters of CTA 21 and OF+247 are close to those of the hot spots in the radio galaxy Cygnus A. On this basis, it is suggested that CTA 21 and OF+247 are radio galaxies at an early stage of their evolution, when the hot spots (dominant compact radio components) have appeared, and the radio lobes (weak extended components) are still being formed.  相似文献   

10.
We present the temperature dependence of the specific heat of CoCr2O4 between 2.08 K and 306 K in zero magnetic field. The lattice component can be described by the Komada–Westrum model with a characteristic temperature ΘKW = 541 K. The entropy of the magnetic component amounts to 33.51 J mol?1 K?1 at T = 298.15 K, in good agreement with the magnetic entropy of Co2+ and Cr3+ ions with completely quenched orbital moments. We compare our results with data available in literature.  相似文献   

11.
The motion of electrons and positrons in the vacuum magnetosphere of a neutron star with a surface magnetic field of B ≈ 1012 G is considered. Particles created in the magnetosphere or falling into it from outside are virtually instantaneously accelerated to Lorentz factors γ ≈ 108. After crossing the force-free surface, where the projection of the electric field onto the magnetic field vanishes, a particle begins to undergo ultra-relativistic oscillations. The particle experiences a regular drift along the force-free surface simultaneous with this oscillatory motion.  相似文献   

12.
The electron spin resonance (ESR) spectrum of Cr3+ in a synthetic single crystal of forsterite doped with Cr2O3 was studied at room temperature in the X-band frequency range. The dependence of the observed spectra on the crystal orientation with respect to the applied magnetic field was investigated. The ESR spectra are described by the spin Hamiltonian \(H = \beta HgS + D(S_Z^{\text{2}} - {\text{1/3}}S{\text{(}}S{\text{ + 1)) + }}E{\text{(}}S_x^{\text{2}} - S_y^{\text{2}} {\text{)}}\) with S=3/2. The spin resonance reveals that the chromium ions are located at both the M1 and M2 positions. Other possible substitutional or interstitial Cr3+ positions may be possible, but were not observed. The site occupancy numbers of Cr3+ at M1 and M2 are roughly 1.2×10?4 and 0.8×10?4, respectively, assuming that chromium is oxidized completely. The preference of the chromium ions for M1 was interpreted qualitatively in terms of crystal field criteria. The rhombic and axial spin Hamiltonian parameters, D and E, and the directions of the magnetic axes obtained for M1 and M2 are consistent with the respective oxygen coordination polyhedra.  相似文献   

13.
The electronic and magnetic structure of the chain silicate hedenbergite (CaFe2+Si2O6) has been investigated by a number of experimental methods (neutron diffraction, Mössbauer spectroscopy, low temperature magnetic measurements), as well as by electronic structure calculations for clusters of different size in the local spin density approximation. The calculated size-converged spectroscopic data (d-d excitation energies, hyperfine parameters) are in quantitative agreement with the respective experimental values. The calculated magnetic coupling constants are about +25 cm?1 and ?4 cm?1 for intra-chain and inter-chain coupling, respectively. The latter value shows that weak superexchange via edges of silicon tetrahedra is well reproduced by the calculations, and it is in qualitative agreement with an observed metamagnetic transition at 4.2 K in an external magnetic field with an onset around 4 T but saturation is not achieved in fields up to 14.5 T. The large ferromagnetic intra-chain coupling is attributed to a nearly degenerate ground state. The ratio between the two magnetic coupling constants agrees with earlier estimates on similar compounds. Finally, it is demonstrated how the detailed discussion of the various exchange pathways contributes to an improved understanding of the connection between magnetic properties and the geometrical structure.  相似文献   

14.
Ground-based (Big Bear Solar Observatory) and extra-atmospheric (SOHO/MDI) measurements of the photospheric line-of-sight magnetic field of one active and two quiet regions are used to calculate power spectra of the field, taking into account the characteristic function for the diffraction limit of the telescope resolution. At high frequencies, the physically meaningful linear interval in the spectrum extends to a wave number of k=4.6 Mm?1 (spatial scale l=1.4 Mm) for the quiet regions and k=3.35 Mm?1 (l=1.9 Mm) for the active region. A high-frequency spectral break at k≥3 Mm?1 is associated with the characteristic telescope function; the position of the break and the spectral slope beyond the break do not reflect the turbulent state of the field. As the field recording improves, the break shifts toward higher frequencies. The spectral indices in the physically meaningful linear interval are substantially different for the active and quiet regions: in the active region (NOAA 8375), the spectrum behaves as E(k)≈k ?1.7 (very close to the Kolmogorov index, ?5/3) in the interval 0.78≤k≤3.35 Mm?1, while in the quiet regions E(k)≈k ?1.3 for 0.77≤k≤4.57 Mm?1. This difference can be explained by the additional effect of a small-scale turbulent dynamo in the unperturbed photosphere. In this case, this mechanism can generate at least 6% of the magnetic energy of the photospheric line-of-sight field in quiet regions.  相似文献   

15.
Non-magnetized suspensions of magnetite particles with concentrations in excess of 30% by mass and particle size less than 75 μm exhibit Bingham plastic behaviour. When exposed to external magnetic fields of strengths in excess of 41 × 10−4 T, the rheological behaviour of the suspensions departs from the Bingham model and can be described by a Herschel–Bulkley model of the form τ = τ0 + kγn. The value of the index n was found to range from 0.38 to 0.9, depending on the magnetic field strength, solids concentration and particle size and correlations are proposed for the apparent viscosity of magnetized suspensions as a function of magnetic field strength and solids concentration.  相似文献   

16.
Particles can be accelerated to ultrahigh energies E≈1021 eV in moderate Seyfert nuclei. This acceleration occurs in shock fronts in relativistic jets. The maximum energy and chemical composition of the accelerated particles depend on the magnetic field in the jet, which is not well known; fields in the range ～5–1000 G are considered in the model. The highest energies of E≈1021 eV are acquired by Fe nuclei when the field in the jet is B≈16 G. When B～(5–40) G, nuclei with Z<10 are accelerated to E≤1020 eV, while nuclei with Z≥10 acquire energies E≥2×1020 eV. Only particles with Z≥23 acquire energies E≤1020 eV when B～1000 G. Protons are accelerated to E<4×1019 eV, and do not fall into the range of energies of interest for any magnetic field B. The particles lose a negligible amount of their energy in interactions with infrared photons in the accretion disk; losses in the thick gas-dust torus are also negligible if the luminosity of the galaxy is L≤1046 erg/s and the angle between the normal to the galactic plane and the line of sight is sufficiently small, i.e., if the axial ratio of the galactic disk is comparatively high. The particles do not lose energy to curvature radiation if their deviations from the jet axis do not exceed 0.03–0.04 pc at distances from the center of R≈40–50 pc. Synchrotron losses are small, since the magnetic field frozen in the galactic wind at R≤40–50 pc is directed (as in the jet) primarily in the direction of motion. If the model considered is valid, the detected cosmic-ray protons could be either fragments of Seyfert nuclei or be accelerated in other sources. The jet magnetic fields can be estimated both from direct astronomical observations and from the energy spectrum and chemical composition of cosmic rays.  相似文献   

17.
Data on the global magnetic field (GMF) of the Sun as a star for 1968–1999 are used to determine the correlation of the GMF with the radial component of the interplanetary magnetic field (IMF) |B r|; all data were averaged over a half year. The time variations in the GMF |H| are better correlated with variations in |B r|; than the results of extrapolating the field from the “source surface” to the Earth’s orbit in a potential model based on magnetic synoptic maps of the photosphere. Possible origins for the higher correlation between the GMF and IMF are discussed. For both the GMF and IMF, the source surface actually corresponds to the quiet photosphere—i.e., background fields and coronal holes—rather than to a spherical surface artificially placed ≈2.5 R ⊙ from the center of the Sun, as assumed in potential models (R ⊙ is the solar radius). The mean effective strength of the photospheric field is about 1.9 G. There is a nearly linear dependence between |H| and |B r|. The strong correlation between variations in |H| and |B r| casts doubt on the validity of correcting solar magnetic fields using the so-called “saturation” factor δ?1 (for magnetograph measurements in the λ 525.0 nm FeI line).  相似文献   

18.
Weak, compact radio sources (～100 mJy peak flux, L～1–10 pc) with their spectral peaks at about a gigahertz are studied, based on the complete sample of 46 radio sources of Snellen, drawn from high-sensitivity surveys, including the low-frequency Westerbork catalog. The physical parameters have been estimated for 14 sources: the magnetic field (H ⊥), the number density of relativistic particles (n e), the energy of the magnetic field $(E_{H_ \bot } )$ , and the energy of relativistic particles (E e). Ten sources have $E_{H_ \bot } \ll E_e $ , three have approximate equipartition of the energies $(E_{H_ \bot } \sim E_e )$ , and only one has $E_{H_ \bot } \gg E_e $ . The mean magnetic fields in quasars (10?3 G) and galaxies (10?2 G) have been estimated. The magnetic field appears to be related to the sizes of compact features as $H \sim 1/\sqrt L $ .  相似文献   

19.
Ilvaite samples from six different localities in Japan are found to be members of a solid-solution series varying from Ca(Fe2+,Fe3+)2Fe2+(OH)O Si2O7 to approaximately Ca(Fe2+,Fe3+)2Fe 0.5 2+ Mn 0.5 2+ (OH)O Si2O7, and have been studied by Mössbauer spectrometry and magnetic measurements. The variation in intensity of Mössbauer doublets confirms that Mn substitutes for Fe2+ in the M(B) cation site. An temperatures decreasing from 300 K to 4K, an abrupt change in the reciprocal mass magnetic susceptibility, 1/x g, occurs about 120 K; 1/x g depends linearly upon temperature above 120 K. This change, which is characterized by an unusual mode of decrease in 1/x g, has been interpreted based on Mössbauer spectra at 80 K: the spectra of Fe2+ and Fe3+ in the M(A) site show Zeeman splitting, whereas those of Fe2+ in the M(B) site do not show the effect. This Mössbauer evidence suggests that magnetic spins of Fe in M(A) are in an ordered state, very likely of antiparallel coupling, whereas those of Fe in M(B) are randomly oriented, showing that below 120 K ilvaite has two different magnetic states for Fe ions. As there is a line of evidence that the spins of Fe in M(B) would take an ordered state at extremely low temperatures, ilvaite magnetism may be regarded as basically antiferromagnetic. The magnetic spins of Fe in M(A) and M(B) undergo magnetic transitions at different specific temperatures, thus giving as a whole unusual features of magnetism.  相似文献   

20.
Susceptibility, magnetisation and Mössbauer measurements are reported for a representative selection of 2:1 layer phyllosilicates. Eight samples from the mica, vermiculite and smectite groups include examples diluted in iron which are paramagnetic at all temperatures, as well as iron-rich silicates which order magnetically below 10 K. Anisotropic susceptibility of crystals of muscovite, biotite and vermiculite is quantitatively explained with a model where the Fe2+ ions lie in sites of effective trigonal symmetry, the trigonal axis lying normal to the sheets. The ferrous ground state is an orbital singlet. Ferric iron gives an isotropic contribution to the susceptibility. Fe2+-Fe2+ exchange interactions are ferromagnetic with y ～ 2 K, whereas Fe3+-Fe3+ coupling is antiferromagnetic in the purely ferric minerals. A positive paramagnetic Curie temperature for glauconite may be attributable to Fe2+ → Fe3+ charge transfer. Magnetic order was found to set in inhomogeneously for glauconite at 1–7 K. One biotite sample showed an antiferromagnetic transition at T N =7 K marked by a well-defined susceptibility maximum. Its magnetic structure, consisting of ferromagnetic sheets with moments in their planes coupled antiferromagnetically by other, weak interactions, resembles that found earlier for the 1:1 mineral greenalite.  相似文献   

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