MAS, STMAS and DQMAS NMR Studies of the Thermal Transformation of Kaolinite

Thermal transformations of kaolinite at different temperatures were monitored using X-ray diffraction (XRD), high-resolution solid-state nuclear magnetic resonance (800 MHz for ¹H Larmor frequency) with single-pulse magic-angle spinning, double-quantum filter satellite-transition magic-angle spinning, and double-quantum homo-nuclear correlation under magic-angle spinning experiments. Results show that combined experiments clearly manifest the transitions of silicon and aluminum structures at different thermal treatment stages; and moreover, high magnetic field offers higher sensitivity and resolution, hereby the slim resonances are obtained successfully at less stringent conditions. The dehydroxylation process of kaolinite causes the presence of short-range order in metakaolinite, which is absence of XRD reflections. Particularly, the features of metakaolinite with high concentration of defects are found with dispersive aluminum coordinations; and further, the distorted tetrahedral aluminum is detected in kaolinite-derived mullite because of the locally disorganized structure. The framework structure of kaolinite-derived mullite is considered to be primarily formed by the tetrahedral aluminum bonding with octahedral aluminum. In addition, except for the primary limitation caused by potentially long relaxation time of sample, such approaches are applicable to obtain detailedly structural characteristics of aluminosilicate.     

Fabrication and magnetic properties of hexagonal BaFe12O19 ferrite obtained by magnetic-field-assisted hydrothermal process

High magnetic field effects on the microstructure and magnetic properties of BaFe₁₂O₁₉ hexaferrites synthesized hydrothermal method have been investigated. The obtained results indicate that the lattice constant decreases gradually as the magnetic field strength increases, which may be attributed to the lattice distortion resulted from the high magnetic field. Polycrystalline BaFe₁₂O₁₉ samples prepared under magnetic field strength at zero and 5 T are single phase. It is found that application of external magnetic field during synthesis can induce orientated growth of the hexaferrite crystals along the easy magnetic axis. The magnetic properties can be effectively regulated by an application of high magnetic fields. It is observed that the BaFe₁₂O₁₉ prepared under a 5 T magnetic field exhibits a higher room-temperature saturation magnetization (66.3 emu/g) than that of the sample (43.6 emu/g) obtained without magnetic field. The results can be explained as the enhanced crystalline, improvement of Fe³⁺ ions occupancy and the oriented growth induced by the external magnetic field. The growing orientation of particles gives rise to increased coercivity due to the enhancement in shape anisotropy. It is expected that an application of magnetic field during the formation of magnetic nanoparticles could be a promising technique to modify magnetic properties with excellent performance.     

以巨脉冲红宝石激光为光源的Mach-Zehnder干涉仪应用于θ收缩等离子体的研究

本文应用巨脉冲红宝石激光为光源的Mach-Zehnder干涉仪,研究了23kJ(千焦耳)“θ收缩”(θ-pinch)等离子体的电子密度、形状和不稳定性。从轴向干涉图的照片获得45至120mTorr(毫乇)氘气压范围的二维电子密度分布和电子密度的峰值在6×10¹⁶—2.3×10¹⁷cm^-3范围。等离子体最大收缩出现在主压缩磁场的第一个半周期的1/3附近,在等离子体的最大收缩附近的特征约束时间(粒子数衰减1/e的时间)为1.8—4.3μs,具有负偏磁场的干涉图显示在等离子体中捕获了偏磁场。从这些干涉图的照片上还看到等离子体在主压缩磁场和等离子体捕获场之间作径向磁流体振荡。干涉图照片还表明,在45至80mTorr氘气压范围内,等离子体在主压缩磁场的第一个半周期内基本上都是稳定的。     

Spectroscopic issues in optical polarization of 3He gas for Magnetic Resonance Imaging of human lungs

The Magnetic Resonance Imaging (MRI) of human lungs for diagnostic purposes became possible by using nuclear spin hyperpolarized noble gases, such as ³He. One of the methods to polarize ³He is the Metastability Exchange Optical Pumping (MEOP), which up to now has been performed at low pressure of about 1 mbar and in low magnetic field below 0.1 T (standard conditions). The equilibrium nuclear polarization can reach up to 80%, but it is dramatically reduced during the subsequent gas compression to the atmospheric pressure that is necessary for the lungs examination. Further polarization losses occur during the transportation of the gas to the hospital scanner. It was shown recently that up to 50% polarization can be obtained at elevated pressure exceeding 20 mbar, by using magnetic field higher than 0.1 T (nonstandard conditions). Therefore, following the construction of the low-field MEOP polarizer located in the lab, a dedicated portable unit was developed, which uses the magnetic field of the 1.5 T MR medical scanner and works in the continuous-flow regime. The first in Poland MRI images of human lungs in vivo were obtained on the upgraded to ³He resonance frequency Siemens Sonata medical scanner. An evident improvement in the image quality was achieved when using the new technique. The paper shows how spectroscopic measurements of ³He carried out in various experimental conditions led both to useful practical results and to significant progress in understanding fundamental processes taking place during MEOP.     

Ion-beam induced changes in magnetic and microstructural properties of thin iron films

Changes in magnetic and structural properties of 60–82 nm iron films induced by heavy-ion implantation were studied using the magneto-optical Kerr effect, M?ssbauer spectroscopy, Rutherford backscattering spectroscopy, X-ray diffraction, and X-ray absorption fine structure. The influence of ion-beam parameters (ion mass, fluence) and of sample parameters (external magnetic field and stress during implantation) were investigated. The Fe films, some of them containing a thin ⁵⁷Fe marker layer for M?ssbauer spectroscopy, were deposited on Si(100) substrates, by electron-beam and effusion-cell evaporation. The films were irradiated with ²⁰Ne, ⁵⁶Fe, ⁸⁶Kr and ¹³²Xe ions at energies chosen so that the implantation profiles peaked near the middle of the Fe films. The as-deposited films were magnetically isotropic and had a high coercivity. After ion implantation, the coercivity decreased and magnetic anisotropy developed. Both changes correlated with a decrease in the internal film stress. External mechanical stress applied during the irradiation had hardly any influence on the magnetic texture, opposite to an external magnetic field applied during or before ion implantation. The results are compared with those obtained for ion-irradiated polycrystalline Ni films and epitaxial Fe films and discussed with respect to the role of radiation-induced extended defects as pinning centers.     

Dirac-neutrino magnetic moment and the dynamics of a supernova explosion

The double conversion of neutrino chirality ν_L → ν_R → ν_L has been analyzed for supernova conditions, where the first stage is due to the interaction of the neutrino magnetic moment with plasma electrons and protons in the supernova core, and the second stage, due to the resonance spin flip of the neutrino in the magnetic field of the supernova envelope. It is shown that, in the presence of the neutrino magnetic moment in the range 10^?13 μ_B < μ_ν < 10^?12 μ_B and a magnetic field of ～10¹³ G between the neutrinosphere and the shock-stagnation region, an additional energy of about 10⁵¹ erg, which is sufficient for a supernova explosion, can be injected into this region during a typical shock-stagnation time.     

Random textures of the order parameter of superfluid 3He-B in aerogel

A phenomenological scheme of the observed properties of superfluid ³He in aerogel is proposed in the spirit of the Ginzburg-Landau theory. The effect of the aerogel on the order parameter is described by the random tensor field η_jl(r). The tensor field exerts a considerable disorienting effect on the order parameter in the A phase of ³He, but virtually unaffects the orientation of the order parameter in the B phase in zero magnetic field. The change in the texture of the order parameter emerging in the B phase in aerogel in a magnetic field is considered. It is shown that the mean square of the angle between the magnetic field direction and the anisotropy axis of the B phase is proportional to the third power of the magnetic field strength. The fluctuations of the direction of the magnetic anisotropy axis of ³He-B are correlated over the familiar “healing length”, which is inversely proportional to the field strength and has a macroscopic scale.     

Imaging magnetic responses of nanomagnets by XPEEM

The Spin-resolved Photoelectron Emission Microscope (SPEEM) is a permanently installed set-up at Helmholtz-Zentrum Berlin (HZB). Due to its specific contrast it is mainly used for magnetic imaging and micro-spectroscopy with quantitative analysis. A crucial point in magnetic imaging is the application of magnetic fields. Many experiments require observation of magnetic responses or the preparation of a certain magnetic state during the measurement. We present a dedicated magnetic sample holder combining magnetic field during imaging with additional temperature control. This set-up enables SPEEM to measure magnetization curves of individual Fe nanocubes (18 nm)³ in size. If additionally alternating magnetic fields are applied we can image the local magnetic AC susceptibility (χ_AC) as a function of temperature. The latter is ideally suited to visualize local variations of the Curie temperature (T_C) in nano- and microstructures.     

NEGATIVE DONOR CENTER QUANTUM DOTS IN MAGNETIC FIELDS

The method of few-body physics is applied to treat a D^{-<\sup> center quantum dot system in a magnetic field. The magnetic field is applied in the z direction. Using this method, we investigate the energy spectra of low-lying states of D-<\sup> center quantum dots as a function of magnetic field. The dependence of the binding energies of the ground-state of the D-<\sup> center are calculated as a function of the dot radius with a few values of the magnetic field strength and compared with other results.}     

The magnetic hyperfine interaction of iron atoms isolated in a xenon matrix in the presence of an external magnetic field

The Mössbauer effect has been used to measure the magnetic hyperfine interaction of isolated ⁵⁷Fe atoms in solid xenon with an applied external magnetic field. A field dependent Mössbauer absorption spectrum is observed. The ground state of these iron atoms is a triplet, which is split in the external field. The Mössbauer spectrum was analyzed taking into consideration relaxation effects. For an applied external field of 28 kOe an internal magnetic field at the ⁵⁷Fe nucleus of 700± 15 kOe was observed (external field included).     

Effect of strong magnetic field on chemical potential and electron capture in magnetar

The chemical potential of electrons in a strong magnetic field is investigated. It is shown that the magnetic field has only a slight effect on electron chemical potential when B 〈 10^11 T, but electron chemical potential will decrease greatly when B 〉 10^11 T. The effects of a strong magnetic field on electron capture rates for ^60Fe are discussed, and the result shows that the electron capture sharply decreases because of the strong magnetic field.     

Temperature Dependence of the Magnetic Hyperfine Field at 140Ce on Gd Sites in GdAg Compound

Perturbed gamma–gamma angular correlation technique was used to measure the magnetic hyperfine field at Gd sites in the intermetallic compound GdAg using the ¹⁴⁰La→¹⁴⁰Ce nuclear probe. A major and well-defined magnetic interaction is observed at ¹⁴⁰Ce substituting Gd sites in GdAg below 130 K, corresponding to a ferromagnetic ordering of Gd moments. The temperature dependence of magnetic hyperfine field, however, shows a sharp deviation from an expected Brillouin-like behavior for temperatures below 75 K. This additional magnetic interaction is believed to result from the polarization of Ce spin moments induced by the magnetic field from Gd atoms.     

Mössbauer and mineral magnetic studies on archaeological potteries from Adhichanallur, Tamilnadu, India

Megalithic potteries collected from Adhichanallur, Tamilnadu, India (Lat. 8°44′ N; Long. 77°42′ E) have been subjected to various spectroscopic and rock magnetic studies. The type of clay, their origin, level of structural deformation due to firing, firing temperature and atmospheric condition followed during making the potteries are analyzed. The potteries were subjected to Mössbauer and X-ray diffraction studies to analyze the iron phases in them. It is found that the samples were made of local clay (red clay), fired above 600°C under open atmospheric and/or reduced atmospheric conditions and air has been allowed during cooling. The Mössbauer spectra reveal the presence of Fe^3?+?, Fe^2?+? and iron oxides of hematite and magnetite. The firing temperature and firing conditions established from Mössbauer studies are similar to the observation made from FT-IR studies. The magnetic mineral types, the mass fractions and the domain states of the constituent magnetic grains were elucidated from a range of rock magnetic measurements including variation of susceptibility with low field, frequency and temperature, hysteresis parameters and isothermal remanence acquisition data. The magnetic mineralogy of most pottery samples was dominated by magnetite/(titano) magnetite, while magnetic grain size spectrum varies from very fine (super paramagnetic) to fine (stable single domain, pseudo single domain). The reversible thermo magnetic behavior reflects no mineralogical transformations during reheating and all the samples show same Curie temperature 580°C due to magnetite. From the above information it is demonstrated that these samples are suitable for determining the reliable ancient geomagnetic field intensity values existed during that period.     

Meissner effect in supercoducting cores of neutron stars

Evolution of the magnetic field of a neutron star (pulsar) under the influence of the superconductivity of a proton plasma in the stellar core is considered. The magnetic field expulsion from the core due to the Meissner effect for the second-type npe-superconductor is shown to be inefficient for the magnetic fields B ₀<10¹⁴ G. Therefore, neglecting other expulsion mechanisms (e.g., related to the buoyancy of the Abrikosov vortices or their pinning to the Onsager-Feynman vortices in the superfluid interior of the rotating star), such magnetic fields should be frozen into the core for>10¹⁰ years, i.e., during the entire lifetime of the star.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Radiofizika, Vol. 39, No. 1, pp. 26–33, January, 1996.In conclusion, authors wish to thank V. V. Kurin, Yu. V. Petukhov, D. A. Ryndyk, and V. Yu. Trakhtengerts for discussion of the problems of vortex dynamics and magnetic field evolution on neutron stars.     

Effect of high magnetic field annealing on the microstructure and magnetic properties of Co-Fe layered double hydroxide

The effects of high magnetic field (10 T) on the products obtained by calcination of Co-Fe LDH precursors at different temperatures were investigated. The XRD results indicated that Fe^III substituted for Co^III in Co₃O₄ to yield Co^IICo^IIIFe^IIIO₄ under the calcination of Co-Fe LDH precursors at 400 °C. The products obtained by magnetic field annealing at 400 °C had a porous plate-like morphology, whereas the products without magnetic field annealing were composed of nanoparticles. It was seen that CoFe₂O₄ phase could be formed at low temperature (about 500 °C) under the magnetic field annealing. The grain size of products obtained by magnetic field annealing at 800 °C was larger than that of zero magnetic field. It was found that the saturation magnetization was significantly enhanced after magnetic field annealing, especially at lower temperature (≤600 °C). The possible reason for the effects on the microstructure and magnetic properties of products obtained by magnetic field annealing was discussed.     

Double resonance experiments in low magnetic field: Dynamic polarization of protons by N and measurement of low NQR frequencies

The possibilities of dynamically polarizing proton spin system via the quadrupole ¹⁴N spin system in low magnetic field are analyzed. The increase of the proton magnetization is calculated. The polarization rate of the proton spin system is related to the transition probabilities per unit time between the ¹⁴N quadrupole energy levels and proton energy levels. The experiments performed in 1,3,5-triazine confirm the results of the theoretical analysis. A new double resonance technique is proposed for the measurement of nuclear quadrupole resonance frequencies ν_Q of the order of 100 kHz and lower. The technique is based on magnetic field cycling between a high and a low static magnetic field and observation of the proton NMR signal in the high magnetic field. In the low magnetic field the quadrupole nuclei and protons resonantly interact at the proton Larmor frequency ν_H = ν_Q/2. The quadrupole nuclei are simultaneously excited by a resonant rf magnetic field oriented along the direction of the low static magnetic field. The experimental procedure is described and the sensitivity of the new technique is estimated. Some examples of the measurement of low ¹⁴N and ²H nuclear quadrupole resonance frequencies are presented.     

Photoinduced dynamic magnetic structure in FeBO3:Ni and its main properties

Main properties of the photoinduced dynamic structure in FeBO₃:Ni have been studied magneto-optically. The oscillations of the ferromagnetic moment are found to follow a quasiharmonic law. The deviation of the equilibrium direction of m in the structure from the direction of the applied magnetic field and the amplitude of its oscillations are found to decrease and the cyclic frequency of the oscillations to increase non-linearly, when the magnetic field applied along the wave vector of the structure during illumination is growing. The phase velocity of the photoinduced structure is found to increase linearly, when the intensity of the exciting illumination is growing. The discovered state of the magnetic system in FeBO₃:Ni is considered to be due to the Ni^III+ ions in the crystal and their interaction with photo-exited ions of the Fe³⁺ matrix.     

Cyclotron mechanism of electron acceleration in subpicosecond laser plasma

The effect of ultrastrong magnetic fields generated in a relativistic-intensity subpicosecond laser plasma on the acceleration of fast electrons was studied. It is shown that resonance electrons can continuously accumulate energy from the circularly polarized laser field in the presence of a longitudinal magnetic field. For the linear polarization and a transverse magnetic field, energy accumulation has a pulse-periodic character, and the electron trajectories correspond to electron rotation in the Larmor orbit in a quasi-stationary magnetic field, while the energy strongly oscillates. In both cases, electron energy may attain values higher than 100 MeV for intensities of 10²⁰ W/cm².     

Magnetic texturing of xenon-ion irradiated nickel films

Thin polycrystalline Ni films of typically 75 nm thickness evaporated on Si or SiO₂ substrates were irradiated with 30-900 keV Xe-ions to fluences of 2.5 x 10¹³ - 4 x 10¹⁴/cm². The magnetization of the Ni films was measured using the longitudinal Magneto-Optical Kerr Effect and Vibrating Sample Magnetometry. The Ni-film thickness and Xe-concentration profiles were determined with Rutherford backscattering spectroscopy and the lattice dilation with X-ray diffraction. The Xe-irradiations were found to induce an in-plane uniaxial magnetic anisotropy within the Ni-films. This magnetic texture was investigated in relationship to the microstructure as function of the ion energy and fluence, the sample temperature, the presence of an external magnetic field during the irradiation and the stress field produced before, during and after the implantations.Received: 30 September 2004, Published online: 14 December 2004PACS: 61.82.Bg Metals and alloys - 68.55.Ln Defects and impurities: doping, implantation, distribution, concentration, etc. - 75.30.Gw Magnetic anisotropy - 75.70.-i Magnetic properties of thin films, surfaces, and interfaces