Influence Of High Magnetic Field Research Articles

Magnetization for lower temperature, selective diamond and carbon nanotube formation: A milestone in carbon physicochemical condensation

Diamonds and carbon nanotubes (CNTs) have extraordinary properties with the potential for vast technological and scientific advancements. However, the syntheses of these super materials have required extreme conditions. Recent synthetic developments surrounding catalytic chemical vapor deposition (CCVD) have contributed to more suitable, practical and economical preparations, but more progress is needed for better selectivity, purity, and mass production of CNTs and diamonds. Such synthetic advancements require a deeper understanding of the mechanisms of formation on the atomic scale. A recent comprehensive mechanism of Little suggests the importance of high-spin electronic states and the rehybridization mechanics of carbon atoms and metal atoms (for catalytic synthesis) during the nucleation and growth of CNTs and diamonds. The significance of these predictions is demonstrated in this work by using an intense, static magnetic field of several teslas to influence carbon fixation events during carbon CCVD. Single crystalline diamonds are discovered to nucleate and grow under the influence of the static magnetic field (19.3 T) under catalytic CVD conditions that normally result in carbon nanotubes. Furthermore, this technique results in a bottom-up approach for creating diamond nucleation sites on the basis of a so-called chemical preabrasion of the silicon substrate with the potential advantage of the control of seeding nucleation-site density and nonrandom patterning for larger single crystal diamond syntheses. This technique also provides a basis for diamond–CNT composite super materials. Moreover, the observed influence of high magnetic field on diamond formation provides implications concerning natural diamond genesis in the earth’s mantle and core in comparison to celestial diamond formation. Furthermore, these findings suggest uses of polarized and nonpolarized neutron irradiation for static and dynamic magnetic interactions, leading to diamond and CNT productivity, respectively.

On the hyperfine interaction in rare-earth Van Vleck paramagnets at highmagnetic fields

The influence of high magnetic fields on the hyperfine interactionin rare-earth ions with a non-magnetic ground state (Van Vleckions) is investigated theoretically for the case of Tm3+ions in an axially symmetrical crystal electric field (in an ethylsulphate crystal). Itis shown that magnetic-field-induced distortions of the 4f-electron shell lead tocrucial changes in the hyperfine magnetic field at the nucleus. The proposedtheoretical model is in agreement with recent experimental data.

Influence of high magnetic field on the corrosion of carbon steel

An experimental study of the corrosion pattern of steel exposed to intense magnetic fields is reported. Experiments were performed on two different shaped surfaces, cylinders and grooves. Uniform magnetic fields of up to 10 T were applied to these samples. Non-uniform corrosion patterns on the steel cylinder surface were observed. Further, when the magnetic field was applied parallel to the plane of the groove, it was found that the corrosion increased on the inside of the groove. In addition, the interpretation of the corrosion pattern is supported by an experiment using aerobic bacteria (i.e., escherichia colis). The relationship between the corrosion pattern and a magnetic field gradient near the steel is discussed.

Investigations of dielectric Van Vleck paramagnets at high magnetic fields and low temperatures

The influence of high magnetic fields on the magnetic properties of dielectric Van Vleck paramagnets is discussed. The dynamics of coupled electron–nuclear spin system is investigated at high magnetic fields and low temperatures.

Impact ionization breakdown of n-GaAs in high magnetic fields

The influence of high magnetic field and its orientation on the impact ionization breakdown of n-type GaAs has been investigated experimentally. Hysteretic behaviour of current-voltage characteristics was observed for longitudinal magnetic field orientation to the current direction and no hysteresis was observed for transverse. The threshold voltage was much higher for transverse than for longitudinal magnetic field orientation to the current direction. The dependencies of transverse and longitudinal magnetoresistance on magnetic fields in the electric fields in pre- and post-breakdown regimes were also measured. In the pre-breakdown regime (hopping conduction) there is no great difference between the transverse and longitudinal magnetoresistance in the case when the magnetic length is much higher than the radius of an electron in the ground state on the impurity atom (low magnetic fields). In the opposite case (high magnetic fields) the transverse magnetoresistance is much higher than the longitudinal one. Such a behaviour of magnetoresistance in the pre-breakdown regime is, possibly, due to magnetic-field-induced shrinkage of the electron wavefunction. The observed great difference between the transverse and longitudinal magnetoresistance in the post-breakdown regime can be explained in terms of the Lorentz force influence on the filamentary current flow.

Unusual magnetism of U 2Rh 3Si 5 in high magnetic fields

Our former investigations have shown U 2Rh 3Si 5 to exhibit at 25.5 K a spin-lattice coupled first-order transition into an antiferromagnetically ordered state. Large ordered moments of 2.3 μ B (at 9 K) align in a canted antiferromagnetic structure. In this publication we address the influence of high magnetic fields on the unusual magnetism. Specific heat measurements (<12 T), magnetization measurements (<30 T), and longitudinal magnetoresistance measurements (<20 T) on single crystalline material have been performed. The magnetic phase diagram is determined. The b -axis is the easy magnetic axis and for B>14.5 T along the b -direction the antiferromagnetic transition is suppressed. The first-order character of the magnetic ordering persists in high magnetic fields as shown by the specific heat measurements. The Fermi-surface is strongly affected by the magnetism.

Small-polaron transport inLa0.67Ca0.33MnO3thin films

We present a detailed study of the activated resistivity of ${\mathrm{La}}_{0.67}{\mathrm{Ca}}_{0.33}{\mathrm{MnO}}_{3}$ films up to 600 K under the influence of high magnetic fields. Data in zero field can be explained by small polaron hopping as treated in the Friedman-Holstein theory. Based on the spin orientation of ferromagnetic clusters in a magnetic field, we develop a phenomenological model describing the temperature and field dependence of the resistivity with a minimum of free parameters. We find that the polarons have a magnetic contribution to their activation energy for hopping which depends on the variation of the spin order with increasing temperature and can be modified by applied magnetic fields. The average magnetic clusters contain 4--6 ions. Hall measurements show a temperature dependent electronlike linear low field slope predicted by the theory of the Hall mobility of small polarons.

Field-Induced Itinerant Metamagnetism in Iron Monosilicide

Influence of high magnetic fields and pressure on band structure of FeSi has been studied by the full-potential LMTO–ASA method. It is found that the semiconducting ground state of FeSi is stable in the wide volume range of ±15% of V / V exp . At the same time, as follows from the LSDA calculations, the applied external magnetic fields induce the metallic magnetic state. At low fields our results agree with the measured magnetization. There is steep increase of calculated magnetization at high critical field H cr = 170 T (itinerant metamagnetic transition).

Magneto-optical effects on shallow donor states in 6H-SiC in high magnetic fields.

Impurity excitation spectra in 6H-SiC doped with nitrogen donors are studied by infrared absorption measurements in the frequency range 200--1400 ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}1}$ under the influence of high magnetic fields up to 20 T. No line splitting is observed for any orientation of the magnetic field. However, four transitions show a diamagnetic shift if the magnetic field is oriented along the crystal c axis. This diamagnetic shift is found to be anisotropic, depending on the orientation of the viewing direction towards c. From these results we conclude that the effective mass ellipsoids describing the conduction-band minima must be oriented along lines of the Brillouin zone, which are parallel to the crystal c axis, and that the effective mass tensor has three independent components. Using the symmetry oproperties of the Brillouin zone it is shown that the conduction-band minimum must be located somewhere along the line M-L in the Brillouin zone. From the experimental results at B=20 T it is estimated that the average effective mass ${\mathit{m}}^{\mathrm{*}}$=(${\mathit{m}}_{\mathit{x}}$${\mathit{m}}_{\mathit{y}}$${\mathit{m}}_{\mathit{z}}$${)}^{1/3}$ must be ${\mathit{m}}^{\mathrm{*}}$&gt;0.3${\mathit{m}}_{0}$.

Raman study of crystal-field-phonon coupling in NdBa2Cu3O7- delta in a magnetic field.

We study the influence of high magnetic fields on the coupled phonon--crystal-field excitation in the high-temperature superconductor ${\mathrm{NdBa}}_{2}$${\mathrm{Cu}}_{3}$${\mathrm{O}}_{7\mathrm{\ensuremath{-}}\mathrm{\ensuremath{\delta}}}$ by Raman scattering. With wave functions obtained from inelastic neutron scattering, the Zeeman splitting of the Kramers degenerate crystal-field ground state is evaluated for magnetic fields along the main crystallographic axes. The twofold degeneracy of that excitation, which at zero field was found to couple with a phonon of the same symmetry, is lifted at finite fields. This is used to tune the energy of the crystal-field excitation with respect to that of the phonon. In Raman spectra for magnetic fields up to 16 T we observe broadenings, frequency shifts, and relative intensity changes for the lines of the zero-field doublet. This behavior is in agreement with theoretical predictions for the Zeeman splitting of the crystal-field excitation and the varying phonon content of the coupled wave functions.

Current transfer in HTSC and at high pressures

The authors have measured the pressure effects on the superconducting Tc and the current density Ic of the YBaCuO (123) phase and the BiSrCaCuO (2223) phase up to 10 kbar. They have found that for Bi ceramics the increase in Jc with pressure is related to the Tc change, while for YBCO it is due to the Josephson medium weak-link effects. This result provides the evidence that in Bi compounds strong-current Josephson contacts are possible. The influence of high magnetic fields on the Josephson critical current is briefly discussed.

Influence of high magnetic fields on the classical and quantum-mechanical transport in point contacts.

On spear-anvil-type point contacts of As both positive and negative ${\mathit{d}}^{2}$V/${\mathit{dI}}^{2}$ spectra are observed, corresponding to, respectively, an increase and a decrease in the contact resistance at characteristic phonon energies. Under the influence of an applied magnetic field, contacts with positive electron-phonon-interaction spectra show a positive magnetoresistance indicating ballistic metallic transport, while contacts with negative spectra give a negative magnetoresistance indicating the presence of a localization area inside the contact. The spectral intensity depends on the applied magnetic field (\ensuremath{\le} 20 T) due to the magnetic-field-induced one-dimensional current spreading through the contact and the magnetic breakdown of weak localization.

The influence of high magnetic fields on the first order magneto-elastic transition in MnFe(P1−yAsy) systems

Magnetic properties of the MnFeAsyP1−y system (y = 0.2; 0.25; 0.30; 0.50) were examined in high dc magnetic fields of up to 20 T in the 4.2–400 K temperature range. Different first-order magnetic transitions were induced by the intense field and the magnetic phase H = T diagrams were deduced. These transitions were associated with magneto-elastic transitions observed in the absence of a field.

Influence of high magnetic fields (10 T) on paramagnons in rare-earth intermetallic compounds

Low temperature (0.5–20 K) high field (0–10T) heat capacity and low field (0–2T) magnetic susceptibility (60 μK−300K) measurements were used to study and characterize the quenching of spin fluctuations (paramagnons) in the highly exchange enhanced RCo 2 phases, where R = Sc, Y and Lu. These RCo 2 compounds are characterized by large many-body mass enhancements (2.54 to 5.38) and large Stoner factors (15.5 to 25.3). The application of magnetic fields lowers their electronic specific heat constants by 7 to 10% at 10 T. For YCo 2 the coefficient of the T 3 term (β) in the normal heat capacity equation is found to increase with increasing field, but for ScCo 2 and LuCo 2 β remains constant.

Flocculation of diamagnetic particles in high magnetic fields

The combined influence of high magnetic fields and concentration of a paramagnetic electrolyte on the stability of a suspension of diamagnetic quartz particles has been investigated. For the first time, formation of magnetically induced binary pairs of particles has been observed over a broad range of electrolyte concentration and is explained in terms of an extended Derjaguin–Landau–Verwey–Overbeek (DLVO)-theory. Anomalous effects at low fields are shown to be caused by particle rotation due to an anisotropic magnetic susceptibility.

Particle aggregation in colloids in high magnetic fields

The influence of high magnetic fields on the stability of colloidal dispersions of paramagnetic and diamagnetic particles has been investigated. The theory of Svoboda is extended to allow examination of the dynamics of particle aggregation in colloids. Particle trajectories are calculated leading to binary pair formation. Experimental results are presented on particle aggregation in high magnetic fields (flocculation). Here binary pairs seem to form and to be stable at enhanced rates of settling.

Influence of high magnetic fields on meiosis

In a homogeneous, high magnetic field, the number of chromosomal aberrations during meiotic division in the sporogenic tissue of lily is increased.

Application of a vibrating sample magnetometer to magnetic measurements under hydrostatic pressure

A vibrating sample magnetometer has been adapted for use with nonmagnetic high hydrostatic pressure clamps of the type described by Wohlleben and Maple. With this apparatus, the magnetic moment of 0.2 cm3 samples can be measured vs temperature under the simultaneous influence of high magnetic fields (to ≃7 T) and high hydrostatic pressure (to ≃8 kilobar). The sensitivity of the apparatus is at present ≃2×10−3 emu, limited by the reproducibility from run to run of the background moment of the ≃40 g pressure clamp.

Does partial ineffectivity of the energy gap in superconductors occur under the influence of high magnetic fields?

An examination of the Gibbs function of a type II superconductor as a function of the magnetic field for different field and temperature relations indicates that the energy gap is at least partially ineffective. (D.C.W.)