Antisite defects and valence state of vanadium in Na<Subscript>3</Subscript>V<Subscript>2</Subscript>(PO<Subscript>4</Subscript>)<Subscript>3</Subscript>

Electron paramagnetic resonance (EPR) studies have been performed with the aim of determining the valence state and local crystal structure of the nearest environment of vanadium ions in the initial, charged, and discharged samples of the cathode material Na_xV₂(PO₄)₃ (1 ≤ x ≤ 3). It has been found that the charged sample (x = 1) is characterized by an intense signal corresponding to V⁴⁺ ions located in a highly distorted octahedral crystal field. An EPR signal with the g-factor close to the g-factor of the V⁴⁺ ion has also been observed in the initial sample (x = 3), where the intensity of the resonance signal is one order of magnitude lower than that in the charged sample. It has been revealed that the resonance signal under consideration is associated with the formation of antisite defects when a part of vanadium ions are located in sites of sodium ions. It has also been found that the intensity of this signal increases after a complete charge–discharge cycle (x = 3).     

Anomalous dependence of the magnetic resonance signal of cesium atoms in the afterglow in an N<Subscript>2</Subscript>-Ar mixture

The investigation of spin-exchange collisions between optically oriented cesium atoms in the ground ² S _1/2 state and nitrogen atoms in the ground ⁴ S _3/2 state reveals an anomalous behavior of the magnetic resonance signal of cesium atoms in the afterglow in an N₂-Ar mixture, namely, the magnetic resonance signal is slowly enhanced during the time interval between the high-frequency pulses exciting a discharge in the absorption cell. It is found that such a behavior of the magnetic resonance signal is explained by a slow change in the concentration of nitrogen atoms in the absorption cell, which affects the magnetic resonance of cesium atoms via efficient spin exchange.     

Microwave response to a magnetic phase transition in a molecular magnet based on [Mn12O12( Me CO2)16(H2O)4] clusters and tetramethyltetrathiafulvalene molecules

The electron paramagnetic resonance spectra of a molecular magnet synthesized from [Mn₁₂O₁₂(MeCO₂)₁₆(H₂O)₄] high-spin clusters and tetramethyltetrathiafulvalene donor organic molecules are investigated. It is revealed that, in the temperature range 5–7 K, there appears an additional signal against the background of the ferromagnetic resonance spectrum. The additional signal consists of a large number of lines and has a resonance nature, because it is observed in the well-reproduced narrow ranges of the constant magnetic field of the spectrometer (300–650 and 850–1300 Oe) at a frequency of 9.4152 GHz. The appearance of the additional signal at a temperature of 5 K is attended by a multiple increase in the magnetic susceptibility of the sample. There is a correlation between the spectral characteristics of the additional signal (such as the average amplitude, the spectral power, etc.) and the parameters of the ferromagnetic resonance spectrum (the resonance field, the linewidth, and the integrated intensity). The assumption is made that the additional signal is associated with the magnetic-field-induced transitions between spin states of Mn₁₂ high-spin clusters for different orientations of crystallites with respect to the external magnetic field. Original Russian Text ? R.B. Morgunov, V.L. Berdinskiĭ, A.I. Dmitriev, Y. Tanimoto, 2007, published in Fizika Tverdogo Tela, 2007, Vol. 49, No. 5, pp. 945–950.     

Theory of optical ramsey resonance in three separated fields produced by a corner reflector

In this paper we study the optical Ramsey resonance signal of two-level molecules in three separated fields consisting of one standing wave and two traveling waves of opposite directions with each other. Their separations and phase differences are given by a corner reflector. The transition probability of a molecule that traverses the three interaction regions is calculated by using an optical Bloch vector formalism. It is then integrated over distributions of molecular velocities and positions to evaluate the signal observed with a gas cell. The result is applied to compute the optical Ramsey resonance signal in the absorption line of CH₄ at 3.39 μm. Variations of the Ramsey resonance lineshape and the intensity with the gas pressure and the field intensity are obtained.     

Ferromagnetism in Thermally Decomposed Polyimide Films

Magnetic resonance studies of polyimide films thermally decomposed in flowing N₂ at 520°C reveal the presence of two very different magnetic resonance spectra at room temperature. One spectra is a sharp temperature independent paramagnetic resonance line having a g value of 1.990, typical of a free radical. The other much broader line centered at lower field displays a marked broadening and shift to lower magnetic field as the temperature is lowered, characteristic of a ferromagnetic resonance (FMR) signal. Measurements of the AC susceptibility as a function of magnetic field strength confirm the existence of ferromagnetism at room temperature. Magnetic force microscope (MFM) imaging at room temperature show evidence of long thin ferromagnetic regions in the decomposed polymer.     

利用高温超导直流量子干涉器件进行10$lt;sup$gt;-6$lt;/sup$gt; T量级磁场下核磁共振的研究

研究了水样品在10^-6 T量级磁场下的核磁共振谱.核磁共振信号由一个工作在液氮温度的高温超导直流量子干涉仪记录,测量在一个简易磁屏蔽室中进行.在7—70 μT的磁场范围内都观察到了15 ml水样品的核磁共振信号.相应的^1H的核磁共振频率为300—3000 Hz.在实验中获取的单次测量信噪比约为4,通过对信号的100次平均,信噪比可达到约40.进一步讨论了剩余磁场、预极化时间和采样时间对结果的影响.最后用数字滤波之后平均的方法初步得到了时域的自由感应衰减信号. 关键词： 超导量子干涉仪 核磁共振     

Phase error elimination in the M x magnetometer and resonance line shape control in an unstable field using the technique of invariant mapping of a spin precession signal

A method of two-dimensional mapping of a magnetic resonance signal is proposed that allows one to exclude the effects of field variations on the processes of monitoring of the magnetic resonance spectra and phase correction in the feedback loop of the M _x magnetometer.     

Control of the longitudinal relaxation timeT 1 of a flowing liquid in NMR flowmeters

The influence of a strong inhomogeneous magnetic field of a magnet-polarizer on the longitudinal relaxation timeT ₁ of a flowing liquid is investigated. The increased inhomogeneity of this field causesT ₁ to decrease. The magnitude of the polarizer field and its inhomogeneity exhibit an optimum whose criteria are the signal/noise ratio in a system for registering the signal of nuclear magnetic resonance (NMR), the mass, and the overall dimensions of the device. St. Petersburg State Technical University. Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 9, pp. 50–53, September 1999.     

Application of the spherical tensor method for Two-Frequency pure NQR of spinI=1 nuclei

The results of a theoretical investigation of two-frequency excitation in pure nuclear quadrupole resonance (NQR) for a spinI=1 nucleus with a nonaxial elecric field gradient are presented. The multipole tensor operator technique is used for the treatment of the one- and two-frequency pulse excitations. The results are applied to the characterization of the two-frequency signal of nitrogen¹⁴N nuclei. The experiments on sodium nitrite, NaNO₂, confirm the presence of additional (two-frequency) echo in the NQR signal. The effect of resonance offsets on two-frequency NQR is also considered.     

Investigation of magneto-optical effect in transverse magnetic field in He

The non-linear Voigt effect has been studied in He discharge under resonance laser interaction with the He transition. The range of non-linear signal existence was determined. The contribution of the lower and upper states to the overall signal was analyzed. The cross-section for depolarizing collisions with ground state He atoms was estimated for the 3³ D _2,3 He state. Received: 16 June 1998 / Received in final form: 14 December 1998     

ELECTRON SPIN RESONANCE CHARACTERISTICS OF NITROGEN-DOPED FULLERENE

The nitrogen-doped fullerene has been obtained by arc discharge between two high-purity graphite rods in the atmosphere of N₂ and He, The electron spin resonance(ESR) characteristics of N-doped fullerene have been investigated. The results show that the ESR spectra of N-doped fallerene axe composed of two parts, a paramagnetic signal of N-center and a paramagnetic signal of C-center, For comparing with N-doped fullerene, we have also studied ESR spectra of C₆₀ powder, C₆₀ sublimed film and H-doped C₆₀ film. For C₆₀ powder and H-doped C₆₀ film, their in situ ESR. measurements are carried out at various temperatures, and reasonable explanations are proposed.     

Detection of vortex excitations in a Bi<Subscript>2</Subscript>Sr<Subscript>2</Subscript>Ca<Subscript>2</Subscript>Cu<Subscript>3</Subscript>O<Subscript>10</Subscript> crystal at temperatures above the critical temperature by EPR of the surface layer

Vortex excitations have been detected at temperatures both below and above the critical temperature when investigating local magnetic fields on the surface of a Bi₂Sr₂Ca₂Cu₃O₁₀ single crystal by means of an electron paramagnetic resonance (EPR) probe. A thin layer of a diphenyl picrylhydrazyl organic radical deposited on the crystal surface is used as the EPR probe. A narrow EPR signal makes it possible to detect weak distortions of the magnetic field appearing at T ≿ T _c. The analysis of the temperature dependences of the resonance field and the EPR linewidth is thebasis of the assumption of the vortex nature of magnetic excitations in this temperature range.     

Features of the dispersion characteristics of nonuniform plasma waveguides in a longitudinal magnetic field

We study the features of the dispersion curves and field structures of the fundamental axisymmetric mode of nonuniform layered plasma waveguides in a longitudinal magnetic field. It is shown that the presence of sharp boundaries between layers leads to the appearance of additional branches of the dispersion curves in the frequency range ω _Be < ω < ω_UH(0), where ω_Be is the electron gyrofrequency and ω_UH(0) is the upper-hybrid resonance frequency for the near-axis region of a nonuniform waveguide. The fields of eigenmodes corresponding to these branches comprise resonance structures near the sharp plasma-density variation at which the upper-hybrid resonance conditions are satisfied and plasma waves are excited. The frequency interval of such a branch is limited by the resonant frequencies of the neighboring uniform layers. It turns out that in the case of a strong magnetic field ({ie392-01}, where ω_p is the plasma frequency having the value {ie392-02} in the near-axis region of a nonuniform waveguide), the fundamental-mode field is localized in the near-axis region of a nonuniform waveguide, whereas in the opposite case {ie392-03}, the maximum wave fields are localized in either the upper-hybrid resonance region or the outer (near-boundary) layer of the waveguide if there is no resonance region. It is found that the whistler (helicon) contribution to the field structure of the fundamental axisymmetric mode is very small for narrow nonuniform waveguides (b < λ₀, where b is the waveguide radius and λ₀ is the wavelength in free space) if the plasma density on the axis is high compared with the cutoff density {ie392-04}. We present one of the possible explanations for the effect of narrowing of the plasma channel of a high-frequency whistler-range discharge with distance from a source in the increasing magnetic field. __________ Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Radiofizika, Vol. 51, No. 5, pp. 434–446, May 2008.     

CO2 Laser Discharge Influenced by a Rotating Magnetic Field

This paper deals with experimental investigations of the effect of a rotating magnetic field on a diffusion cooled CO₂ laser discharge. Some properties relevant to laser activity are discussed, e.g. neutral gas temperature, magnetically driven gas rotation, pumping rate, and distribution of small signal gain within the discharge tube. By means of an intracavity method the radial distribution and the time behavior of the laser output power were measured.     

FMR in thin ferromagnetic granular film

The ferromagnetic resonance is considered for a thin granular film consisting of spherical metal particles. It is shown that the interparticle dipole interactions lead to a noticeable shift of the resonance field and a strong asymmetry of absorption line. The free induction signal is time modulated and its envelopem _ind(t) obeys the law ln (m _ind) ≈t ^2/3. The role of the particle shape in considered system is also discussed.     

Interrelationship between the polarization moments of different parity upon optical pumping of atoms under magnetic resonance conditions: III. Comparison of theory with experiment

The optical pumping of Cs atoms with light of the D ₁ line under magnetic resonance conditions is numerically calculated. This calculation is done to check the suggestion that an unexpectedly strong influence of the polarization of the pumping light on the resonance signals of transverse alignment, which we observed experimentally, is caused by the coupling between orientation and alignment by means of the magnetic field and/or the pumping light. This suggestion has been confirmed: upon circularly polarized pumping, the shape of the signal line of transverse alignment proved to be similar, as well as in the experiment, to the shape of the signal line of transverse orientation and was sharply different from the shape of the line of alignment observed upon linearly polarized pumping. For metastable 2³ S ₁ ⁴He atoms, in accordance with the experimental data, the calculated shape of the signal line of transverse alignment is found to be independent of the polarization of the pumping light. The calculations also confirm the possibility of a reverse effect: the influence of alignment on orientation, which manifests itself in the occurrence of orientation upon pumping with unpolarized light under magnetic resonance conditions. For Cs atoms exposed to a field of ～0.6 Oe, the largest contribution to this effect is yielded by the field coupling of orientation and alignment, whereas, for metastable 2³ S ₁ ⁴He atoms, the largest contribution is made by the light coupling of these polarization moments.     

Magnetoresonance EMF in thin manganite films

The effect of magnetoresonance emf (MREMF) in thin epitaxial films of rare-earth manganites La_0.67Sr_0.33MnO₃ is detected and investigated. The effect is manifested in the occurrence of a constant voltage under the action of microwave pumping in magnetic fields corresponding to ferromagnetic resonance conditions. The MREMF signal includes symmetric and antisymmetric components and changes its polarity upon switching of the external magnetic field. The temperature dependence of the effect (including the range in the neighborhood of the phase transition) is analyzed. The experimental data including the shape of the signal and its dependence of the field orientation are in good agreement with the results obtained in the theoretical model based on the mechanism of anisotropic magnetoresistance. It is shown that the magnetoresistance anisotropy in the manganite under investigation is negative and sharply attenuates as the temperature approaches the Curie point, almost vanishing in the paramagnetic phase.     

A simple model for the velocity modulation detection technique of molecular ions

We present a simple model to describe the line-profile of velocity modulated ions. This model is based on Langevin's theory of the mobility and on the independence of the electric field strength on the discharge current in the ideal positive column. A comparison of time constants responsible for the drift of the ions, the concentration, and the ignition and extinction of the discharge itself is essential for a rough understanding of the velocity modulation. The behaviour of H₃O⁺ ions in an ac glow discharge was examined. Under our experimental conditions the line-profile can be explained by a temporal dependence of the velocity which is close to a square-wave. Due to this particular temporal dependence the amplitude of the signal only depends on the concentration modulation, while the line-shape is a simple difference line-profile.     

The first Trichel pulse of negative corona discharge in N2 with a small admixture of SF6

Current growth waveforms of transient negative point-to-plane discharge in N₂ and in N₂ with 0·025-0·1% admixture of SF₆ at a pressure of 40 kPa have been measured and compared. The transient glow discharge regime in pure N₂ was found to be preceded by a peaked current signal of conspicuous similarity to the first Trichel pulse rise and its initial decay in N₂+SF₆ mixture. The results are in a basic agreement with a hypothesis on streamer mechanism for Trichel pulse and indicate occurrence of field emission during the rise of Trichel pulse current.     

Optogalvanic detection of negative ions in discharges

With the aid of rf, microwave and dc discharges, an optogalvanic (OG) signal, due to the photodetachment of negative oxygen ions, has been observed. The OG signal intensity in an rf discharge was successfully estimated from the discharge parameters. Thus, the OG technique may be potentially useful as a plasma diagnostic method. Furthermore, the OG signal from negative oxygen ions is found to be strong in the diffusion-controlled positive column, while the OG signal due to the excited levels of atomic oxygen is strong in the cathode fall region. Preliminary results for the observation of the OG signal in discharges of H₂, CO, H₂O₂, and (COCH₃)₂ are also described.