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.     

Observation of magnetic Mn sites in cubic and hexagonal HoMn2: 55Mn NMR study

⁵⁵Mn nuclear magnetic resonance has been measured for both cubic C15 and hexagonal C14 HoMn₂. In the ordered state, we found a high-frequency signal, which can be assigned to magnetic Mn atoms, for both C15 and C14 phases together with a low-frequency signal from non-magnetic Mn atoms. The results of the spin-spin relaxation time T₂ in the ordered state and the NMR spectra in the paramagnetic state are also given to discuss the magnetic instability and the magnetic structure.     

<Superscript>53</Superscript>Cr NMR study of CuCrO<Subscript>2</Subscript> multiferroic

The magnetically ordered phase of the CuCrO₂ single crystal has been studied by the nuclear magnetic resonance (NMR) method on ⁵³Cr nuclei in the absence of an external magnetic field. The ⁵³Cr NMR spectrum is observed in the frequency range ν_res = 61–66 MHz. The shape of the spectrum depends on the delay tdel between pulses in the pulse sequence τ_π/2–t _del–τ_π–t _del–echo. The spin–spin and spin–lattice relaxation times have been measured. Components of the electric field gradient, hyperfine fields, and the magnetic moment on chromium atoms have been estimated.     

Nuclear magnetic relaxation induced by the relaxation of electron spins

A physical mechanism responsible for the relaxation of nuclear spins coupled by the hyperfine interaction to relaxed electron spins in materials with spin ordering is proposed. The rate of such induced nuclear spin relaxation is proportional to the dynamic shift of the nuclear magnetic resonance (NMR) frequency. Therefore, its maximum effect on the NMR signal should be expected in the case of nuclear spin waves existing in the system. Our estimates demonstrate that the induced relaxation can be much more efficient than that occurring due to the Bloch mechanism. Moreover, there is a qualitative difference between the induced and Bloch relaxations. The dynamics of nuclear spin sublattices under conditions of the induced relaxation is reduced to the rotation of m₁ and m₂ vectors without any changes in their lengths (m ₁ ² (t) = m ₂ ² (t) = m ₀ ² (t)= const). This means that the excitation of NMR signals by the resonant magnetic field does not change the temperature T _n of the nuclear spin system. This is a manifestation of the qualitative difference between the induced and Bloch relaxations. Indeed, for the latter, the increase in T _n accompanying the saturation of NMR signals is the dominant effect.     

Nuclear spin diffusion effects in optically pumped quantum wells

We studied the influence of the nuclear spin diffusion on the dynamical nuclear polarization of low dimensional nanostructures subject to optical pumping. Our analysis shows that the induced nuclear spin polarization in semiconductor nanostructures will develop both a time and position dependence due to a nonuniform hyperfine interaction as a result of the geometrical confinement provided by the system. In particular, for the case of semiconductor quantum wells, nuclear spin diffusion is responsible for a nonzero nuclear spin polarization in the quantum well barriers. As an example we considered a 57 Å GaAs square quantum well and a 1000 Å Al _x Ga_1?x As parabolic quantum well both within 500 Å Al_0.4Ga_0.6As barriers. We found that the average nuclear spin polarization in the quantum well barriers depends on the strength of the geometrical confinement provided by the structure and is characterized by a saturation time of the order of few hundred seconds. Depending on the value of the nuclear spin diffusion constant, the average nuclear spin polarization in the quantum well barriers can get as high as 70% for the square quantum well and 40% for the parabolic quantum well. These results should be relevant for both time resolved Faraday rotation and optical nuclear magnetic resonance experimental techniques.     

X-Band ESR and51V NMR study of the Haldane system PbNi2−xMgxV2O8

The temperature and angular dependence of the X-band electron spin resonance (ESR) and⁵¹V nuclear magnetic resonance (NMR) spectra have been measured in a recently discovered Haldenegap system, PbNi_2-xMg_xV₂O₈ (0≤x≤0.24). The angular dependence of the ESR signal suggests that both the spin diffusion as well as the magnetic anisotropy determine the electronic spin correlation functions. However, in doped samples the magnetic anisotropy increasingly dominates the spin dynamics on cooling. The huge broadening of the⁵¹V NMR spectra in doped samples at low temperatures provides evidence for localized magnetic moments in the vicinity of the Mg impurities. Locally distorted structure around each Mg impurity may slightly modify the magnetic interactions and be potentially responsible for the antiferromagnetic ordering (belowT _N≈ 3.5K) in doped compositions.     

Temperature-scanned magnetic resonance and the evidence of two-way transfer of a nitrogen nuclear spin hyperfine interaction in coupled NV-N pairs in diamond

New method for the detection of magnetic resonance signals versus temperature is developed on the basis of the temperature dependence of the spin Hamiltonian parameters of the paramagnetic system under investigation. The implementation of this technique is demonstrated on the nitrogen-vacancy (NV) centers in diamonds. Single NV defects and their ensembles are suggested to be almost inertialess temperature sensors. The hyperfine structure of the ¹⁴N nitrogen nuclei of the nitrogen-vacancy center appears to be resolved in the hyperfine structure characteristic of the hyperfine interaction between NV and an N _s center (substitutional nitrogen impurity) in the optically detected magnetic resonance spectra of the molecular NV-N _s complex. Thus, we show that a direct evidence of the two-way transfer of a nitrogen nuclear spin hyperfine interaction in coupled NV-N _s pairs was observed. It is shown that more than 3-fold enhancement of the NV optically detected magnetic resonance signal can be achieved by using water as a collection optics medium.     

Mechanical detection of nuclear spin relaxation in a micron-size crystal

A room temperature nuclear magnetic resonance force microscope (MRFM), fitted in a 1 tesla electromagnet, has been used to measure the nuclear spin relaxation of ¹H in a micron-size (70 ng) crystal of ammonium sulfate. NMR sequences, combining both pulsed and continuous wave radio-frequency fields, have allowed us to measure mechanically T₂ and T₁, the transverse and longitudinal spin relaxation times. Because two spin species with different T₁ values are measured in our 7 μm thick crystal, magnetic resonance imaging of their spatial distribution inside the sample section have been performed. To understand quantitatively the measured signal, we carefully study the influence of spin-lattice relaxation and non-adiabaticity of the continuous-wave sequence on the intensity and time dependence of the detected signal. Received 23 February 2000     

Surface-NMR relaxation and echo of aquifers in geomagnetic field

Macroscopic samples of near-surface water in pores or fractures of rocks down to 100 m and deeper are studied by the measurement of proton relaxation and echo in the Earth’s magnetic field. The excitation and reception of the surface nuclear magnetic resonance (SNMR) signal is accomplished with the help of an antenna, circle or 8-shaped (for the minimization of the outer electromagnetic jamming influence), placed at the surface. The frequency of magnetic resonance in the case considered amounts to several kilohertz, the dead time of the instrumentation to several milliseconds. Water in extremely small pores of water-resisting rocks (e.g., in argillaceous grounds), is chemically bound, crystallization or frozen water has smaller times of spin relaxation and is not registered. The distribution of water concentration with depth is determined by inversion of an integral equation, including the model and measured dependences of the SNMR signal against the intensity of excitation. The current state of the art of the SNMR sounding and perspectives of this method on the basis of free induction decay and spin echo detection and relaxation times measurement are presented. Free induction decayT ₂ ^* equal to 60 ms, spin-echoT ₂ equal to 220 ms, and inversion-recoveryT ₁ equal to 700 ms relaxation times have been measured for medium-to coarse-grained sand aquifer. Microscopic characteristics of the aquifer — longitudinal relaxivity (7·10^?3 cm/s), transverse relaxivity (3.5·10^?2 cm/s), and local magnetic field gradient (2·10^?2 G/cm) — have been estimated from experimental data. The importance of spin relaxation and echo measurements for obtaining the information about the microstructure of pores and fractures, as well as filtration, properties of aquifers and diamagnetic, paramagnetic and hydrocarbon contamination, is emphasized.     

Magnetoacoustic resonance on nuclear spin waves in the cubic antiferromagnet RbMnF3

Magnetoacoustic resonance on nuclear spin waves is measured in the cubic antiferromagnet RbMnF₃. A resonance change with respect to a constant magnetic field H ₀ with maximum damping at H ₀≈4×10³ Oe is observed in the amplitude of an acoustic pulse passing through a sample owing to excitation of nuclear spin waves under nuclear magnetoacoustic resonance conditions. A study of the angular dependence of the damping revealed a 90° periodicity consistent with the fact that the [001] direction, around which the rotation takes place, is a four-fold axis of the crystal. An analysis of the dispersion law for nuclear spin waves shows that longitudinal ultrasound propagating along the [001] axis perpendicular to H ₀ excites a branch of nuclear spin waves whose frequency depends on the magnitude of the constant magnetic field. Fiz. Tverd. Tela (St. Petersburg) 41, 297–300 (February 1999)     

Determination of the polarization of deuterium atoms following optical orientation

The redistribution of the electronic polarization in deuterium atoms is analyzed theoretically and the various polarization moments are shown to influence the magnetic resonance signal of deuterium. The analysis gives expressions that relate the amplitudes of the magnetic resonance signals for various Zeemann sublevels of the D atom to the electronic and nuclear polarizations of these atoms and their nuclear alignment. Experimental data on the optical orientation and spin exchange in a D-Cs mixture are used to determine the electronic and nuclear orientation and nuclear alignment of the D atoms, which are found to be 〈S _z〉=0.1, 〈I _z〉=0.27, and 〈Q _zz=0.027. Zh. Tekh. Fiz. 67, 22–26 (January 1997)     

Surface-enhanced nuclear spin conversion in CH3F

The nuclear spin conversion of a molecule is the modification of the total nuclear spin I of its equivalent atoms. This phenomenon is observed by measuring the relaxation rate of a gas sample initially prepared with a population of spin isomers far from the equilibrium given by nuclear spin statistics. New experimental data obtained at low pressure show a surface-induced enhancement of the nuclear spin conversion in ¹³CH₃F. Contrary to binary collisions in the gas phase, hitting the surface induces direct conversion. Several mechanisms are proposed.     

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.     

Single-crystal like NMR spectra of CeCu 5 Au powder samples

Single-crystal like nuclear magnetic resonance spin-echo spectra are obtained for powder samples of the antiferromagnet CeCu₅Au in the paramagnetic phase. Line shifts and quadrupolar splittings are analyzed for ⁶³Cu and ⁶⁵Cu. The influence of the extreme magnetic anisotropy of the CeCu_6-xAu_x compounds on nuclear spin relaxation is discussed. Received 19 June 2000     

Magnetic rotation observation of the C2b3Σg− ← a3Πu transition using a color center laser

The magnetic rotation observation of the C₂b³Σ_g^? ← a³Π_u Ballik-Ramsay system using a color center laser is reported. This is the first detection of this system in absorption. Three bands, 0 ← 1, 1 ← 2, and 2 ← 3, were identified in the spectral range 3650–4030 cm^?1. The last two bands were observed for the first time. In magnetic rotation many satellite lines (ΔN ≠ ΔJ) which would be very weak in normal absorption have been observed with intensity comparable to the main branch lines. This permits a slight improvement in the accuracy of some of the fine structure constants. A variety of lineshapes are observed for the various branches by magnetic rotation. Because the b³Σ_g^? fine structure is small, giving a partial overlap, the peak frequency of a magnetic rotation signal usually does not correspond to the center frequency of the normal absorption signal of that transition. A computer program has been written to predict magnetic rotation lineshapes and obtain the peak frequency displacements. Various observed and calculated lineshapes are displayed and compared.     

Electronic structure and indirect spin-spin interactions in bournonite (CuPbSbS3) according to antimony nuclear quadrupole resonance

A complex sulfide CuPbSbS₃ (bournonite) has been studied by the nuclear quadrupole resonance on ^121,123Sb. The temperature dependences of the spectroscopic and relaxation parameters in the temperature range of 10–295 K have been obtained. The crystallochemical features of the environment of the two non-equivalent Sb positions in the unit cell have been revealed from the nuclear quadrupole resonance spectra. The existence of the lattice vibrations with the frequency ω = 110 cm^?1 has been demonstrated on the basis of the temperature dependence of the nuclear quadrupole resonance frequencies. Slow beats have been observed on the decay curve of the spin echo signal. Experimental data have been analyzed in order to reveal the existence of the indirect spin-spin interactions involving Sb atoms. The indirect spin-spin coupling constant has been estimated as J = 2.5 ± 0.5 kHz.     

Pulse EPR spectroscopy of Cu2+-doped inorganic glasses

Two-frequency continuous-wave and pulse EPR (electron paramagnetic resonance) spectroscopical techniques are applied to determine static and dynamic EPR parameters of Cu²⁺ ions in oxide and fluoride glasses. The investigations are focussed on the analysis of strain effects in the glassy matrices, the identification of the magnetic nuclei in the vicinity of Cu²⁺ ions as well as the determination of the dependence of the phase memory timeT _M on temperature and resonance field. The results obtained by X-band continuous-wave EPR, X- and S-band echo-detected EPR, and X- and S-band electron spin echo envelope modulation studies of Cu²⁺-doped inorganic glasses yield information on the local symmetry of the Cu²⁺ coordination polyhedra, the chemical nature of the atoms in the second and higher coordination spheres, the distribution of the parameters of the static spin Hamiltonian and the low-temperature motions of the dopant-containing structural units. Special techniques like 2-D Mims ENDOR (electron nuclear double resonance) and hyperfine-correlated ENDOR are applied for the first time to doped inorganic glasses. From the spin relaxation measurements a stronger tendency of the Cu²⁺ ions to aggregate is found for fluoride glasses in comparison to aluminosilicate and phosphate glasses.     

NMR studies of magnetic properties in Heusler-type Mn 3 Si

In order to microscopically investigate the magnetic properties of both paramagnetic and antiferromagnetic phases in Mn₃Si (T _N?=?23 K), the ⁵⁵Mn NMR has been carried out at temperatures between 2.2 K and 300 K. The temperature dependences of the spectrum, Knight shift (or resonance frequency shift) and spin-lattice relaxation time T ₁ of ⁵⁵Mn NMR have been measured. In the paramagnetic phase, only one resonance spectrum can be obtained. The observed spectrum is identified to be a signal corresponding to the Mn(II) site. In the antiferromagnetic phase, two different spectra corresponding to the Mn(I) and Mn(II) sites are found at the resonance frequencies of 145 and 6 MHz, respectively, by the zero field NMR at 4.2 K. From these results, the internal magnetic fields on the ⁵⁵Mn(I) and ⁵⁵Mn(II) nuclei are found to be 13.6 and 0.6 T, respectively. According to the NMR results, the helical structure in incommensurate Mn spin states is better explained compared with the transverse sinusoidal structure.     

Magnetic resonance study of the two-dimensional spin diffusion in the Heisenberg paramaget (C18H37NH3)2MnCl4

¹H nuclear magnetic resonance (NMR) and electron paramagnetic resonance (EPR) techniques were employed to study the perovskite-type layered structure compound (C₁₈H₃₇NH₃)₂MnCl₄ undergoing structural phase transitions. The spin relaxation was found to sensitively reflect the two-dimensional electron spin diffusion.     

Efficient spin diffusion of transverse13C magnetization

Relatively efficient spin diffusion among unprotonated carbons with large chemical-shift anisotropies can be achieved by a¹³C nuclear magnetic resonance multiple-pulse sequence with a lowduty cycle of ～5% on the¹³C channel, which minimizes sample heating and reduces cumulative effects of pulse imperfections. The spin diffusion occurs among transverse-magnetization isochromats, while the total transverse magnetization is a conserved quantity under the average Hamiltonian. The “flip-flop” term of the dipolar-coupling average Hamiltonian is the same as in the full dipolar coupling, i.e., its scaling factor is unity. For a sample of 40%¹³COO-labeled poly(vinyl acetate), with¹³C in ester groups accounting for 7% of all heavy atoms, magnetization equilibrates within 20 ms over a volume of (0.9 nm)³, corresponding to a molecular mass of 500 Da, while the T₂ relaxation time of the total transverse magnetization is ～40 ms. The spin diffusion coefficient is estimated asD = 3 ± 1.5 nm²/s.