N.A. Sergeev

Size dependence of 13C nuclear spin-lattice relaxation in micro- and nanodiamonds.

Size dependence of physical properties of nanodiamond particles is of crucial importance for various applications in which defect density and location as well as relaxation processes play a significant role. In this work, the impact of defects induced by milling of micron-sized synthetic diamonds was studied by magnetic resonance techniques as a function of the particle size. EPR and (13)C NMR studies of highly purified commercial synthetic micro- and nanodiamonds were done for various fractions separated by sizes. Noticeable acceleration of (13)C nuclear spin-lattice relaxation with decreasing particle size was found. We showed that this effect is caused by the contribution to relaxation coming from the surface paramagnetic centers induced by sample milling. The developed theory of the spin-lattice relaxation for such a case shows good compliance with the experiment.

13C spin-lattice relaxation in nanodiamonds in static and magic angle spinning regimes

We report on (13)C nuclear spin-lattice relaxation time (T1) dependence on the magic-angle-spinning (MAS) rate in powder nanodiamond samples. We confirm that the relaxation is caused by interaction of nuclear spins with fluctuating electron spins of localized paramagnetic defects. It was found that T1 is practically not affected by MAS for small particles, while for larger particles with lower defect density T1 is different in static and MAS regimes and reveals elongation with increasing MAS rate. This effect is attributed to suppression of nuclear spin diffusion by MAS. We propose an approach that describes T1 dependence on the MAS rate and allows quantitative analysis of this effect.

Relaxation of spin echo signals of 53Cr nuclei in Ag-doped CdCr2Se4

Abstract The frequency dependences of the relaxation times of NMR spin echo signals of the quadrupole nuclei 53Cr were measured in the ferromagnetic semiconductor Cd0.985Ag0.015Cr2Se4 at the temperature T=4.2 K . The experimental results were well explained by the developed theory of the two-pulse echoes relaxation. The main assumption of this theory is the assumption that the temporal fluctuations in the electron magnetization due to the fluctuations in the hyperfine and quadrupole Hamiltonians lead to the relaxation of the echo signals. It was shown that in Cd0.985Ag0.015Cr2Se4 there are two kinds of the quadrupole nuclei 53Cr, which have quite different relaxation times. The existence of two kinds of the nuclei 53Cr (53Cr(I) and 53Cr(II)) was connected with doping of the cadmium selenochromite with Ag+ ions. The nuclei 53Cr(II) are sited in the crystal ranges where the rapid electron exchange between the Cr4+ and Cr3+ ions leads to the rapid fluctuations in the local electron magnetization vector. The nuclei 53Cr(I) are located far from these dynamical defects. The observed frequency dependence of the relaxation rate of the usual Hahns echo signal from the nuclei 53Cr(I) was explained by the secular theory of the echo relaxation. The nonsecular relaxation theory well explains the frequency dependence of the relaxation rate of multiquantum echo signal from the nuclei 53Cr(II).

Effects of the finite pulse widths on solid echo signals

The quadrupolar and dipolar interactions of spins during the radio frequency (RF) pulses are considered. It is shown that due to these interactions the two-pulse echo signal is observed at the shifted time t(e) = tau + t1/2 (t1 = width of the first RF pulse, tau = time interval between the pulses).

Solid-echo in solids with molecular motions: Effects of nonzero pulse widths

The effects of nonzero pulse widths on the solid-echo signals in solids with molecular motions have been investigated. It has been shown that in the slow-motion region (M2τc2 ≈ 1) the amplitude of the echo signal is reduced and the maximum of the echo signal is shifted to the end of the second pulse. Comparison of the developed theory with experimental results obtained on polycrystalline C6H6 and NH4Cl demonstrates good agreement between them.

1H NMR study of water molecules confined in nanochannels of mordenite

Behavior of water molecules entrapped in nanochannels of zeolite mordenite has been investigated by (1)H NMR technique. The (1)H spectra and spin-lattice relaxation times in the laboratory and rotating frames, T1 and T1ρ, respectively, as well as the dipolar relaxation time T1D have been measured in the temperature range from 96 to 351K. Diffusion of water molecules along the channels was observed above ~200K. While in bulk liquid the dipolar ordered state of nuclear spins is not formed owing to complete motional average of dipolar interactions, we show that such a state is observed for mobile molecules confined in a restricted geometry. At temperatures below ~140K the relaxation was found to be mainly caused by interaction of (1)H nuclear spins with paramagnetic impurities. Complete lost of the fine structure of (1)H spectra above ~320 K is attributed to isotropic molecular reorientation or/and proton exchange. We show that the dipolar relaxation in mordenite is responsive to slow 180° reorientations of water molecules. The correlation times of nuclear and electron spin fluctuations were determined.

11B MAS NMR study of Ga1−xFexBO3 mixed crystals

Mixed iron-gallium borate crystals Ga1-xFexBO3 have been studied by Magic Angle Spinning (MAS) NMR of (11)B isotope. Experimental MAS NMR spectra have been computer simulated using a laboratory-developed code. The quadrupole parameters and isotropic chemical shift for (11)B are consistent with threefold-coordination of boron atoms. A detailed fitting to the experimental NMR spectra reveals the existence of a certain local disorder in Ga1-xFexBO3 crystals.

Effect of magic angle spinning on (13)C spin-lattice and spin-spin relaxation in nanodiamonds.

We report on (13)C spin-lattice (R 1) and spin-spin (R 2) relaxation rate dependence on magic-angle-spinning (MAS) rate in highly purified synthetic nanodiamonds. Noticeable slowdown of both relaxation processes and reduction of nuclear spin diffusion coefficient D with increasing MAS rate was obtained. This effect is attributed to suppression of nuclear spin diffusion by MAS. We developed a theoretical approach that describes the MAS rate dependence of R 1, R 2 and D, allows quantitative analysis of the data and shows good compliance with the experiment.

Influence of molecular motions on NQR echoes

The influence of thermal molecular motions on spin echo decay in pure nuclear quadrupole resonance (NQR) is considered. Our calculations show that the Hahn echo decay is caused by dipole-dipole interaction of the nuclear spins and is strongly affected by molecular mobility that can lead to the shortening of the echo decay with increased temperature. Slow molecular motion yields an exponential τ3 time dependence, while fast motion yields an exponential decay. The outlined theory allows us to explain an unusual shortening of the35Cl NQR echo decay on heating in thiourea-C2Cl6 inclusion compound.

Kubo-Anderson oscillator and NMR of solid state

The analytical solution for the Kubo-Anderson oscillator with a fluctuating frequency omega for arbitrary distribution function p(omega) has been obtained. The obtained theoretical expression has been applied to consideration of some dynamical problems of solid state NMR, namely (1) dynamical transformation of NMR line shape and spin-echo signal and (2) the temperature transformation of the second moment of NMR line for the case, when the potential barrier for the mobility of magnetic nuclei is a stochastic function of time.