169Tm NMR study of the high-Tc superconductor TmBa2Cu3O6.92

The ground state of Tm³⁺ ion (4t²,³H₆) in a crystal electric field (CEF) of TmBaCuO is a singlet, separated from the lowest excited states by an energy gap of 100÷130 cm^?1. The pulsed NMR of¹⁶⁹Tm nuclei (spinI=1/2) in a magnetically-oriented TmBa₂Cu₃O_6.92 power is studied at temperatures 1.5÷4.2 K. Two sorts of Tm nuclei are observed: rapidly relaxing Tm1 (≥80% of total amount, having relaxation timesT ₁ ⁽¹⁾ ≈35 ms,T ₂ ⁽¹⁾ =80÷250 μs) and slowly relaxing Tm2 nuclei (≦20%,T ₁ ⁽²⁾ =300÷1000 μs). The¹⁶⁹Tm NMR spectra are described by the spin-Hamiltonian ?₁ = ??Σγ _i H _i I _i (i = x,y,z), where |γ _x ⁽¹⁾/2π| = 5.3(1), |γ _y ⁽¹⁾/2π| = 6.6(1), |γ _x ⁽²⁾/2π| = |γ _y ⁽²⁾/2π| = 5.1(1), |γ _z ⁽¹⁾/2π| = |γ _z ⁽²⁾/2π| = 2.3(1) kHz/Oe, and axesx, y, z coincide with the crystal axesa, b, c. The Tm1 and Tm2 nuclei are identified as those belonging to orthorhombic (CEF of D_2h symmetry) and tetragonal (D_4h) phases of the TmBaCuO compound, respectively. Two-exponential and nearly temperature-independent nuclear relaxation is observed at temperatures 1.5÷4.2 K. The smallestT ₂ ⁽¹⁾ -values (≈80 μs) found for the orthorhombic (superconducting) phase in an external field H perpendicular to the crystalc-axis agree fairly well with those one could expect due to the dipole-dipole interaction of thulium nuclei. However, the slowing down of this rapid relaxation of the nuclear transverse magnetization in a field H‖c as well as the origin of unusually largeT ₂ ⁽²⁾ -values remain unclear.     

NMR and AFM investigations of nanocavities on the double rare-earth fluoride crystal surface

Nanocavities with a size of less than 30 nm on the crystal surface of superfine LiYF₄ powders and single crystals of the Van Vleck paramagnet LiTmF₄ were detected by nuclear magnetic resonance (NMR) cryoporometry and atomic-force microscopy (AFM) and liquid³He NMR cryoporometry. NMR investigations show that the nanocavities are evidently present on the surface of the LiYF₄ powder particles. The distribution of nanocavities has two maxima corresponding to the specific porous sizes of 3–5 and 10–15 nm. AFM investigations detect the presence of the nanocavities with sizes in the range 50–300 nm on the surface of powder particles and single crystals. The cases of powder microparticles and of a single crystal differ in the value of the parameter “surface roughness”, which is 14.5 and 11 nm, respectively. The mechanism of creation of nanocavities is proposed and verified by additional investigations with CaF₂ powders.     

Mechanism of the formation of a diamond nanocomposite during transformations of C60 fullerite at high pressure

The results of an investigation of the transformation of C₆₀ fullerite to diamond under pressure through intermediate three-dimensionally polymerized and amorphous phases are reported. It is found that treatment of fullerite C₆₀ at pressures 12–14 GPa and temperatures ∼1400°C produces a nanocrystalline graphite-diamond composite with a concentration of the diamond component exceeding 50%. At lower temperatures (700–1200°C) nanocomposites consisting of diamondlike (sp ³) and graphitic (sp ²) amorphous phases are formed. The nanocomposites obtained have extremely high mechanical characteristics: hardness comparable to that of best diamond single crystals and fracture resistance two times greater than that of diamond. Mechanisms leading to the transformation of C₆₀ fullerite into diamond-based nanocomposites and the reasons for the high mechanical characteristics of these nanocomposites are discussed. Pis’ma Zh. éksp. Teor. Fiz. 69, No. 11, 822–827 (10 June 1999)     

Sound field of a turbulent conducting fluid flow

The influence of fluctuations of the Joule dissipation and ponderomotive forces on the generation of sound by a tubulent conducting fluid flow is investigated on the basis of Light-hill's theory.Kiev. Translated from Izvestiya Akademii Nauk SSSR. Mekhanika zhidkosti i Gaza, No. 2, pp. 9–14, March–April, 1972.     

Effect of surface magnetism of solid-state substrates on the NMR of liquid 3He

The results of investigations of the longitudinal relaxation rate of the nuclear spins of liquid helium-3 in contact with finely dispersed LiYF₄ dielectric powders with various degrees of filling of the microcracks on the crystal surface by water molecules are reported. It is found that exchange correlations appear between paramagnetic defect centers on the surface as a result of spin-density transfer via the molecular orbitals of oxygen in the water molecule. Pis’ma Zh. éksp. Teor. Fiz. 69, No. 7, 503–509 (10 April 1999)     

Magnetic resonant and non-resonant investigations of LiLnF4 (Ln = Y, Tm) powders

The properties of dielectric powders of the Van Vleck paramagnet LiTmF₄ and its diamagnetic analogue LiYF₄ have been investigated by both resonant methods (EPR, NMR, and the mass-spectroscopy) and non-resonant ones (conductometry and magnetization measurement). On the basis of experimental data and theoretical calculations a self-consistent model for the magnetic and other properties of these powders is suggested. Two structural phase transitions induced by the magnetic field are discovered in fine LiTmF₄ powder at low temperature in a high magnetic field.     

Ultrahigh-frequency NMR of Tm3+ ions in single crystals of thulium ethylsulfate in high magnetic fields

Resonant transitions predicted earlier between low-lying electron-nuclear sublevels of the Tm³⁺ ground state were observed at frequencies up to 700 MHz in a dielectric Van Vleck paramagnet—thulium ethylsulfate single crystal. It is shown that, due to the distortion of the 4f-electron shell of a rare-earth ion in an applied magnetic field, the parameters of electron-nuclear interaction become field-dependent.     

1H NMR study of static and fluctuating internal Magnetic fields in Tb(C2H5SO4)3 · 9 H2O ising ferromagnet

The pulsed NMR method is applied to an analysis of a complicated structure of inhomogeneous internal fields in a ferromagnetic crystal. Proton magnetic resonance in the Ising ferromagnet TbES at a temperature range from 1.6 K down to 35 mK is studied at frequencies of 10–35 MHz. A complicated picture of static and fluctuating internal magnetic fields in the crystal is presented. Interatomic distances are shown to have an uncertainty of the order of 0.2% due to defects in the crystal lattice. The fluctuations of internal magnetic fields produced by thermal excitation and spin-spin relaxation of Tb³⁺ ions give rise to the effective nuclear magnetic relaxation: 1/T₁₍₂₎～exp (δ/kT), where δ is the energy splitting of the lowest Tb³⁺ quasi-doublet. The rate of these fluctuations in TbES at low temperatures is approximately equal to 2×10⁷ s^?1 being independent of temperature and magnetic field.     

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