Mechanism of nuclear spin-lattice relaxation and its field dependence for ultraslow atomic motion

The contribution of ultraslow self-diffusion of polycrystalline benzene molecules to the spin-lattice relaxation of protons is studied as a function of effective magnetic field H ₂ in a doubly rotating frame (DRF). Proton relaxation time T _1ρρ is measured by direct recording of NMR in a rotating frame (RF). The effective fields have a “magic” orientation corresponding to angles arccos(1/√3) in the RF and π/2 in the DRF so that the secular part of the dipole-dipole interactions of protons is suppressed in two orders of perturbation theory, while the nonsecular part becomes predominant. It is found that the diffusion contribution of benzene molecules to proton relaxation time T _1ρρ is a linear function of the square of field H ₂ and exhibits all peculiarities typical of the model of strong collisions generalized to only fluctuating nonsecular dipole interactions in fields exceeding the local field. This means that the model can also be employed in the given conditions. It is shown that perfect agreement with such a dependence can also be obtained in the model of weak collisions if we take into account the concept of the locally effective quantization field, whose magnitude and direction are controlled by the vector sum of field H ₂, and the nonsecular local field perpendicular to it.     

Nuclear spin-lattice relaxation in single domain particles

Small precipitates of bcc Fe were made by tempering foils of Cu₆₉Au₃₀Fe₁ and Cu₆₇Au₃₀Fe₃. By Mö\bauer source experiments using⁵⁷Fe, hyperfine fields and particle sizes, deduced from their superparamagnetism, were investigated. Nuclear spin lattice relaxation (SLR) was measured by nuclear orientation and thermal cycling method. We found an increase in SLR rates with decreasing external magnetic field and a drastic increase with decreasing particle size. This may be explained by a uniformly fluctuating particle magnetization.     

Soliton dynamics and nuclear spin-lattice relaxation in incommensurately modulated crystals

The nuclear spin-lattice relaxation rate in incommensurate systems is analysed for the the so-called soliton limit which often can be applied near the transition to a subsequent commensurate phase. Previous calculations are corrected by taking into account adequately the eigenfunctions of the relevant fluctuations. In contrast to previous conclusions, it is shown that for nuclei in the discommensurations the spin-lattice relaxation rate is expected to increase on approaching the phase transition. The theoretical predictions are confirmed by experimental data obtained from NMR measurements on the prototype incommensurate systems Rb₂ZnCl₄ and BCCD. b]References     

Magnons and solitons in CsNiF3 investigated by nuclear spin-lattice relaxation

By nuclear spin-lattice relaxation of¹³³Cs we studied linear and nonlinear magnetic excitations in the one-dimensional easy-plane ferromagnet CsNiF₃. The measurements were performed in the temperature range from 3 to 20 K and with external magnetic fields from 3 to 55 kOe applied perpendicular to the chain direction. Universal dependence on was obtained forT/T ₁. A quantitative interpretation of theT ₁ data in terms of two-magnon and soliton contributions could be achieved by considering renormalization effects resulting from magnon-magnon and soliton-magnon interaction as well as from quantum corrections. The instability of solitons due to out-of-plane fluctuations expected in high magnetic fields is discussed.     

Nuclear orientation and nuclear spin-lattice relaxation in insulating magnetically ordered crystals

The mechanism of nuclear spin-lattice relaxation in insulating magnetically ordered crystals at low temperatures is discussed. In 3-dimensional systems the relaxation time T₁ is long, but in the lower dimensional systems it can be quite short making them possible candidates for hosts in on-line experiments. An experiment in 2-dimensional⁵⁴Mn−Mn (COOCH₃)₂·4H₂O is discussed with particular reference to the relatively short value of T₁ in low applied fields. A preliminary experiment in the 1-dimensional system⁵⁴Mn−(CH₃)₄NMnCl₃ (TMMC) is also described. Pulsed NMRON measurements in⁵⁴Mn−MnCl₂·4H₂O are outlined and the advant-systems with fast relaxation emphasized.     

Spin-wave contribution to the nuclear spin-lattice relaxation in triplet superconductors

We discuss collective spin-wave excitations in triplet superconductors with an easy axis anisotropy for the order parameter. Using a microscopic model for interacting electrons, we estimate the frequency of such excitations in Bechgaard salts and ruthenate superconductors to be 1 and 20 GHz, respectively. We introduce an effective bosonic model to describe spin-wave excitations and calculate their contribution to the nuclear spin-lattice relaxation rate. We find that, in the experimentally relevant regime of temperatures, this mechanism leads to the power law scaling of 1/T1 with temperature. For two- and three-dimensional systems, the scaling exponents are 3 and 5, respectively. We discuss experimental manifestations of the spin-wave mechanism of the nuclear spin-lattice relaxation.     

Random distribution of paramagnetic centers in the nuclear spin-lattice relaxation

A numerical solution is presented for the differential equation governing the nuclear spin-lattice relaxation via paramagnetic centers for spherically symmetric spin-diffusion constantD and direct relaxation transition probabilityC in the one-paramagnetic-center approximation. An interpolation function is given which reproduces the computedT ₁ values within ±5% for both the rapid diffusion and diffusion-limited cases. The introduction of a random distribution of the paramagnetic centers over the sample causes the relaxation to occur as exp(?at ^h ) with 0.5<h<1.0 forβ=(C/D)^1/4<R _av=the average radius of the influence spheres. This differs from the experimentally observed exp(?a ₁ t) behaviour. However, the random distribution seems to explain the difference between the theoretically calculated fluorine spin-diffusion in CaF₂ and that extracted from experimental NMR data by using the uniform-distribution model.     

Anisotropic intermolecular potential from nuclear spin-lattice relaxation in hexafluoride gases

The data on fluorine spin-lattice relaxation times per unit densityT _jσ in pure SF₆ and UF₆ gases can be analyzed to obtain information on the anisotropic part of the intermolecular potential in these systems. A new and more performant potential, Morse-Morse-Spline-van der Waals potential (J. Chem. Phys.94, 1034 (1991)) was used for the isotropic part of the intermolecular interaction. The analysis was made using the Bloom-Oppenheim theory, assuming, that the correlation time of the spin-rotation interaction can be approximated by the average lifetime of a molecule in a givenJ state. We have obtained the strengths of the repulsive and attractive terms in the anisotropic potential. From the strength of the attractive term, the hexadecapole moment of SF6 and UF6 were also obtained, being in good agreement with the values reported earlier, based on other potentials and techniques.     

Mechanism of 3He-mediated nuclear spin-lattice relaxation at surfaces

We measured the nuclear spin-lattice relaxation time T₁, of several surface-bound nuclei, ¹H, ¹⁹F, ¹¹B, ¹³C, ²⁹Si, and ²H, immersed in liquid ³He over the temperature range 0.01 K ⩽ T < 1 K. The Larmor frequencies of these nuclei in a 3.39 T field extended from 22 to 144 MHz. All T₁ values were temperature-independent and ranged from a few seconds to several hours, depending on the particular nucleus and the surface geometry of the sample. The results indicate that the coupled relaxation of surface spins is a phenomenon occurring in all solids immersed in ³He and thus provides a general mechanism for obtaining high nuclear polarization in solids, that the relaxation is controlled by direct dipole-dipole interactions between the surface spins and ³He in the first surface layer, that the ³He motion dynamics do not change appreciably from one surface to another, and that measurements of T₁ may thus be useful for determining the structure of surfaces.     

Magnetoplastic effect and spin-lattice relaxation in a dislocation-paramagnetic-center system

A magnetic induction threshold B _c above which the magnetoplastic effect — depinning of dislocations from paramagnetic pinning centers — can be observed in samples placed in a magnetic field is predicted and observed in Al, NaCl, and LiF crystals. The existence of a threshold is associated with the fact that for B<B _c the spin-lattice relaxation time τ_sl in a dislocation-paramagnetic-center system is less than the time required for spin evolution in a magnetic field resulting in the removal of the spin forbiddenness of an electronic transition that “switches off” the dislocation-pinning-center interaction. It is shown that the threshold field B _c is sensitive to temperature and x-ray irradiation of the samples. A new method for measuring the spin-lattice relaxation time in paramagnetic centers on dislocations is proposed on the basis of the data obtained. Pis’ma Zh. éksp. Teor. Fiz. 63, No. 8, 628–633 (25 April 1996)     

Temperature dependence of nuclear spin-lattice relaxation in the heavy-fermion superconducting state

Some features of the experimental data on nuclear spin relaxation time T₁ in the heavy-fermion superconducting state can be explained by taking into account the effect of the electron Zeeman energy. It is found that at intermediate temperatures the usual quasiparticle spin-flip scattering dominates, while at very low temperatures a new process, pair creation (annihilation), dominates and gives T^-1₁ ∝ T.     

Critical behavior of a ferroelectric studied by nuclear spin-lattice relaxation

Nuclear spin-lattice relaxation measurements have been performed on ¹¹B nuclei in colemanite CaB₃O₄(OH)₃·H₂O in the ferroelectric and paraelectric phases. Results obtained with a temperature accuracy of 0.01° C show a logarithmic behavior of T₁^?1 in the very vicinity of T_c.     

Deuterium nuclear spin-lattice relaxation times and quadrupolar coupling constants in isotopically labeled saccharides

(13)C and (2)H spin-lattice relaxation times have been determined by inversion recovery in a range of site-specific (13)C- and (2)H-labeled saccharides under identical solution conditions, and the data were used to calculate deuterium nuclear quadrupolar coupling constants ((2)H NQCC) at specific sites within cyclic and acyclic forms in solution. (13)C T(1) values ranged from approximately 0.6 to 8.2 s, and (2)H T(1) values ranged from approximately 79 to 450 ms, depending on molecular structure (0.4 M sugar in 5 mM EDTA (disodium salt) in (2)H(2)O-depleted H(2)O, pH 4. 8, 30 degrees C). In addition to providing new information on (13)C and (2)H relaxation behavior of saccharides in solution, the resulting (2)H1 NQCC values reveal a dependency on anomeric configuration within aldopyranose rings, whereas (2)H NQCC values at other ring sites appear less sensitive to configuration at C1. In contrast, (2)H NQCC values at both anomeric and nonanomeric sites within aldofuranose rings appear to be influenced by anomeric configuration. These experimental observations were confirmed by density functional theory (DFT) calculations of (2)H NQCC values in model aldopyranosyl and aldofuranosyl rings.     

Electric quadrupolar contribution to the nuclear spin-lattice relaxation of Ir in Fe

We report on the first quantitative determination of the electric quadrupolar contribution to the nuclear spin-lattice relaxation in a transition metal. For 186Ir and 189Ir in Fe we have determined the magnetic and the electric quadrupolar part of the relaxation for magnetic fields between 0.01 and 2 T. The quadrupolar part gives information on the role of the orbital motion of the electrons for the relaxation process. Our results prove that the unexpected high relaxation rates in Fe and their magnetic field dependence are due to a nonorbital relaxation mechanism.     

Spin-wave description of nuclear spin-lattice relaxation in Mn12O12 acetate

In response to recent nuclear-magnetic-resonance (NMR) measurements on the molecular cluster Mn12O12 acetate, we study the nuclear spin-lattice relaxation rate 1/T(1), developing a modified spin-wave theory. Our microscopic new approach, which is distinct from previous macroscopic treatments of the cluster as a rigid spin of S=10, not only excellently interprets the observed temperature and applied-field dependences of 1/T(1) for 55Mn nuclei but also strongly supports the 13C NMR evidence for spin delocalization over the entire molecule.     

Nonexponential nuclear spin-lattice relaxation in the isotropic phase of thermotropic liquid crystals

In the isotropic phase of nematic and smectic liquid crystals T₁ of CH3 and chain protons is larger than that of ring and ring-neighboured protons being caused by fast CH₃ reorientation and internal motions in chains, respectively.