Mechanism Of Nuclear Spin-lattice Relaxation Research Articles

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 H2 in a doubly rotating frame (DRF). Proton relaxation time T1ρρ 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 T1ρρ is a linear function of the square of field H2 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 H2, and the nonsecular local field perpendicular to it.

Nuclear magnetic relaxation studies of 69Ga, 71Ga, and 75As nuclei in GaAs single crystals doped with paramagnetic impurities

The GaAs, GaAs:In,Cr, and GaAs:Mn single crystals were grown by the vertical gradient freeze (VGF) technique. The spin-lattice relaxation processes of 69Ga, 71Ga and 75As have been studied for the undoped GaAs single crystals and crystals doped with paramagnetic impurities Cr and Mn. The relaxation processes in all samples are well described by a single exponential function. The T 2 dependence of the relaxation rate was in accordance with that predicted for the Raman mechanism of nuclear spin-lattice relaxation for all crystals investigated. The manganese doping showed a tendency to shorten slightly the spin-lattice relaxation times.

Low-Temperature Nuclear Spin-Lattice Relaxation in Amorphous Materials

The low temperature nuclear spin–lattice relaxation mechanism caused by the effective interaction between nuclear spins and two-level systems (TLS), arising from a paramagnetic centre interaction with TLS by means of g-factor modulation and the electron–nuclear dipole–dipole interaction, is investigated. It is shown, that this mechanism of the nuclear spin–lattice relaxation is effective under definite conditions.

Observation of a Nuclear-Magnon Contribution to the Nuclear Spin-Lattice Relaxation ofP191tin Ferromagnetic Cobalt

We report the first indication for a nuclear spin-lattice relaxation mechanism originating from the coupling of the impurity nuclei to nuclear magnons. The magnetic hyperfine interaction and the nuclear spin-lattice relaxation of $^{191}P\mathrm{t}$ $({T}_{1/2}\phantom{\rule{0ex}{0ex}}=\phantom{\rule{0ex}{0ex}}2.8\mathrm{d})$ in fcc Co were measured with NMR on oriented nuclei at temperatures between 8 and 14 mK in external magnetic fields ${B}_{\mathrm{ext}}\phantom{\rule{0ex}{0ex}}=\phantom{\rule{0ex}{0ex}}0.1--10\mathrm{kG}$. Between ${B}_{\mathrm{ext}}\phantom{\rule{0ex}{0ex}}=\phantom{\rule{0ex}{0ex}}5$ and $1\mathrm{kG}$ the relaxation rate increases by 2 orders of magnitude. This increase is attributed to the interaction of ${}^{191}\mathrm{Pt}$ with nuclear magnons originating from the ${}^{59}\mathrm{Co}$ nuclear spin system.

Reply to "Comment on 'Mechanism of nuclear spin-lattice relaxation in insulators at very low temperatures' "

The authors explain, in the language of their original paper, the error discovered by R\~o\~om. The error consists of using an expression for the electron spin-lattice relaxation time which is based on the high-temperature approximation. Use of the exact expression gives R\~o\~om's result. The so-called electron-wobble mechanism proposed by the authors remains correct, but it turns out to compete less effectively than the authors stated previously with the mechanism in which the electron-spin flips rather than wobbles.

Comment on "Mechanism of nuclear spin-lattice relaxation in insulators at very low temperatures"

We show that the coupling of nuclear spins to the electron spins of paramagnetic impurities in insulators in the presence of a static magnetic field leads to nuclear-spin relaxation, which at millikelvin temperatures still satisfies the detailed-balance requirement for nuclear-spin levels.

Nuclear spin-lattice relaxation times for mixtures of ortho- and para-H2. II. Low ortho-H2 concentration.

We study the longitudinal relaxation time ${T}_{1}$ for the nuclear spins of ortho-hydrogen molecules in a para-hydrogen or a hydrogen deuteride (HD) matrix. Phonon-driven molecular reorientations give rise to both a temperature- and concentration-dependent modulation to the electric-quadrupole-quadrupole-induced electric-field-gradient couplings at ortho sites and provide an important mechanism of nuclear spin-lattice relaxation for experimentally accessible concentrations and temperatures. The well-known ${c}^{5/3}$ law for ${T}_{1}$ is recovered for the ortho-para system. We calculate explicit distributions of relaxation rates as a function of concentration, temperature, and magnetic field. These functions are then applied to ortho and para mixtures and HD. For both systems we find satisfactory agreement with experiment, including temperature and concentration dependence that has been previously unexplained.

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.

Mechanism of nuclear spin-lattice relaxation in insulators at very low temperatures.

Nuclear spin-lattice relaxation by paramagnetic impurities in insulating crystals is expected in conventional theory to be frozen out in the millikelvin temperature region owing to the freezing of the electron-spin orientation into the lowest-energy spin state in the presence of a static field. Experimental relaxation times are much shorter than the conventional theory predicts. The authors show that by extending the normal relaxation theory to include the slight change in electron-spin quantization arising from a nuclear spin flip, the electron effectively gains a new degree of freedom, which we call wobble, which takes much less energy to excite than a complete electron-spin flip. The electron-spin orientation is, therefore, effectively unfrozen even at millikelvin temperatures. This new mechanism is expected to dominate in high field at temperatures of tens of millikelvin and below.

NMR relaxation in ferro- and antiferromagnets

We discuss the new nuclear spin-lattice relaxation mechanism in magnetic materials which is caused by the indirect coupling through magnons between the nuclear spins and the rapidly relaxing spins of impurity ions. This mechanism may be effective under low impurity concentrations; at this the impurity spin system is not saturated. A correlation between the spin-spin and spin-lattice relaxations must exist in this case.

Paramagnetic relaxation reagents as a probe for translational motion of liquids

Use of the paramagnetic relaxation reagent tris- (acetylacetonato) chromium(III) [Cr(acac)3] proves capable of probing the solution structure of organic liquids. The theory describing the relaxation effects of this chelate is presented, wherein it is assumed that the nuclear spin–lattice relaxation mechanism is due to the modulation of the electron–nuclear dipole–dipole interaction. The electronic relaxation time Te1 (or τs) and the mutual translational motion of the paramagnetic species and the solvent molecules, in our case carbon tetrachloride, are considered the only causes of this modulation. Since for the paramagnetic molecules it is possible to calculate the electronic relaxation times, such systems are well defined for the description of translational diffusion. It is found that a jump diffusion model for translational motion explains the experimental data with jump distances comparable to the diameter of the solvent molecules.

Nuclear spin relaxation in the presence of internal rotation II. Nuclear-spin-internal rotational coupling in benzotrifluoride and hexafluorobutyne-2

A magnetic nucleus located on an internal rotor is coupled not only to the rotational magnetic field generated by the end-over-end rotation of the molecule but also to the magnetic field produced by the internal rotation of the top relative to the frame. We have examined the importance of this nuclear-spin-internal-rotational coupling as a nuclear spin-lattice relaxation mechanism for the fluorine spins in benzotrifluoride and hexafluorobutyne-2. In order to elucidate the nature of the stochastic processes which modulate the spin-rotation interaction and are thereby responsible for the coupling of the nuclear-spin system to the lattice in these molecules, both the temperature and the viscosity dependences of the fluorine spin-lattice relaxation times in benzotrifluoride as well as the viscosity dependence in hexafluorobutyne-2 have been investigated. It is concluded that the rotational magnetic fields generated by overall and internal rotation are fluctuating independently and that each field is described by a distinct correlation time. It is also concluded that, contrary to previous proposals, the nuclear-spin-internal-rotation interaction contributes significantly to the spin-lattice relaxation of the methyl group protons in molecules such as toluene at room temperature.

On the mechanism of nuclear spin-lattice relaxation in bloch walls

On the mechanism of nuclear spin-lattice relaxation in bloch walls

An Experimental Study of the Nuclear Relaxation Mechanism in Several Crystals

The effective nuclear spin-lattice relaxation mechanism has been determined in a number of crystals by observing the relative saturation behaviour of the lines of their quadrupole-split nuclear magnetic resonance spectra. It is shown that all lines and combinations of lines should have the same saturation behaviour if the mechanism is magnetic, and differing behaviour if the mechanism is quadrupolar. The 23Na relaxation in pure synthetic crystals of sodium nitrate, sodium chlorate and sodium thiosulphate was found in each case to be quadrupolar. The 7L1 and 27Al relaxation in natural mineral crystals of spodumene and the 27Al relaxation in a mineral crystal of euclase were found to be magnetic. The 11B relaxation in a pure synthetic crystal of borax was also found to be magnetic. Chemical analysis showed a sufficient concentration of paramagnetic impurity in the mineral specimens to account for the dominance of the magnetic relaxation mechanism, and a sufficiently small concentration of paramagnetic impurity in the three sodium salts to account for the dominance of the quadrupolar mechanism. Despite the purity of the borax crystal, the quadrupole moment of the 11B nuclei is too small to enable the quadrupolar mechanism to compete with the magnetic mechanism.