Nuclear Magnetic Resonance Enhancement Research Articles

Frequency-chirped dynamic nuclear polarization with magic angle spinning using a frequency-agile gyrotron.

We demonstrate that frequency-chirped dynamic nuclear polarization (DNP) with magic angle spinning (MAS) improves the enhancement of nuclear magnetic resonance (NMR) signal beyond that of continuous-wave (CW) DNP. Using a custom, frequency-agile gyrotron we implemented frequency-chirped DNP using the TEMTriPol-1 biradical, with MAS NMR at 7 T. Frequency-chirped microwaves yielded a DNP enhancement of 137, an increase of 19% compared to 115 recorded with CW. The chirps were 120 MHz-wide and centered over the trityl resonance, with 7 W microwave power incident on the sample (estimated 0.4 MHz electron spin Rabi frequency). We describe in detail the design and fabrication of the frequency-agile gyrotron used for frequency-chirped MAS DNP. Improvements to the interaction cavity and internal mode converter yielded efficient microwave generation and mode conversion, achieving >10 W output power over a 335 MHz bandwidth with >110 W peak power. Frequency-chirped DNP with MAS is expected to have a significant impact on the future of magnetic resonance.

The effect of 13C enrichment in the glassing matrix on dynamic nuclear polarization of [1-13C]pyruvate

Dimethyl sulfoxide (DMSO) can effectively form a glassy matrix necessary for dynamic nuclear polarization (DNP) experiments. We tested the effects of 13C enrichment in DMSO on DNP of [1-13C]pyruvate doped with trityl radical OX063Me. We found that the polarization build-up time τ of pyruvate in 13C-labeled DMSO glassing solution is twice as fast as the unenriched DMSO while the nuclear magnetic resonance enhancement was unchanged. This indicates that 13C–13C spin diffusion is a limiting factor in the kinetics of DNP in this system, but it has a minimal effect on the absolute value of polarization achievable for the target.

Enhancement of nuclear magnetic resonance in microtesla magnetic field with prepolarization field detected with high-Tc superconducting quantum interference device

We applied prepolarization field and high-Tc superconducting quantum interference device (SQUID) detector to enhance nuclear magnetic resonance signal in a microtesla magnetic field. The minimum measuring magnetic field is 8.9μT at which the proton resonance frequency is 380Hz. The specificity instrumentation and the difficulty of using a high-Tc SQUID with prepolarization field were investigated. We applied gradient field to perform one-dimensional proton imaging in a microtesla magnetic field. Additionally, low field high-Tc SQUID-based NMR systems are promising in biomagnetic research due to its use, for example, in imaging with hyperpolarized noble gas.

Nuclear spin polarization transfer across an organic-semiconductor interface

Motivated by Tycko’s proposal to harness optically pumped nuclear spin polarization for the enhancement of nuclear magnetic resonance (NMR) signals from biological macromolecules, we investigate the transfer of thermal nuclear spin polarization between H1 or F19 in an organic overlayer and P31 at the surface of micron-sized InP particles by Hartmann–Hahn cross polarization. Comparison with analytic and numerical models indicates that the total quantity of polarization transferred across the semiconductor-organic interface is limited by the relatively short room-temperature H1 T1ρ (11 ms) and the slow diffusion of nuclear spin polarization in the semiconductor. Models and spin-counting experiments indicate that we are able to transfer approximately 20% of the total nuclear spin polarization originating in the organic overlayer to the semiconductor, supporting the feasibility of transferred optically pumped NMR.

Paramagnetically induced nuclear magnetic resonance relaxation in solutions containing S⩾1 ions: A molecular-frame theoretical and physical model

The enhancement of nuclear magnetic resonance (NMR) relaxation rates produced by paramagnetic solutes is physically rather different for electron spin S=1/2 paramagnetic species than for S⩾1 species due to the presence of zero-field splitting interactions in the electron spin Hamiltonians of the latter. When the zfs energy is larger than the electronic Zeeman energy, the electron spin precessional motion is spatially quantized with respect to the molecule-fixed principal axis system (PAS) of the zfs tensor rather than along the external laboratory magnetic field. An analytical theory of the orthorhombic zfs limit has been derived in which the motion of the electron spin variables is described in the zfs-PAS and that of the nuclear spin variables in the laboratory coordinate frame. The resulting theoretical expressions are simple in form and suggest a physically transparent interpretation of the experiment. The NMR relaxation enhancement R1p results from additive contributions, R1x, R1y, and R1z, arising from the molecular-frame Cartesian components of the time-dependent electron spin magnetic moment operator μr(t). Each Cartesian component R1r depends on the dipolar power density at the nuclear Larmor frequency that is produced by the corresponding Cartesian component of μr(t). The theory displays the dependence of the relaxation enhancement on the variables of molecular structure in a very simple and physically transparent form: R1r∝r−6[1+P2(cos θr)], where r is the interspin distance and cos θr is the direction cosine of the interspin vector with the rth principal axis of the zfs tensor. New experimental data are presented for the model S=1 complex [trans-Ni(II)(acac)2(H2O)2] (acac=acetylacetonato) in dioxane solvent. The magnetic field dependence of the proton T1 of the axial water ligands has been measured over the range 0.15–1.5 T, the lower end of which corresponds to the zfs limit. The experimental data have been analyzed using the new analytical theory for the zfs-limit regime in conjunction with spin dynamics simulations in the intermediate regime. Dipolar density power plots are presented as graphical devices which clearly exhibit the physical information in the experiment, and which permit a rapid differentiation of the sensitive and insensitive parameters of theory. The data analysis depends strongly on the zfs parameter |E| and on the electron spin relaxation time τS,z along the zfs-PAS z-axis, but only very weakly on the other parameters of theory. A fit of the data to theory provided the values |E|=1.8±0.1 cm−1 and τS,z=8.0±0.3 ps.

Drastic enhancement of nuclear magnetic resonance in a hcp cobalt single crystal magnetized perpendicular to the c axis.

Nuclear magnetic resonance on oriented nuclei was performed for the first time using as host matrix a hexagonal Co single crystal magnetized perpendicular to the c axis. The radioactive isotope $^{191}\mathrm{Pt}$ was implanted using the mass-separator ISOLDE-3 at CERN. In comparison to the case of magnetization parallel to the c axis, a drastic enhancement of the resonance signal was observed. This observation opens new possibilities for precise measurements of nuclear moments of short-lived isotopes far from stability.

Enhanced Multiple Resonance by Overhauser Effect in Paramagnetic Crystals

The Overhauser effect in paramagnetic ions is studied theoretically for arbitrary values of electron and nuclear spins. The nuclear magnetic resonance enhancement and the degree of nuclear polarization are computed in the high-temperature approximation. The results show the possibility of obtaining simultaneous and enhanced NMR at more than one characteristic detection frequency when the hyperfine interaction is dominating. The NMR enhancement and nuclear polarization are found to be independent of the electron spin, indicating the suitability of samples with S > 12 for dynamic nuclear orientation.

Enhancement of Nuclear Magnetic Resonance in Bulk Rubidium in the Size-Effect Regime

The intensity of nuclear magnetic resonance absorption of ${\mathrm{Rb}}^{87}$ nuclei in a thin slab of purified, cooled metal is observed to increase significantly in the range of magnetic-field strength and orientation where nearly sinusoidal size-effect variations in the surface resistance are found. Though no quantitative description of the effect is presently available, it clearly results from enhanced penetration of radio-frequency currents and fields into the bulk of the metal slab due to the cyclotron motion of the electrons. The phenomenon is expected to be generally observable in pure metals at low temperatures.

Quantum Theory of Domain-Wall Motion

Several workers have examined the enhancement of nuclear magnetic resonance within a Bloch wall, and have demonstrated the existence of both bound and free excitations on the Bloch wall structure. The free states correspond to precessional excitations akin to ordinary spin-wave excitations, while the bound states form a covenient basis for the representation of domain-wall motion. We derive the spectrum of both types of excitations, including exchange, anisotropy, and dipole field contributions for an infinite uniaxial ferromagnet. In contrast to earlier treatments, we treat the dipole field exactly (in the magnetostatic approximation), and show that this leads to a translational spectrum in which many states are degenerate with the translation, which is the translational mode excited by a uniform external magnetic field. The existence of such degeneracy is required for damping by imperfections to occur. The precessional spectrum is greatly different from the usual spin-wave spectrum, and, in particular, is not a symmetric function of k. The dipole fields lead to strong interactions, not conserving momentum, between the precessional modes; such interactions may explain the increase in ferromagnetic-resonance linewidth which is observed experimentally in the presence of a domain wall (in low dc magnetic fields). The motion of the domain wall, when it is bound to a certain position in the crystal by linear restoring forces, is studied by a Green's function technique. The domain-wall effective mass so obtained is identical to the expression given by D\oring, and the domain-wall damping parameter proves to be simply related to the energy dispersion of the uniform translational mode. We calculate this energy dispersion due to scattering by the dipole fields, and due to fluctuations, as used by Clogston et al. to explain the linewidth in disordered systems, such as the ferrites. The damping due to intrinsic scattering processes is proportional to ${T}^{2}$, while the damping due to fluctuations is essentially temperature-independent. In disordered systems, such as ferrite, the resonance line-width and domain-wall damping due to fluctuations should agree to within a factor of order unity. The motion is not describable by the Landau-Lifshitz equation. This communication is intended to demonstrate that a formulation for the quantum-mechanical study of domain-wall motion exists, and has the properties necessary to explain the losses which occur during such motion; it is not intended to lead to any quantitative results which can be directly compared with experiment. We also consider the specific heat contribution due to the domain wall, and we find that this is proportional to $T$ above about ${10}^{\ensuremath{-}2}$ \ifmmode^\circ\else\textdegree\fi{}K. It should be possible to observe such a specific heat contribution in YIG below 1 \ifmmode^\circ\else\textdegree\fi{}K.