51V nuclear magnetic resonance study of vanadium oxide nanotubes

We report on a ⁵¹VNMR study of vanadium oxide - decylamine nanotubes. ⁵¹V NMR spectra show three lines resulting from a diamagnetic V^5?+? and two paramagnetic V^4?+? ions, respectively. The V^4?+?/V^5?+? ratio is estimated to be ~0.47. ⁵¹V spin-lattice relaxation comprises both magnetic and quadrupolar contributions.     

1H nuclear magnetic resonance study of mixed-valent dodecylamine-intercalated vanadium oxide nanosheets

Alkyl-chain dynamics in the mixed-valent dodecylamine-intercalated vanadium oxide nanosheets (C12VONS) was investigated by means of 400-MHz ¹H nuclear magnetic resonance. Two C12VONS samples with distinct alkylammonium-to-alkylamine ratios showed distinct alkyl-chain dynamics, the alkylammonium chains being apparently more disordered than the alkylamine chains. The high-temperature spin–lattice relaxation is dominated by the conformational chain defects and the reorientational motions of the alkyl chains, respectively, in the alkylammonium-abundant system and in the alkylamine-abundant system. The reorientational and conformational changes of the alkyl chains appear to be uncorrelated with the magnetic transitions.     

Magnetic susceptibility and superconductivity of cubic vanadium nitrides

The superconducting transition temperature T_C and the magnetic susceptibility from 77 to 300°K have been measured on five cubic vanadium nitrides: VN, VN_0.91, VN_0.82, VN_0.84 and VN_0.75. The materials were carefully prepared to exclude oxygen and ferromagnetic impurities.The value of T_C, falls from 8.1°K for VN to 2.3°K for VN_0.75. The mass-susceptibility decreases from +3.94 × 10^?6e.m.u./g for VN to 1.88 × 10^?6e.m.u./g for VN_0.75 at 300°K. All samples showed a small positive slope for the susceptibility temperature curve.The results are discussed in terms of the rigid band model. The main features are a high density of states of d electrons, 2.4 states/atom eV for VN that drops off as the nitrogen content decreases, to 0.8 states/atom eV.Preliminary considerations indicate that many-body effects could reduce this density of states by as much as a factor of 2. Lack of experimental results on Knight shifts and low-temperature specific heats prevent a more quantitative estimate being made.     

Magnetic fields associated with a nuclear magnetic resonance medical imaging system

Measurements were made of magnetic and electric field levels in and around a nuclear magnetic resonance imaging system undergoing a clinical trial. Magnetic field levels ranged from 0.04 tesla (T) in the imaging volume down to about 0.0006 T at the end of the patient table. The peak radio-frequency magnetic field level was 15 amperes per meter (A/m) in the imaging volume, while the rms value was 4.6 A/m. The specific absorption rate resulting from the radio-frequency magnetic field was calculated to be no more than 0.017 watts per kilogram (W/kg). The radio-frequency electric field was detectable only within a few centimeters of the coil assembly, and does not significantly contribute to the specific absorption rate. These exposure levels were much lower than existing guidelines for clinical NMR procedures.     

Near-zero-field nuclear magnetic resonance

We investigate nuclear magnetic resonance (NMR) in near zero field, where the Zeeman interaction can be treated as a perturbation to the electron mediated scalar interaction (J?coupling). This is in stark contrast to the high-field case, where heteronuclear J?couplings are normally treated as a small perturbation. We show that the presence of very small magnetic fields results in splitting of the zero-field NMR lines, imparting considerable additional information to the pure zero-field spectra. Experimental results are in good agreement with first-order perturbation theory and with full numerical simulation when perturbation theory breaks down. We present simple rules for understanding the splitting patterns in near-zero-field NMR, which can be applied to molecules with nontrivial spectra.     

Magnetic field distribution in superconducting niobium by nuclear magnetic resonance fourier spectroscopy

Nuclear magnetic resonance measurements of the superconducting mixed-state field distribution in niobium metal are reported. It is demonstrated that pulsed NMR Fourier spectroscopy (defined in the text) can resolve considerable detail in the field distribution functionf(h). A simple analytic model forf(h) was used for numerical calculation of the expected spectrum shape. An ordered vortex lattice was always observed. Far from the transition the spectra were most consistent with triangular lattice symmetry. The temperature variation of the Maki parameter? ₂ was measured. The shift of spectrum position is compatible with appreciable reduction of spin susceptibility in the superconducting state.     

Magnetic susceptibility of dilute magnetic alloys

The magnetic susceptibility of dilute magnetic alloys is calculated using the Nagoaka approximation to the Kondo problem. We use the exact solutions of the Nagaoka equations, or equivalently Suhl's dispersion relations, as obtained recently. Our result is represented by a universal function of a certain temperature parameter. In the case of ferromagnetic coupling no appreciable change of the free spin susceptibility is found over the whole temperature range. In the case of antiferromagnetic coupling we find that the free spin susceptibility is greatly reduced. In fact, for spin 1/2, the result indicates the breakdown of the expansion in terms of the impurity concentration and suggests the onset of impurity ferromagnetism.     

Two-dimensional nuclear magnetic resonance petrophysics

Two-dimensional nuclear magnetic resonance (2D NMR) opens a wide area for exploration in petrophysics and has significant impact to petroleum logging technology. When there are multiple fluids with different diffusion coefficients saturated in a porous medium, this information can be extracted and clearly delineated from CPMG measurements of such a system either using regular pulsing sequences or modified two window sequences. The 2D NMR plot with independent variables of T2 relaxation time and diffusion coefficient allows clear separation of oil and water signals in the rocks. This 2D concept can be extended to general studies of fluid-saturated porous media involving other combinations of two or more independent variables, such as chemical shift and T1/T2 relaxation time (reflecting pore size), proton population and diffusion contrast, etc.     

170 nm nuclear magnetic resonance imaging using magnetic resonance force microscopy

We demonstrate one-dimensional nuclear magnetic resonance imaging of the semiconductor GaAs with 170 nm slice separation and resolve two regions of reduced nuclear spin polarization density separated by only 500 nm. This was achieved by force detection of the magnetic resonance, magnetic resonance force microscopy (MRFM), in combination with optical pumping to increase the nuclear spin polarization. Optical pumping of the GaAs created spin polarization up to 12 times larger than the thermal nuclear spin polarization at 5K and 4T. The experiment was sensitive to sample volumes of 50 microm(3) containing approximately 4 x 10(11)71 Ga/Hz. These results demonstrate the ability of force-detected magnetic resonance to apply magnetic resonance imaging to semiconductor devices and other nanostructures.     

Magnetic phase separation in La1-xSrxCoO3 by 59Co nuclear magnetic resonance

59Co NMR measurements on La1-xSrxCoO3 reported here establish unequivocally, for the first time, the coexistence of ferromagnetic regions, spin-glass regions, and hole-poor low spin regions at all x values from 0.1 to 0.5. A zero external field NMR spectrum, which is assigned to the ferromagnetic regions, has a spectral shape that is nearly x independent at 1.9 K, as are the relaxation times, T1 and T2. The integrated spectral area increases rapidly with x up to x = 0.2 and then decreases slightly for larger x. In a field of 9.97 T, a narrow NMR line is observed at 102 MHz, identical to that found in x = 0 samples in previous work. The integrated intensity of this spectrum decreases rapidly with increasing x, and is ascribed to hole-poor low spin regions. Beneath this spectrum, a third broad line, with a peak at 100 MHz, is assigned to a spin- or cluster-glass-like phase.     

Stripline probes for nuclear magnetic resonance

A novel route towards chip integrated NMR analysis is evaluated. The basic element in the design is a stripline RF 'coil' which can be defined in a single layer lithographic process and which is fully scalable to smaller dimensions. The sensitivity of such a planar structure can be superior to that of a conventional 3D helix. The basic properties, such as RF field strength, homogeneity and susceptibility broadening are discussed in detail. Secondary effects related to the thermal characteristics are discussed in simplified models. Preliminary NMR tests of basic solid and liquid samples measured at 600 MHz confirm the central findings of the design study. It is concluded that the stripline structure can be a valuable addition to the NMR toolbox; it combines high sensitivity with low susceptibility broadening and high power handling capabilities in a simple scalable design.     

89Yttrium nuclear magnetic resonance studies

With a Fourier-transform spectrometer, especially developed for nuclei with weak NMR signals, the lines of⁸⁹Y have been investigated in aqueous solutions of Y(NO₃)₃, YCl₃, and Y(ClO₄)₃. The concentration dependence of the chemical shifts of the⁸⁹Y resonance frequencies in these solutions were measured. Using this dependence, the Larmor frequency of the⁸⁹Y³⁺ ion solely surrounded by water was determined by extrapolation. The Larmor frequency of⁸⁹Y was referred to those of²H,³⁹K, and⁷³Ge with high accuracy. The magnetic moment of the⁸⁹Y³⁺ ion purely surrounded by H₂O molecules is μ(⁸⁹Y³⁺) = ?0.1368523(4) μ_N. The concentration dependence of Y(NO₃)₃ solutions in D₂O yields the solvent isotope effect δ(⁸⁹Y³⁺ in D₂O)?δ(⁸⁹Y³⁺ in H₂O)= ?(4.3±0.5)ppm. The⁸⁹Y relaxation times T₁ and T₂ of a 3 molal aqueous Y(NO₃)₃ solution were determined in the pH range ?0.5...+1.25. T₁ 190...90 s is nearly constant in this range, whereas the transverse relaxation rate T₂ ^?1 increases strongly with increasing pH; this effect seems to be due to the chemical exchange of the hydrated Y³⁺ ion between a monomer and a polymer site.     

Magnetic resonance of magnetic fluid and magnetoliposome preparations

In this study, magnetic resonance was used to investigate lauric acid-coated magnetite-based magnetic fluid particles and particles which are surrounded by a double layer of phospholipid molecules (magnetoliposomes). The data reveal the presence of monomers and dimers in both samples. Whereas evidence for a thermally induced disruption of dimers is found in the magnetic fluid, apparently, the bilayer phospholipid envelop prevents the dissociation in the magnetoliposome samples.     

Distinctive features of nonlinear nuclear magnetic resonance

Nonlinear NMR modes are classified and the limits of realization are established for each of them. The unstable behavior of magnetization near the separatrix of relaxationless motion of nonlinear NMR is studied and ways of stabilizing this instability are considered.Tbilisi State University. Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Radiofizika, Vol. 34, No. 7, pp. 731–737, July, 1991.