No abstract

13C nuclear relaxation times were measured by a high‐pressure sapphire NMR cell for neat CO2 and a CO2–CH4 mixture over a wide range that covers gas, liquid and supercritical densities. The nuclear spin–lattice relaxation times at liquid and supercritical densities for the CO2–CH4 mixture were obtained for the first time. Although the spin–rotation interactions still dominate the spin–lattice relaxation processes in CH4, the intramolecular dipolar interaction contributes approximately 5% to the overall relaxation rates at liquid densities for this mixture. The chemical shift anisotropy and the intermolecular contributions to the relaxation rates of CO2 were experimentally determined to be negligible. A movable wall cell model was modified and used to relate the relaxation time data to the fluid structures of the first solvation shell around the central molecules in the liquid and supercritical fluid states. The coordination number in the first shell was found to be 12 for both the neat CO2 and CO2–CH4 mixture at liquid and supercritical fluid densities.

No abstract

We study the third harmonic signal of magnetic nanoparticles (MNPs) for use in narrow-band magnetic nanoparticle imaging. We measured the properties of the third harmonic signal, such as frequency and magnetic field dependencies, when the behavior of MNPs was dominated by Neel relaxation. It was shown that the third harmonic signal had both real and imaginary parts, although only the real part is expected from the conventional Langevin function. The real and imaginary parts exhibited different dependences on the frequency and magnetic field. The dynamic behavior of MNPs was analyzed by taking into account the Neel relaxation of MNPs. It was shown that the imaginary part was generated due to Neel relaxation. We obtain an analytical expression for the third harmonic signal, in which distributions of magnetic moment and anisotropic energy of MNPs in the sample were also considered. We show that the analytical results quantitatively explain the experimental results. Our results indicate that the properties of the third harmonic signal of immobilized MNPs are significantly affected by Neel relaxation.