Spin-Spin and Spin-Lattice Relaxation of Protons in Ferrofluids Characterized With a High- SQUID-Based NMR Spectrometer in Microtesla Fields

In this work, the relaxation rates of ferrofluids are characterized using a high-Tc SQUID-based nuclear magnetic resonance spectrometer in different field strengths and temperatures. It was found that the longitudinal relaxation rate, 1/T1, of ferrofluids measured in a high field strength is significantly higher than that measured in a low field strength. We attribute this to the magnetic-field gradients from the magnetization of magnetic nanoparticles that accelerate the T1-relaxation more in a high strength of magnetic fields than they are in a low strength of magnetic fields. Furthermore, T1 and T2 decrease when temperature increases, where T2 is the transverse relaxation time. This is due to the improved field-homogeneity seen by protons' spins at high temperatures, attributed to the enhanced Brownian motion of magnetic nanoparticles. Characterizing the relaxation rates will be helpful for further use of ferrofluids as contrast agents in low-field MR imagings.