Spin-Spin and Spin-Lattice Relaxation of Protons in Ferrofluids Characterized With a High- $T_{\rm c}$ SQUID-Based NMR Spectrometer in Microtesla Fields

In this work, the relaxation rates of ferrofluids are characterized using a high-T_c 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.