Spin-Spin and spin-lattice relaxation of protons in ferrofluids characterized with a high-T_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.