Temperature and field dependences of transverse relaxivity of Co-Zn ferrite nanoparticles coated with silica: the role of magnetic properties and different regimes

Abstract The transverse relaxivity r2, characterizing the performance of negative contrast agents like magnetic nanoparticles in T2-weighted MRI, depends on multiple factors, most importantly on magnetic properties and the size of the particles, including both the size of magnetic cores and the effective size of whole particles present as individual entities in the suspension. The current study probes the transverse relaxivity of ferrimagnetic CoxZnyFe3-x-yO4 nanoparticles whose behavior varies from the blocked state, across the superparamagnetic regime to the vicinity of the paramagnetic state, while other parameters, namely the size of the magnetic cores (≈10 nm), the size of whole colloidally stable particles, and their geometry (≈30 nm-sized clusters of crystallites uniformly coated with ≈12 nm thick silica shell) are kept the same. Among the samples studied, the silica-coated clusters of Co0.66Zn0.47Fe1.87O4 crystallites show high values of r2 = 534 s-1 mmol(Me3O4)-1 L at room temperature in the magnetic field of 1.5 T. Importantly, the field and temperature dependences of transverse relaxivity are discussed in relation to relevant theoretical models, i.e., motional averaging and static dephasing regimes. The increase in r2 in higher magnetic fields is related to the notable paraprocess in the employed cores, whereas the gradual decrease of r2 with increasing temperature is dominated by the significant increase of the self-diffusion coefficient of water.