On the magnetic anisotropy and nuclear relaxivity effects of Co and Ni doping in iron oxide nanoparticles

We report a systematic experimental study of the evolution of the magnetic and relaxometric
properties as a function of metal (Co, Ni) doping in iron oxide nanoparticles. A set of five samples,
having the same size and ranging from stoichiometric cobalt ferrite (CoFe2O4) to stoichiometric nickel
ferrite (NiFe2O4) with intermediate doping steps, was ad hoc synthesized. Using both DC and AC
susceptibility measurements, the evolution of the magnetic anisotropy depending on the doping is
qualitatively discussed. In particular, we observed that the height of the magnetic anisotropy barrier is
directly proportional to the amount of Co, while the Ni has an opposite effect. By Nuclear Magnetic
Resonance Dispersion (NMR-D) experiments, the experimental longitudinal r1 and transverse r2
relaxivity profiles were obtained, and the heuristic theory of Roch et al. was used to analyze the data
of both r1 and, for the first time, r2. While the experimental and fitting results obtained from r1 profiles
were satisfying and confirmed the anisotropy trend, the model applied to r2 hardly explains the
experimental findings.