Intra- and interparticle magnetism of cobalt-doped iron-oxide nanoparticles encapsulated in a synthetic ferritin cage

We present an in-depth examination of the composition and magnetism of cobalt (Co super(2+)) -doped iron-oxide nanoparticles encapsulated in Pyrococcus furiosus ferritin shells. We show that the Co super(2+) dopant ions were incorporated into the gamma -Fe sub(2) O sub(3)/Fe sub(3) O sub(4) core, with small paramagnetic-like clusters likely residing on the surface of the nanoparticle that were observed for all cobalt-doped samples. In addition, element-specific characterization using Mossbauer spectroscopy and polarized x-ray absorption indicated that Co super(2+) was incorporated exclusively into the octahedral B sites of the spinel-oxide nanoparticle. Comparable superpara magnetic blocking temperatures, coercivities, and effective anisotropies were obtained for 7%, 10%, and 12% cobalt-doped nanoparticles, and were only slightly reduced for 3% cobalt, indicating a strong effect of cobalt incorporation, with a lesser effect of cobalt content. Due to the regular particle size and separation that result from the use of the ferritin cage, a comparison of the effects of interparticle interactions on the disordered assembly of nanoparticles was also obtained that indicated significantly different behaviors between undoped and cobalt-doped nanoparticles.