Magnetic Interaction in Superparamagnetic Co-Pt Nanoparticles Synthesized in Protein Crystal

We synthesized Co-Pt nanoparticles in a crystal composed of proteins with a cage structure [i.e., Pyrococcus furiosus virus-like particle (PfV)] and investigated their magnetic interactions. When the soaking concentrations of metal, Co, and Pt ions in the PfV crystal were below 5.5 mM, isolated Co-Pt nanoparticles were formed. The size of the primary particles was 2–3 nm, which was smaller than the inner core size of the cage structure. When the soaking concentrations increased above 11 mM, the Co-Pt nanoparticles formed aggregates exceeding 30 nm in size. The synthesized nanoparticles showed superparamagnetic behavior at 300 K, independent of the soaking concentration. The temperature dependence of the ac magnetic susceptibility and remanent magnetization curve verified the occurrence of magnetic interactions between the Co-Pt nanoparticles. These measurements revealed that the dipolar-dipolar interaction was significant in the closely packed isolated nanoparticles, whereas it was weaker in the aggregated Co-Pt nanoparticles, probably due to the increased distance between the particles.