Abstract
Prof. J. van Lierop et al and we have showed that the magnetism of core-shell nanoparticles (made of maghemite, -Fe2O3, cores and transition-metal and metal-oxide shells) is altered substantially by the interface, which is a doped iron-oxide layer formed naturally during the seed-mediated synthesis process, a route used typically to produce core-shell nanoparticles. Characteristics fundamental to useful applications, such as the anisotropy and superparamagnetic blocking temperature, were altered substantially with Cu, CoO, MnO, and NiO shells. To ascertain the origin of this behavior, the prototype -Fe2O3/CoO core-shell nanoparticles are described in detail. The magnetism originates essentially from an interfacial doped iron-oxide layer formed via migration of shell ions,e.g., Co2+, into octahedral site vacancies in the surface layers of the γ -Fe2O3 core. For this system, an overall Fe morb/mspin = 0.15 ± 0.03 is measured (morb ∼ 0 for the Fe-oxides) and an enhanced Co morb/mspin = 0.65 ± 0.03 elucidates the origin of the unexpectedly high overall anisotropy of the nanoparticle. This interfacial layer isresponsible for the overall (e.g., bulk) magnetism and provides a perspective on how the magnetism of core-shell nanoparticles manifests from the selected core and shell materials. Within this work, TEM and first-principles calculations to prove the core/shell intermixing and magnetic property were performed in our group. By analyzing the HRTEM and EDS, the intermixing was confirmed, mainly by the doping of Co into the octahedral site vacancies of -Fe2O3. The average Co doping depths in different processing temperatures (150°C and 235°C) were 0.47nm and 0.67nm, respectively. The error of this measurement is within 5 percent through a simulation. By first-principles calculations, the intermixing phase of -Fe2O3 with Co doping is ferromagnetic, with even higher magnetization as compared to that of pure -Fe2O3. Besides, Co doping (same numbers) into different octahedral sites can cause different magnetizations.