Magnetic Interactions Versus Magnetic Anisotropy in Spinel Ferrite Nanoparticles

We investigated the flexibility of high-temperature thermal decomposition synthesis to design magnetic nanoparticles (NPs) with engineered properties. We prepared spinel iron oxide particles with the desired composition (Fe
₃
O
₄
and CoFe
₂
O
₄
) and a well-controlled average size (5-8 nm) by tuning the synthesis procedure. The substitution of Fe by Co produces a dramatic increase of the magnetocrystalline anisotropy: for 5 nm particles, the anisotropy constant increased from ~3.9×10
⁴
to ~7.5×10
⁵
J/m
³
. The magnetocrystalline anisotropy has shown a major effect on the magnetic behavior of highly crystalline particles, except for the smallest iron oxide sample, where the surface anisotropy plays a dominant role. The presence of surfactant (oleic acid) on the NPs surface prevents direct exchange coupling among them, whereas the dipolar interaction was predominant, with an estimated temperature-equivalent energy in the range of 4-60 K.