Magnetic Anisotropy and Interactions in Hard/Soft Core/Shell Nanoarchitectures: The Role of Shell Thickness

In this work, the influence of shell thickness on the magnetic behavior of hard cobalt ferrite (CoFe2O4) nanoparticles coated with one or more layers of magnetically soft nickel ferrite (NiFe2O4) was investigated. The materials were chosen as model soft and hard magnetics due to their significant difference in the magnetic anisotropy constant, which spans 2 orders of magnitude. The obtained magnetic nanoarchitectures are compositionally graded single crystals, i.e., they comprise a crystallographic coherence of core and shell with no visible core-shell interface at the atomic level. The CoFe2O4 core exhibited an average size of ∼9 nm, while the NiFe2O4 shells were engineered with varying thicknesses of ∼1, 3, and 6 nm. The core/shell nanoarchitectures behave as a single magnetic unit, i.e., in the rigid exchange coupling regime. Nonmonotonic variation of the coercivity with the shell thickness is observed that is attributed to the competition of the different magnetic anisotropies, such as magneto-crystalline and surface, interplaying with the interparticle dipolar interactions. The experimental findings are in good qualitative agreement with Monte Carlo simulation results for a mesoscopic model that includes both the nanoparticle morphology and interparticle interactions.