Magnetic Anisotropy\nand Interactions in Hard/Soft\nCore/Shell Nanoarchitectures: The Role of Shell Thickness

In this work, the influence of shell thickness on the\nmagnetic\nbehavior of hard cobalt ferrite (CoFe<sub>2</sub>O<sub>4</sub>) nanoparticles\ncoated with one or more layers of magnetically soft nickel ferrite\n(NiFe<sub>2</sub>O<sub>4</sub>) was investigated. The materials were\nchosen as model soft and hard magnetics due to their significant difference\nin the magnetic anisotropy constant, which spans 2 orders of magnitude.\nThe obtained magnetic nanoarchitectures are compositionally graded\nsingle crystals, i.e., they comprise a crystallographic coherence\nof core and shell with no visible core–shell interface at the\natomic level. The CoFe<sub>2</sub>O<sub>4</sub> core exhibited an\naverage size of ∼9 nm, while the NiFe<sub>2</sub>O<sub>4</sub> shells were engineered with varying thicknesses of ∼1, 3,\nand 6 nm. The core/shell nanoarchitectures behave as a single magnetic\nunit, i.e., in the rigid exchange coupling regime. Nonmonotonic variation\nof the coercivity with the shell thickness is observed that is attributed\nto the competition of the different magnetic anisotropies, such as\nmagneto-crystalline and surface, interplaying with the interparticle\ndipolar interactions. The experimental findings are in good qualitative\nagreement with Monte Carlo simulation results for a mesoscopic model\nthat includes both the nanoparticle morphology and interparticle interactions.