Magnetic Traits in CoO‐Core@CoFe₂O₄‐shell Like Nanoparticles

Abstract Antiferromagnetic (AF) CoO pseudo‐single crystals 125±20 nm in diameter were prepared by forced hydrolysis in polyol and used as seeds to grow a poly‐ and nano‐crystalline (5 nm in diameter) ferrimagnetic (F) CoFe₂O₄ shell. Transmission electron microscopy (TEM) showed a certain epitaxy between the face‐centered cubic lattices of the AF rock‐salt core and the F spinel shell. Magnetometry measurements do not evidence any exchange bias feature between the two phases in direct contact. The hysteresis loops recorded between 7 and −7 T after cooling the composite particles from 400 K (between the Néel and the Curie temperatures of the AF and F phases, respectively) to 5 K, with and without a cooling magnetic field (μ₀H_FC=+7 T), are the same, meaning that the AF core does not allow any spin pinning on the CoFe₂O₄ nanocrystals. Nevertheless, differences exist between these magnetic data and those collected on two CoFe₂O₄ reference particles: the low temperature coercive field of the composite is between those of 5 nm sized single‐crystals and their assembly as submicrometer polycrystals. This order was also observed for the blocking temperature, although the constituting CoFe₂O₄ nanocrystals of all these particles, the composite and the two references, were the same. Advanced analysis by X‐ray diffraction and TEM established that the coherent crystallographic domain length of the ferrite phase and the magnetic behavior of the three systems are interdependent. Depending on the growth conditions of the CoFe₂O₄ phase, a relative epitaxy between the nanocrystals may exist. It is much greater in the polycrystalline CoFe₂O₄ particles than in their CoO−CoFe₂O₄ core‐shell counterparts, and does not occur in free single crystal CoFe₂O₄ particles.