Cubic bi-magnetic hard-soft core-shell nanoarchitectures were prepared starting from cobalt ferrite nanoparticles, prevalently with cubic shape, as seeds to grow a manganese ferrite shell. The combined use of direct (nanoscale chemical mapping via STEM-EDX) and indirect (DC magnetometry) tools was adopted to verify the formation of the heterostructures at the nanoscale and bulk level, respectively. The results showed the obtainment of core-shell NPs (CoFe2O4@MnFe2O4) with a thin shell (heterogenous nucleation). In addition, manganese ferrite was found to homogeneously nucleate to form a secondary nanoparticle population (homogenous nucleation). This study shed light on the competitive formation mechanism of homogenous and heterogenous nucleation, suggesting the existence of a critical size, beyond which, phase separation occurs and seeds are no longer available in the reaction medium for heterogenous nucleation. These findings may allow one to tailor the synthesis process in order to achieve better control of the materials' features affecting the magnetic behaviour, and consequently, the performances as heat mediators or components for data storage devices.

Consorzio Interuniversitario Nazionale per la Scienza e Tecnologia dei Materiali Via Giuseppe Giusti 9 50121 Florence Italy

Department of Chemical and Geological Sciences University of Cagliari Cittadella Universitaria S S 554 Bivio per Sestu 09042 Monserrato Italy

Department of Inorganic Chemistry Charles University Hlavova 2030 128 40 Prague 2 Czech Republic

Department of Physics and Astronomy University of California Irvine CA 92617 USA

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