Magnetic nanoparticles are being extensively studied due to their wide range of potential applications, spreading from biomedicine to magnetic recording. While some applications require superparamagnetic particles, others, like recording, need the particles to remain ferromagnetic. Moreover, the use of magnetic nanoparticles for sensing purposes calls for the particles to be magnetically soft without being superparamagnetic. In this context, we have investigated oxide bi-magnetic core-shell nanoparticles with ferrimagnetic cores and antiferromagnetic (or ferrimagnetic) shells. Two examples of core-shell structures will be discussed. These have been fabricated by two different post-synthesis methods using previously prepared ferrimagnetic γ-Fe₂O₃ cores: i) Co(II) ions were adsorbed on the cores to form a shell with a composition near CoFe₂O₄ and ii) a MnO layer was formed by using the cores as seeds for the heterogeneous growth. It is shown that both ferrimagnetic and antiferromagnetic shells allow to controllably increase the blocking temperature, T_B, of the system with respect to that of untreated particles. The presence of the shells also brings about a remarkable increase of the coercivity. Moreover, in the case of antiferromagnetic shells a shift of the hysteresis loop along the field axis, i.e., exchange bias, is also observed after field cooling from above the Néel temperature. These results open the door to the possibility to synthesize a single, generic, type of particle, which can later be modified to adapt to the needs of a specific application.

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