The result is an increase of the coercive field (H C) and a horizontal shift of the hysteresis loop, characterized by the exchange bias field, H E. This coupling, indeed, entails an extra energy barrier for the F(i)M uncompensated spins, making more difficult to reverse the magnetization toward the direction opposite to the cooling field. The exchange bias originates from the pinning force exerted by the AFM phase on the magnetic moments in the first atomic layer of the interfaced F(i)M material, leading to an additional unidirectional anisotropy energy. One of the most interesting phenomenon encountered in coupled systems is the so called exchange bias effect, generally observed in binary systems comprising an antiferromagnetic (AFM) and a ferro(i)magnetic (F(i)M) ordered phases, when they are cooled in the presence of a magnetic field through the Néel temperature of the AFM component. The tuning of the physical properties of traditional spinel ferrites by coupling them with different magnetic materials at the nanoscale is a promising strategy that has been extensively investigated in the last decade to optimize their behavior for several high-tech applications. The dual-doping is revealed to be an efficient way for tuning the magnetic properties of the final system, obtaining Ni-Co doped iron oxide NPs with high coercivity (and therefore, high energy product), and increased antiferromagnetic ordering transition temperature, close to room temperature. An extensive morphological and crystallographic characterization of the obtained NPs shows how a higher stability against the oxidation process in ambient condition is attained when divalent cation doping of the iron oxide lattice with Co (II) and Ni (II) ions is performed. To this aim, a series of 20 nm NPs with a wüstite-based core and a ferrite shell, with tailored composition, (Co 0.3Fe 0.8Fe 2.2O 4 and Ni 0.17Co 0.21Fe 0.4Co 0.3Fe 2.3O 4) is synthetized through a thermal-decomposition method. Here, the effect of the combined substitution of Fe (II) with Co (II) and Ni (II) on the crystal structure and magnetic properties is studied. Nanometric (Fe 1− 3O 4) has been extensively studied because of the emergence of exchange bias phenomena. Since their actual implementation in modern technologies is hampered by the low temperature at which bias is operating, the critical issue to be solved is to obtain exchange-coupled nanoparticles (NPs) with ordering temperature close to 300 K by replacing the divalent iron with other transition-metal ions.
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