Competitive Size Effects in Antiferromagnetic|Ferrimagnetic Core|Shell Nanoparticles for Large Exchange Bias

A family of exchange-coupled core-shell (CS) nanoparticles composed of an antiferromagnetic (AFM) core (Co_0.3Fe_0.7O) and a ferrimagnetic (FiM) shell (Co_0.6Fe_2.4O₄) was investigated to unravel the role played by the dimension of the two components on the magnetic properties of the system. The series comprises three samples with different core diameters (2, 5, and 16 nm) and fixed shell thickness of ∼2 nm. Although a strong core and shell magnetic coupling occurs in all the samples, the final properties of the hybrid nanosystems are greatly influenced by the size of the two counterparts. Indeed, while the larger sample can be described as a classic <i>T</i> _C > <i>T</i> _N exchange-bias, where <i>T</i> _C and <i>T</i> _N denote the ordering temperature of the FiM and AFM phases, respectively, on reducing the size, the blocking transition of the FiM shell decreases to values well below the <i>T</i> _N of the AFM. In the first case, the FiM-AFM exchange-bias effect is determined by the magnetic ordering of the AFM core; in the other cases, it is due to the reduction of the thermal-driven magnetic fluctuations of the ordered FiM shell. On the other hand, the AFM properties of the core regions also are extremely sensitive to the particle size reduction, showing, for the smallest sample, the effect of the coupling between the two phases to appear at temperature well below <i>T</i> _N displayed by the bulk system, indicating the potential presence of a blocking transition in the AFM core for small particles. These findings highlight the significant influence of the size of the AFM and FiM components on the hybrid system's ultimate properties. This result is potentially relevant for defining the working conditions of nanodevices exploiting exchange-bias phenomena, which have been recently proposed in the literature for application in several technological fields, ranging from rare-earth free magnets, spintronics, optoelectronics, and magnetic-refrigeration.