Competitive Size\nEffects in Antiferromagnetic|Ferrimagnetic\nCore|Shell Nanoparticles for Large Exchange Bias

A family of exchange-coupled core–shell (CS) nanoparticles\ncomposed of an antiferromagnetic (AFM) core (Co<sub>0.3</sub>Fe<sub>0.7</sub>O) and a ferrimagnetic (FiM) shell (Co<sub>0.6</sub>Fe<sub>2.4</sub>O<sub>4</sub>) was investigated to unravel the role played\nby the dimension of the two components on the magnetic properties\nof the system. The series comprises three samples with different core\ndiameters (2, 5, and 16 nm) and fixed shell thickness of ∼2\nnm. Although a strong core and shell magnetic coupling occurs in all\nthe samples, the final properties of the hybrid nanosystems are greatly\ninfluenced by the size of the two counterparts. Indeed, while the\nlarger sample can be described as a classic <i>T</i><sub>C</sub> > <i>T</i><sub>N</sub> exchange-bias, where <i>T</i><sub>C</sub> and <i>T</i><sub>N</sub> denote\nthe ordering temperature of the FiM and AFM phases, respectively,\non reducing the size, the blocking transition of the FiM shell decreases\nto values well below the <i>T</i><sub>N</sub> of the AFM.\nIn the first case, the FiM-AFM exchange-bias effect is determined\nby the magnetic ordering of the AFM core; in the other cases, it is\ndue to the reduction of the thermal-driven magnetic fluctuations of\nthe ordered FiM shell. On the other hand, the AFM properties of the\ncore regions also are extremely sensitive to the particle size reduction,\nshowing, for the smallest sample, the effect of the coupling between\nthe two phases to appear at temperature well below <i>T</i><sub>N</sub> displayed by the bulk system, indicating the potential\npresence of a blocking transition in the AFM core for small particles.\nThese findings highlight the significant influence of the size of\nthe AFM and FiM components on the hybrid system’s ultimate\nproperties. This result is potentially relevant for defining the working\nconditions of nanodevices exploiting exchange-bias phenomena, which\nhave been recently proposed in the literature for application in several\ntechnological fields, ranging from rare-earth free magnets, spintronics,\noptoelectronics, and magnetic-refrigeration.