Influence of the nanocrystallinity on exchange bias in Co/CoO core/shell nanoparticles

This study presents a comparison of the magnetic properties of core/shell nanoparticles composed of cobalt (Co) and cobalt oxide (CoO), with variations in their crystalline structure. Specifically, we investigated poorly crystallized core/shell nanoparticles (with a diameter of 8.8 nm) prepared from face-centered cubic (fcc) cobalt polycrystals synthesized via the micellar approach, as well as highly crystallized nanoparticles (with a diameter of 9.9 nm) prepared from hexagonal (hcp) cobalt single-crystals formed using the organometallic decomposition approach. The oxidation process is conducted in a solution. To examine the structural characteristics, high-resolution transmission electron microscopy (HRTEM) was employed. Our findings revealed that the Co fcc polycrystals yield a poorly fcc crystallized CoO shell, whereas the Co hcp nanoparticles exhibited a highly crystallized CoO shell. Furthermore, we conducted a magnetic investigation utilizing a Vibrating Sample Magnetometer (VSM). The results clearly demonstrate the significant influence of the core/shell nanoparticle's crystalline structure on the interfacial magnetic coupling. Specifically, we observed an exchange bias exclusively in the Cohcp/CoO nanoparticles. Notably, the measured exchange bias field (655 mT) surpasses previously reported values in literature for similar nanoparticles with an hcp cobalt core. This enhancement can be attributed to several factors, including the high-quality Co/CoO interface, the disparate magnetic anisotropy between the core (Co hcp) and the shell (CoO), as well as the optimal ratio between the core and shell, thereby inducing a heightened interfacial magnetic coupling.