Core Size and Interface Impact on the Exchange Bias of Cobalt/Cobalt Oxide Nanostructures

• Main Authors: Maral Ghoshani, Morteza Mozaafari, Peter S. Normile, Jose A. De Toro, Abdulrahman Al-Nabhani
• Format: Article
• Language: English
• Published: MDPI AG 2021-03-01
• Series: Magnetochemistry
• Subjects: magnetic nanostructures; exchange bias; core/shell structure; cobalt oxide
• Online Access: https://www.mdpi.com/2312-7481/7/3/40

Two series of Co/Co-oxide nanostructures have been synthesized by the co-precipitation method followed by different reduction and oxidation processes in an attempt to optimize their exchange bias (EB) properties. The samples are characterized by X-ray diffraction, scanning and transmission electron microscopy, and SQUID (superconducting quantum interference device) magnetometry. The two series differ with respect to their average Co core grain sizes: in one (the <i>l</i>-series), the size is ≈100 nm, and in the other (the <i>s</i>-series, obtained using lower synthesis temperatures than the <i>l</i>-series), it is ≈10 nm. In the <i>l</i>-series, progressive oxidation yields an increase in the EB field together with a reduction in Co core size. In contrast, progressive oxidation in the <i>s</i>-series results in growth of the Co-oxide fraction at the expense of the Co core upon oxidation, which is accompanied by a decrease in the EB effect that is attributed to an ordering of the ferromagnetic–antiferromagnetic interface and therefore a reduction of uncompensated spins density. These results illustrate how the interface details become relevant only for small enough ferromagnetic cores.