Exploring the Enhancement of Exchange Bias in Innovative Core/Shell/Shell Structures: Synthesis and Magnetic Properties of Co-Oxide/Co and Co-Oxide/Co/Co-Oxide Inverted Nanostructures

In this study, we investigate the enhancement of exchange bias in core/shell/shell structures by synthesizing single inverted core/shell (Co-oxide/Co) and core/shell/shell (Co-oxide/Co/Co-oxide) nanostructures through a two-step reduction and oxidation method. We evaluate the magnetic properties of the structures and study the effect of shell thickness on the exchange bias by synthesizing various shell thicknesses of Co-oxide/Co/Co-oxide nanostructures. The extra exchange coupling formed at the shell–shell interface in the core/shell/shell structure leads to a remarkable increase in the coercivity and the strength of the exchange bias by three and four orders, respectively. The strongest exchange bias is achieved for the sample comprising the thinnest outer Co-oxide shell. Despite the general declining trend of the exchange bias with Co-oxide shell thickness, we also observe a nonmonotonic behavior in which the exchange bias oscillates slightly as the shell thickness increases. This phenomenon is ascribed to the dependence of the antiferromagnetic outer shell thickness variation at the expense of the simultaneous opposite variation in the ferromagnetic inner shell.