Certain pairs of ferromagnets (FMs) and antiferromagnets (AFMs), such as Co/CoO and Ni/NiO, couple magnetically when they share a common interface, which is known as exchange bias (EB). In exchange biased systems, the AFM pins the orientation of the moment in the FM, thereby enhancing the magnetic anisotropy and coercivity of the FM. EB is routinely utilized in spintronic devices, in which two layers with differing magnetic hardness are employed. Despite such technological applications, a microscopic understanding of EB remains incomplete. EB is of particular importance for magnetic nanoparticles, for which many compositions and sizes lose their static magnetization through superparamagnetism at room temperature. Superparamagnetism poses a grain size limit for ultrahigh-density magnetic recording, but anisotropy enhancement through EB can overcome this limit.

We will report results for Co(core)/CoO(shell) and Ni(core)/NiO(shell) nanoparticles. In Co(core)/CoO(shell) nanoparticles, a CoO shell thickness greater than 1 nm is required for EB, and a 3 nm thick shell raises the blocking temperature, at which the nanoparticles become superparamagnetic. At low temperature, moments in the CoO shell associated with grain boundaries and crystal or stoichiometric defects dominate EB and give rise to a thermal memory effect.