Order–Disorder Transitions Confined at the Interface of Pd@Co Core–Shell Nanoparticles: Implications for Magnetic Recording

We observed an order–disorder transition confined to the interface of Pd@Co core–shell nanoparticles with a Pd core and Co shell. A local ordered region with the L1 0 - and L1 2 -type ordered structure 2–3 nm in size was formed in a ∼12 nm-sized particle. The local atomic order was only achieved in the narrow temperature range of 573–723 K, where the diffusion length is limited to a short-range comparable to the nearest-neighbor distance of the Co–Pd solid solution. Once the atom migration was activated ( T ≥ 773 K), alloying of Co and Pd proceeded rapidly, and the ordered phase disappeared along with the core–shell structure. Namely, the stability of the ordered phase was diffusion controlled. Chemically sensitive atomic-resolution electron microscopy enabled detection of the local atomic order formed in a nanoparticle. Such local atomic order has the potential to enable tuning of the magnetic anisotropy of bimetallic nanomagnets, which may open a new route to realize ultrahigh-density magnetic storage media.