Electron Holography of Core-Shell Co/CoO Spherical Nanocrystals

Phase profiles of core-shell Co/CoO nanocrystals are imaged using off-axis electron holography. Contributions of mean inner potential and magnetic induction to the phase shift are extracted, respectively, to quantitatively characterize the nanocrystal geometry and magnetic spin arrangement in a self-assembled nanocrystal chain. It is also found that the spin of 11 nm diameter Co core encapsulated with 5-nm-thick CoO shell remains stable even at an elevated temperature of 200°C, indicating that the magnetically thermal stability of cobalt nanocrystals is remarkably improved due to the exchange coupling between the ferromagnetic core and antiferromagnetic shell.