Observation and elimination of broken symmetry in L1₀ FePt nanostructures

An unexplained surface anisotropy effect was observed and confirmed in the magnetization reversal process of both L1₀ phase FePt nanoparticles with octahedral shape and (001) textured L1₀ FePt thin films with island nanostructures. We suggest that the nature of the observed surface effect is caused by broken symmetry on the FePt surface, which results in weakened exchange coupling for surface atoms. Furthermore, we propose, and experimentally demonstrate, a method to repair the broken symmetry by capping the FePt islands with a Pt layer, which could prove invaluable in understanding fundamental limitations of magnetic nanostructures.