Comparison of magnetic anisotropy and structural properties in chemically ordered CoPt and FePt nanoparticles

The intrinsic magnetic properties of nanoparticles (NPs) can be accurately determined using highly dispersed NPs in a matrix. In this paper, we study chemically ordered CoPt and FePt NPs with a diameter of 3 nm embedded in an amorphous carbon matrix. Although both alloys exhibit almost the same magnetic and crystallographic properties in the bulk materials, they are completely different as NPs. We show that their magnetic anisotropy differs greatly. To understand the origin of such a difference, a fine crystallographic structure study has been performed using extended x-ray absorption fine structure. In that respect, the atomic relaxations appear to be different in both nanoalloys. Ab initio calculations of the atomic relaxation shed light on these experimental results, showing that the wide distance distribution in Co sublattice should strongly alter the magnetic anisotropy of CoPt NPs.