The effects of surface coating on the structural and magnetic properties of CoAg core-shell nanoparticles

We have investigated the impact of Ag surface coating on the microstructure of CoAg core-shell nanoparticles, and the consequences for the magnetic properties. Atomic structures were simulated using a molecular dynamics approach utilizing the embedded atom method. The magnetic properties were then simulated using an atomistic approach using a classical spin Hamiltonian, taking into account the long-range nature, atomic separation and directional and phase dependence of the exchange interactions in Co. For pure cobalt nanoparticles with a diameter less than approximately 3 nm the internal crystal structure showed multiple twinned regions and the morphology of an icosahedron. The addition of a monolayer silver coating alters the internal cobalt crystal structure to a regular planar form along the cubic [111] direction with a mixture of face centred cubic (fcc) and hexagonal close packed (hcp) atomic arrangements. The local atomic environment was used to assign anisotropies to the atoms on a site by site basis. Moreover, the capping layer influences the shape of the particle and yielded a morphology similar to that of a truncated octahedron. Taking into account the effects of surface anisotropy in addition gives an overall picture of anisotropy for CoAg nanoparticles. We present the results of calculations estimating the energy barrier to magnetization reversal and the intrinsic coercivity for these particles.