Surface anisotropy in ferromagnetic nanoparticles

The effect of surface anisotropy on the magnetic ground state of a ferromagnetic nanoparticle is investigated using atomic Monte Carlo simulation for spheres of radius $R=6a$ and $R=15a,$ where a is the interatomic spacing. It is found that the competition between surface and bulk magnetocrystalline anisotropy imposes a “throttled” spin structure where the spins of outer shells tend to orient normal to the surface while the core spins remain parallel to each other. For large values of surface anisotropy, the spins in sufficiently small particles become radially oriented either inward or outward in a “hedgehog” configuration with no net magnetization. Implications for FePt nanoparticles are discussed.