Ground-state configurations in ferromagnetic nanotori

Magnetization ground states are studied in toroidal nanomagnets. The energetics associated to the ferromagnetic, vortex and onion-like configurations are explicitly computed. The analysis reveals that the vortex appears to be the most prominent of such states, minimizing total energy in every torus with internal radius $rgtrsim10,{rm nm}$ (for Permalloy). For $rlesssim10,{rm nm}$ the vortex remains the most favorable pattern whenever $R/ell_{ex}gtrsim1.5$ ($R$ is the torus external radius and $ell_{ex}$ is the exchange length), being substituted by the ferromagnetic state whenever $R/ell_{ex}lesssim1.5$.