  We image the remnant magnetization configurations of CoFeB and permalloy
nanotubes (NTs) using x-ray magnetic circular dichroism photo-emission electron
microscopy. The images provide direct evidence for flux-closure configurations,
including a global vortex state, in which magnetization points
circumferentially around the NT axis. Furthermore, micromagnetic simulations
predict and measurements confirm that vortex states can be programmed as the
equilibrium remnant magnetization configurations by reducing the NT aspect
ratio.
