  Using an optimally coupled nanometer-scale superconducting quantum
interference device, we measure the magnetic flux originating from an
individual ferromagnetic Ni nanotube attached to a Si cantilever. At the same
time, we detect the nanotube's volume magnetization using torque magnetometry.
We observe both the predicted reversible and irreversible reversal processes. A
detailed comparison with micromagnetic simulations suggests that vortex-like
states are formed in different segments of the individual nanotube. Such
stray-field free states are interesting for memory applications and
non-invasive sensing.
