  The unusual band structure of carbon nanotubes (CNs) results in their
remarkable magnetic properties. The application of magnetic field parallel to
the tube axis can change the conducting properties of the CN from metallic to
semiconducting and vice versa. Apart from that B induces (via the Bohm-Aharonov
effect) orbital magnetic moments $\mu_{orb}$ in the nanotube. These moments are
studied both in pure and hole- or electron-doped CNs, isolated or in a circuit.
Remarkably, $\mu_{orb}$ in pure CNs depends uniquely on their original
conducting properties, length, and temperature, but it does not depend on the
nanotube radius or the particular chirality. In doped nanotubes the magnetic
moments can be strongly altered and depend on the radius and
chirality.Temperature can even change their character from diamagnetic at low T
to paramagnetic at high T. A full electron-hole symmetry in doped tubes is also
revealed.
