Is time reversal broken in an atom with nonzero magnetic moment?

[wdlang]

it is obvious that the hamiltonian of an atom is time reversal invariant

but how can an atom have non-zero magnetic moment?

 

[Bill_K]

Time reversal is equivalent to CP. If you imagine the magnetic moment as being produced by tiny little charges running around in a circle, C flips the sign of their charge, while P makes them run around in the opposite sense. So magnetic dipole moment is even under CP, and consequently even under T. It's the electric dipole moment which is odd under CP and T.