  We report an entropy-driven orientational hopping transition in a
magnetically confined colloidal Janus rod. In a magnetic field, the sedimented
rod randomly hops between horizontal and vertical states: the latter state
comes at a substantial gravitational cost at no reduction of magnetic potential
energy. The probability distribution over the angles of the rod shows that the
presence of an external magnetic field leads to the emergence of a metastable
vertical state separated from the ground state by an effective barrier. This
barrier does not come from the potential energy but rather from the vast gain
in phase space available to the rod as it approaches the vertical state. The
loss of rotational degree of freedom that gives rise to this effect is a
statistical mechanical analogue of the phenomenon of gimbal lock from classical
mechanics.
