  An array of spin torque nano-oscillators (STNOs), coupled by dipolar
interaction and arranged on a ring, has been studied numerically and
analytically. The phase patterns and locking ranges are extracted as a function
of the number $N$, their separation, and the current density mismatch between
selected subgroups of STNOs. If $N\geq 6$ for identical current densities
through all STNOs, two degenerated modes are identified an in-phase mode (all
STNOs have the same phase) and an out-of-phase mode (the phase makes a 2$\pi$
turn along the ring). When inducing a current density mismatch between two
subgroups, additional phase shifts occur. The locking range (maximum current
density mismatch) of the in-phase mode is larger than the one for the
out-of-phase mode and depends on the number $N$ of STNOs on the ring as well as
on the separation. These results can be used for the development of magnetic
devices that are based on STNO arrays.
