  Spin torque nano-oscillators realized by magnetization dynamics trapped in a
current-induced potential are reported. We fabricated Ni$_{81}$Fe$_{19}$/Pt
nanostructures and measured current-induced microwave emission from the
structures. The result shows an increase in the magnitude and spectral
narrowing of the microwave emission. We demonstrate that the current-induced
magnetic field suppresses magnon radiation loss and significantly reduces the
linewidth and the threshold current required for the spin torque oscillation.
