We reported microwave measurements on a nanoscale MgO-based magnetic tunnel junction having an elliptical shape with large aspect ratios to obtain enough in-plane shape anisotropy to ensure free layer magnetization along the long axis. Combined with the magnetization of a synthetic antiferromagnet pinned layer along the short axis, this results in the perpendicular configuration between the magnetizations of free and pinned layers. A steady high frequency oscillation up to 5 GHz was achieved in such devices at zero magnetic field. Meanwhile, a large frequency tunability of 0.11 Hz·cm2/mA (2.67 GHz/mA) was obtained. The micromagnetic simulations confirm the origin of the high oscillation frequency of our spin transfer nano-oscillators (STNOs). These results suggest the great possibility of improved integration and potential application of STNOs for developing the next-generation of on-chip oscillators.

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