  The spin-transfer nano-oscillator (STNO) offers the possibility of using the
transfer of spin angular momentum via spin-polarized currents to generate
microwave signals. However, at present STNO microwave emission mainly relies on
both large drive currents and external magnetic fields. These issues hinder the
implementation of STNOs for practical applications in terms of power
dissipation and size. Here, we report microwave measurements on STNOs built
with MgO-based magnetic tunnel junctions having a planar polarizer and a
perpendicular free layer, where microwave emission with large output power,
excited at ultralow current densities, and in the absence of any bias magnetic
fields is observed. The measured critical current density is over one order of
magnitude smaller than previously reported. These results suggest the
possibility of improved integration of STNOs with complementary
metal-oxide-semiconductor technology, and could represent a new route for the
development of the next-generation of on-chip oscillators.
