Ultralow Current Density and Bias-Field-Free Spin-Transfer Nano-Oscillator

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