The generation of oscillations over the microwave frequency range is one of the most important applications of spintronic devices. Such devices exploit electron spin as well as their charge, thus overcoming the increasing limitations of conventional electronics. Of particular interest to wireless communications are spin-transfer nano-oscillators (STNOs). Scientists initiated the EU-funded project MASTER (Microwave amplification by spin transfer emission radiation) to explore the potential of STNOs for use as tunable and ultra-narrow band microwave radiation sources for mobile and wireless telecommunication technology. The focus was on addressing existing challenges related to insufficient power, too much (phase) noise and narrow frequency ranges. Taking advantage of large arrays of coherently coupled oscillators (oscillating together at the same frequency), scientists sought to significantly increase device performance. To optimise results, they studied four different mechanisms of coupling between neighbouring oscillators. Project research resulted in identification of the optimal configuration of N oscillators for synchronisation. Through coupling the magnetisation motion of the two layers that constitute an STNO, scientists achieved the targeted power output and linewidth. They reported enhanced performance up to N=4. The performance characteristics of the optimised array were studied both theoretically and experimentally. The project developed innovative spin-wave spectroscopic techniques that can excite and detect the magnetisation dynamics of individual STNO independently of spin-transfer effects. These techniques were fundamental to understanding the basic mechanisms involved in spin momentum transfer. Another achievement was the development a high-performance solver for performing micromagnetic simulations on a very large array of coherently coupled STNOs. In addition, the project created a simple theoretical framework for transport in magnetic multilayers. SNTOs can cover a different range of frequencies, are easy to fabricate and are compatible with conventional silicon complementary metal-oxide semiconductor technology. These nano-oscillators may soon replace conventional voltage-controlled oscillators that are used in resonant circuits. Another spintronic microwave system could be a dynamic magnetic read head in data storage. A wideband instantaneous frequency detector in the low- and high-frequency ranges for cognitive radio or radar systems is yet another beneficiary of SNTO technology.
Nano-oscillators, microwave frequency, wireless communications, spin-transfer nano-oscillators, emission radiation