27 Jun 2012

US team develops 'world's most powerful' nano-scale microwave oscillators

US-based researchers have created what are described as the most powerful, high-performance nanoscale microwave oscillators in the world, paving the way to cheaper, more energy-efficient mobile communication devices that deliver better signal quality.

Today's cell phones, WiFi–enabled tablets and other electronic gadgets all use microwave oscillators. The transmitter and receiver circuits in a mobile phone for example, contain oscillators that produce radio-frequency signals, which are then converted by the phone's antenna into incoming and outgoing electromagnetic waves.

Current oscillators are silicon-based and use the charge of an electron to create microwaves. However, the team from the University of California, Los Angeles have developed oscillators that use the spin of an electron bringing many advantages over the oscillators commonly in use today.

The research focused on STT-RAM, or spin-transfer torque magnetoresistive random access memory, which has great potential over other types of memory in terms of both speed and power efficiency.

"We realised that the layered nanoscale structures that make STT-RAM such a great candidate for memory could also be developed for microwave oscillators for communications," said principal investigator Kang L. Wang, UCLA Engineering's Raytheon Professor of Electrical Engineering and director of the Western Institute of Nanoelectronics (WIN).

The structures, called spin-transfer nano-oscillators, or STNOs, are composed of two distinct magnetic layers. One layer has a fixed magnetic polar direction, while the other layer's magnetic direction can be manipulated to gyrate by passing an electric current through it. This allows the structure to produce very precise oscillating microwaves.

"Previously, there had been no demonstration of a spin-transfer oscillator with sufficiently high output power and simultaneously good signal quality, which are the two main metrics of an oscillator — hence preventing practical applications," said Pedram Khalili, project manager for the UCLA–DARPA research programs in STT-RAM and non-volatile logic. "We have realised both these requirements in a single structure."

The SNTO was tested to show a record-high output power of close to 1 micro-watt, with a record narrow signal linewidth of 25 megahertz. Output power refers to the strength of the signal, and 1 micro-watt is the desired level for STNOs to be practical for applications.

Also, a narrow signal linewidth corresponds to a higher quality signal at a given frequency. This means less noise and interference, for a cleaner voice and video signal. It also means more users can be accommodated onto a given frequency band.

In addition, the new nanoscale system is about 10,000-times smaller than the silicon-based oscillators used today.

The nano-oscillators can easily be incorporated into existing integrated circuits (computer chips), as they are compatible with current design and manufacturing standards in the computer and electronic device industries.

The oscillators can be used in both analogue and digital communications, which means smart phones could take full advantage of them.

"For the past decade, we have been working to realise a new paradigm in nanoelectronics and nanoarchitectures," said Wang, who is also a member of the California NanoSystems Institute at UCLA. "This has led to tremendous progress in memory research. And along those same lines, we believe these new STNOs are excellent candidates to succeed today's oscillators."

• The paper, "High-Power Coherent Microwave Emission from Magnetic Tunnel Junction Nano-oscillators with Perpendicular Anisotropy," has been published online in the journal ACSNano.

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