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Spin-Torque Nano-Oscillators Eye Frequency-Synthesis ICs

GENERALLY, THE SYNTHESIZERS in today's wireless consumer-electronics products are based on several single-frequency RF voltage-controlled oscillators (VCOs). The leading technology combines standalone high-quality, low-frequency quartzcrystal resonators and integrated high-frequency, low-quality LC tank-based phase-locked loops (PLLs). In future mobile phones, however, a different approach will be needed to cover both current and future communication standards. A potential solution has emerged in an RF nanooscillator based on spintronics, which hails from Frank Badets from STMicroelectronics (Grenoble, France), Jordan Katine and Daniele Mauri from Hitachi Global Storage Technologies (San Jose, CA), Ursula Ebels and Dimitri Houssameddine from CEA-Spintec (Grenoble, France), and Patrick Villard, Bertrand Delaet, Pierre Vincent, Marie- Claire Cyrille, Bernard Viala, Jean-Philippe Michel, and Jrme Prouve from CEA-Lti, MINATEC (Grenoble, France).

Their nano-sized oscillator for RF applications is based on two spintronic effects: the tunnel magnetoresistance (TMR) and spin momentum transfer torque. Negative resistance is provided by the spin momentum transfer, which results from a spin-polarized current flowing into a nano-sized (sub-100-nm) magneto-resistive device. The device's oscillation frequency, which is several gigahertz, can be tuned by both a DC bias current and an external DC magnetic field. The main advantage of this spin torque nano-oscillator (STNO) is that it is more than 50X smaller than a standard LC-tank VCO designed in a CMOS process. In addition, its tuning range can reach 100-percent bandwidth for carrier frequencies from 1 to 10 GHz. Because this oscillator also is fully compatible with standard CMOS processes, it clearly has the potential to serve future RF transceivers.

According to the first measurements on a hybrid built connecting the STNO to a dedicated wideband amplifier, power performance is close but not yet compatible with telecommunicationsstandard requirements. Output power can be enhanced by optimizing the STNO magnetic stack composition and combining tunnel junctions with improved magnetic stack concepts that have so far only been tested for spin valve structures. Using time-domain analysis, the researchers also show that frequency fluctuations are an issue for spectral purity. As a specific characterization method is needed, approaches including time-domain analysis, PLL, and injection lock ring oscillator are currently being investigated. See "A GHz Spintronic-Based RF Oscillator," IEEE Journal Of Solid-State Circuits, January 2010, p. 214.

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