Scientists Create Magnetic Oscillator to Replace Crystal Oscillators

Carnegie Melon University (Pittsburgh, PA) scientists Xiaochun Zhu and Jian-Gang Zhu have developed a perpendicular spin-torque-driven magnetic oscillator that could replace crystal oscillators in electronic equipment.

Crystal (e.g., quartz) oscillators are a common source of time and frequency signals for electronic circuitry. Unfortunately, however, the fabrication of crystal oscillators is not usually compatible with solid state circuitry fabrication processes, and in particular CMOS fabrication processes. According to the inventors there exists a need for an oscillator whose fabrication is compatible with solid state circuitry fabrication processes, but which provides the timing performance of crystal oscillators.

The perpendicular spin-torque-driven magnetic oscillator revealed in U.S. Patent 7,616,412 is comprised of a reference stack, an oscillating stack, and an interlayer between the reference stack and the oscillating stack such that the reference stack and the oscillating stack are exchange coupled. The reference stack may have a sufficient perpendicular anisotropy such that the reference stack causes, via the spin momentum transfer effect, the spin polarization of the conducting electrons in the oscillating stack to produce a spin torque on the local magnetization of the oscillating stack. As such, the oscillating stack may produces a sustained gyromagnetic oscillation around the perpendicular axis of the oscillating stack when (i) the oscillating stack and the reference stack have opposite magnetizations and (ii) there is a direct current flowing in the magnetic oscillator from the oscillating stack to the reference stack. Also, the oscillating stack may produce a sustained gyromagnetic oscillation when (i) the oscillating stack and the reference stack are magnetized in the same direction and (ii) there is a direct current flowing in the magnetic oscillator from the reference stack to the oscillating stack.

Advantageously, the gyromagnetic oscillation of the perpendicular, spin-torque-driven magnetic oscillator may have a very high quality factor (Q), such as Q>1000, with oscillations in the microwave frequency range (e.g., 8 to 35 GHz). Also, the magnetic oscillator may be formed using conventional CMOS fabrication techniques, making it easier to integrate with peripheral CMOS circuitry even at nano-scales.

Tthe magnetic oscillator may comprise a sensor for sensing either voltage or current. The sensor stack may comprise a synthetic anti-ferromagnetic (SAF) reference layer and an anti-ferromagnetic (AFM) layer for pinning the SAF layer. Because the magnetic gyromation of the oscillating stack is around the perpendicular axis of the oscillating stack, the magnetization of the SAF reference layer is preferably perpendicular to the perpendicular axis of the oscillating stack (i.e., along the horizontal axis of the SAF reference layer). In addition, the oscillating stack may further comprise a TMR enhancing layer.

In another general use, the perpendicular, spin-torque-driven magnetic oscillator may be used in a magnetic write head for writing to a magnetic recording medium (e.g., a computer disk or hard drive). It may also be part of an integrated read/write head.