The researchers have developed a small spin torque oscillator with a practical level of output.

(a) Schematic of the high-frequency measuring circuit of the spin torque oscillator; (b) output signal of the oscillator

When constant current is passed through the spin torque oscillator, the center of the magnetic vortex begins to rotate, and an alternating voltage is generated as the electric resistance changes.

The high-frequency output of a spin torque oscillator must be 10 μW for practical use, but has hitherto been on the order of 1 μW. The researchers increased the magnetoresistance ratio to about twice the present level by inserting a crystalline iron-cobalt alloy layer at the interface between the insulator layer and the iron alloy ferromagnetic layer of the oscillator. In addition, they increased the withstand voltage of the oscillator, so that a larger current can be passed through the circuit. Consequently, the output of the oscillator reached 10.1 μW, which is comparable to that of a crystal oscillator.

High-frequency devices, such as next-generation wireless communication devices and vehicle onboard radars, are required to be small and low cost. Existing reference signal sources (crystal oscillators) are large, on the order of 1 mm, and difficult to miniaturize. Spin torque oscillators, which can be miniaturized to approximately 100 nm, have issues to be resolved, including small high-frequency output and low frequency stability.

The researchers will further increase the high-frequency output and frequency stability, to realize a small, low-cost high-frequency oscillator. They will conduct research targeting at an oscillator with a frequency of 2 to 4 GHz, about ten times that of existing oscillators, to increase operating speed.