Abstract:
Spin torque oscillators (STOs) have gained significant interest in the field of spintronics for their potential application in wireless communication, microwave signal generation, and non-conventional computing, i.e., vowel recognition, pattern recognition, neural computation, etc. Spin Oscillators are promising new devices as they have have smaller footprint, lower energy consumption, and greater frequency tunability in the GHz range compared to the conventional CMOS alternatives. The main component of STOs is the magnetic tunnel junction (MTJ) which consists of two magnetic layers separated by a thin insulating layer. Of the two layers, one has a fixed direction, called pinned layer (PL), and the other is free to change direction under the influence of the spin current or external field, called free layer (FL). The precession of the magnetization of the free layer magnet in the MTJ structure results in a fluctuating resistance enabling the real-ization of a compact practical oscillator. Spin precession occurs when the spin torque developed by the spin current or field offsets the damping torque within the magnet. Generally, STOs require a large current density and an external magnetic field to create the conditions for sustained oscillations. Incorporating external fields in nanodevices is difficult and the requirement for high current density leads to reliability issues and also makes the devices energetically inefficient. In this present study, we propose a field-free spin torque oscillator, which comprises of an MTJ with an in-plane magne-tized free and fixed layer. Our approach involves the application of a current pulse to induce spin oscillation without requiring any external magnetic field. We show that for stable sustained oscillation, the initial large pulse amplitude needs to be 3× the critical current of the free layer magnet, and the small DC amplitude needs to be set at 0.4× the critical current. For typical magnet parameters, and energy barriers rang-ing from 25-80 kBT, the device oscillates in the microwave frequency range (6-9 GHz) with sub-mA current in the prescribed conditions. The frequency of oscillation depends primarily on the amplitude of the small DC current applied and can be made to oscillate from a few GHZ upto a few tens of GHz. The small form factor, energy-efficiency, and CMOS compatibility makes the proposed spin-oscillator device a promising candidate for future technologies.