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Frequency shift keying in a MTJ-based Spin Torque Nano-Oscillator

06/02/2018 - Salle S208


Spintec, Grenoble

Spin Torque Nano-Oscillators (STNOs) are a novel type of Radio Frequency (RF) oscillators that make use of the Spin Transfer Torque (STT) effect in a magnetic tunnel junction (MTJ) device to produce high-frequency auto-oscillations. STNOs provide compact solutions for wireless communication due to their large frequency tuning range via the DC current. This frequency tuning permits to encode the information via frequency shift keying (FSK) by digital modulation of the current between two discrete values without the need of an external RF mixer, leading to potentially less complex RF components. In this work, the feasibility of the FSK scheme by current modulation has been studied for an in-plane magnetized MTJ STNO. The parameters addressed are the achievable frequency shift and the maximum modulation rate, up to which the frequency can be shifted between two discrete values. To characterize these parameters, macrospin simulation and experimental studies have been performed.

The simulations reveal that the maximum data rate for FSK by current is limited by the relaxation frequency fp of the STNO, which is on the order of a few hundred MHz for standard in-plane magnetized STNOs. This means that the data rate is limited to a few hundred Mbps, which is targeted here for moderate data rate wireless communication (10-100Mbps) as used in wireless sensor networks. Experimental studies shows a frequency shift around 200MHz (the frequency shift between ≈8.9 GHz and ≈9.1 GHz) at the modulation rate of 10Mbps. This modulation rate is less than the upper limit given by the relaxation frequency fp of the STNO as predicted in the numerical simulation due to the relatively high phase noise of the device measured. Therefore, further improvements in materials and nanofabrication of STNOs are required to enhance the output power and improve the spectral characteristics of the oscillations to push the data rates to higher values with large frequency shift.

Keywords : spin transfer nano-oscillators, magnetic tunnel junction, relaxation frequency, frequency shift keying, data rate.