Magneto Acoustic Spin Hall Oscillators

Tanay Arun Gosavi,Sunil Ashok Bhave,Kerem Yunus Camsari,Mustafa Mert Torunbalci

doi:10.1038/s41598-018-19443-6

Tanay Arun Gosavi, Sunil Ashok Bhave + Show 2 more

Open Access

https://doi.org/10.1038/s41598-018-19443-6

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Abstract

This paper introduces a novel oscillator that combines the tunability of spin Hall-driven nano oscillators with the high quality factor (Q) of high overtone bulk acoustic wave resonators (HBAR), integrating both reference and tunable oscillators on the same chip with CMOS. In such magneto acoustic spin Hall (MASH) oscillators, voltage oscillations across the magnetic tunnel junction (MTJ) that arise from a spin-orbit torque (SOT) are shaped by the transmission response of the HBAR that acts as a multiple peak-bandpass filter and a delay element due to its large time constant, providing delayed feedback. The filtered voltage oscillations can be fed back to the MTJ via (a) strain, (b) current, or (c) magnetic field. We develop a SPICE-based circuit model by combining experimentally benchmarked models including the stochastic Landau-Lifshitz-Gilbert (sLLG) equation for magnetization dynamics and the Butterworth Van Dyke (BVD) circuit for the HBAR. Using the self-consistent model, we project up to ~50X enhancement in the oscillator linewidth with Q reaching up to 52825 at 3 GHz, while preserving the tunability by locking the STNO to the nearest high Q peak of the HBAR. We expect that our results will inspire MEMS-based solutions to spintronic devices by combining attractive features of both fields for a variety of applications.

Highlights

Frequency synthesizers are essential building blocks for modern communication systems such as cell phones, radio receivers, TVs, and GPS
Magnetization of the free layer is controlled using an AC stress generated by the high overtone bulk acoustic wave resonators (HBAR) with in the xand zmdaigrnecettioosntrsi,cetfifoencteivffeecmt4a6g–4n9.eItnictfhieeldprbeesceonmceeosf→Haefuf n=iax(i→Halanmmezch+an→Hiceaxtl)szt−re(s→sHadn+d e→Hxtmeercnha)ml mxxag+ne→Hticn.fTiehlde periodic oscillations on the free layer is sensed by the magnetic tunnel junction (MTJ) as an oscillation voltage
HBAR are fabricated on the opposite sides of the same silicon substrate, and coupled to each other with strain due to the magnetostriction effect. (b) Modular modeling of transport, magnetization, and resonator dynamics. (c) Locking range versus amplifier gain for two different substrate thicknesses (100 μm and 200 μm) simulated with an film bulk acoustic wave resonator (FBAR) where thicker substrate requires larger gain since it can generate lower strain. (d) Tunability is achieved with Vtune by locking the spin torque nano oscillators (STNO) to the nearest HBAR peak

Summary

Introduction

Frequency synthesizers are essential building blocks for modern communication systems such as cell phones, radio receivers, TVs, and GPS. MEMS oscillators are promising candidates to replace quartz crystal oscillators in frequency synthesizers since they are CMOS-compatible, can have kHz to GHz operation, have low phase noise and excellent stability[1,2,3,4,5,6,7,8] They suffer from extremely limited tunability, less than 100 ppm at GHz frequencies, severely restricting their use as a single chip oscillator in the communication systems[9]. Each individual component of the model (sLLG, SHE, MTJ, and BVD) is experimentally benchmarked or equivalent to the-state-of the art theoretical prescription[37] Using this model, we compare the proposed MASH oscillators with a free running STNO using identical STNO and HBAR modules. Strain feedback requires an STNO and a 1-port HBAR stress generator on the opposite sides of the substrate whereas current and magnetic field feedback uses an STNO and 2-port HBAR bandpass filter on the same side

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