Towards Magneto-Acoustic Oscillators
| dc.contributor.author | Gosavi, Tanay Arun | |
| dc.contributor.chair | Bhave, Sunil A. | |
| dc.contributor.chair | Pollock, Clifford Raymond | |
| dc.contributor.committeeMember | Fuchs, Gregory David | |
| dc.contributor.committeeMember | Ralph, Daniel C | |
| dc.date.accessioned | 2017-04-04T20:27:16Z | |
| dc.date.available | 2019-02-01T07:01:52Z | |
| dc.date.issued | 2017-01-30 | |
| dc.description.abstract | Spin torque oscillators (STO) were developed in 2003 and since then great progress has been made in understanding their underlying physics, frequency of operation, frequency tuning range, and functionality. These nanoscale oscillators with their octave spanning frequency tuning range have been touted for many applications including neuromorphic computing, magnetic field sensing, and microwave signal detection. Since their inception, output power of spin torque oscillators has been improved by five orders of magnitude. In spite of all these developments, the adoption of spin torque oscillators as extremely tunable signal source has been hindered due to their large linewidth and frequency stability issues. Solutions such as using spin torque oscillators in a PLL and injection locking of spin torque oscillators has been demonstrated. However, these solutions need an external signal reference for their implementation. Recently, self-locking of spin torque oscillators to their amplified, delayed output signal has been proposed and initial experiments have demonstrated an encouraging progress in the improvement of the linewidth. Further improving on this concept of self-locking, we propose a system that uses spin torque oscillators in a delay line oscillator with a high quality factor micro-electro-mechanical (MEMS) resonator as the delay element. This spin torque oscillator coupled with a MEMS resonator in a feedback system is presented in this thesis as a magneto-acoustic oscillator. This dissertation presents all the components needed for realizing the magneto-acoustic oscillator as well as addresses the issues and challenges in its implementation. To maintain the tuning capability of magneto-acoustic oscillator we use high-overtone bulk acoustic resonators (HBARs) as the MEMS resonator, as they have numerous resonances spread over large range of frequency. We fabricated and characterized one port, high-overtone bulk acoustic resonators to be used as stress transducers for strain feedback-based magneto-acoustic oscillator. We also developed an analytical model to calculate the stress/strain generated by the HBARs for a given drive voltage. We further performed low temperature characterization of the HBAR resonators, fabricated on different substrate materials, in order to better understand the dominant loss mechanism that limits its performance as a strain generator. Two port, laterally coupled high-overtone bulk acoustic resonators were fabricated and characterized with measurements indicating quality factors upwards of 20,000 in the 2 to 8 GHz regime. Two port HBARs were demonstrated as high quality factor, narrow bandwidth filters to be used for current feedback-based magneto-acoustic oscillators that are operational over a large frequency range with nominal transmission loss of -25 dB. We study using strain as the feedback path for implementing a magneto-acoustic oscillator in a macrospin model of a spin torque oscillator. This model incorporates the effect of a uniaxial external AC strain on the spin torque oscillator by considering, the effective field generated by due to the magnetoelastic property of the free layer magnet. We also calculate the threshold amplitude of strain needed to lock the spin torque oscillator and further calculate the gain required for the HBAR to feedback that strain when implemented as a magneto-acoustic oscillator. Comparing the gain needed for locking in current feedback-based magneto-acoustic oscillator and strain feedback-based spin torque oscillator we find that current feedback based magneto-acoustic oscillator performance is superior. CoFeGe/CoFe and Terfenol-D as the free layer based spin valves fabricated at HGST with 6% MR and SHE-MTJs with 30% TMR were characterized as spin torque oscillators for implementing the current feedback-based magneto-acoustic oscillators. We demonstrated an open loop current feedback-based magneto-acoustic oscillator, which reduced the linewidth of the spin valve oscillators from 35.4 MHz to 175 kHz with ∆f_o/f_o ratio as high as 23,657 was measured for in-plane oscillation mode at 4.14 GHz. This is the highest measured ∆f_o/f_o ratio for any spin valve oscillator in an open loop configuration. ∆f_o/f_o ratio for the SHE-MTJ oscillator was enhanced by 1070 fold resulted in reducing the linewidth of the oscillator from 300 MHz to 280 kHz. This is the smallest reported linewidth for SHE-MTJs. Upon implementing a closed loop current feedback-based magneto-acoustic oscillator, we observed HBAR oscillations. This was due to the poor isolation from wide bandwidth circulators and because of the high gain needed for locking owing to the low output power of the spin torque oscillators. We propose using a microwave feedback in magneto-acoustic oscillator that eliminates the implementation problems of closed loop current feedback-based magneto-acoustic oscillator by changing the domain of feedback signal from electrical to microwave. We simulated different closed loop magneto-acoustic oscillators using strain, current, and microwave feedback in HSPICE to compare their power efficiencies. Simulation results show that the microwave feedback-based magneto-acoustic oscillator is most promising alternative due to the low gain required in its implementation. | |
| dc.identifier.doi | https://doi.org/10.7298/X40K26JJ | |
| dc.identifier.other | Gosavi_cornellgrad_0058F_10093 | |
| dc.identifier.other | http://dissertations.umi.com/cornellgrad:10093 | |
| dc.identifier.other | bibid: 9906007 | |
| dc.identifier.uri | https://hdl.handle.net/1813/47760 | |
| dc.language.iso | en_US | |
| dc.subject | auto-oscillator | |
| dc.subject | magnetic tunnel junctions | |
| dc.subject | self locking | |
| dc.subject | Spin Torque Oscillators | |
| dc.subject | Tunable Oscillators | |
| dc.subject | Electrical engineering | |
| dc.subject | Physics | |
| dc.title | Towards Magneto-Acoustic Oscillators | |
| dc.type | dissertation or thesis | |
| dcterms.license | https://hdl.handle.net/1813/59810 | |
| thesis.degree.discipline | Electrical and Computer Engineering | |
| thesis.degree.grantor | Cornell University | |
| thesis.degree.level | Doctor of Philosophy | |
| thesis.degree.name | Ph. D., Electrical and Computer Engineering |
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