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University of Minnesota

1. Delles, James. Non-Equilibrium Two-State Switching in Mesoscale, Ferromagnetic Particles.

Degree: PhD, Physics, 2019, University of Minnesota

There has been much theoretical study attempting to expand upon the Arrhenius law, f=fo exp(U/kT), which describes the switching rate in thermally activated, two-state systems, but few experiments to verify it. This is especially true for ferromagnetic particles. Most of the previous experiments performed attempting to study the Arrhenius law focus on the effect the Boltzmann factor, exp(U/kT), has on the switching rate since it dominates any measurement due to its exponential dependence on temperature. This has made it difficult to probe the underlying physics of the prefactor in front of the exponential. Using square, ferromagnetic particles of sizes 250 nm x 250 nm x 10 nm and 210 nm x 210 nm x 10 nm, controlling the barrier height using an applied field, and measuring the average dwell times in each individual state has allowed us to focus on these prefactors. Our measured prefactors vary by twenty five orders of magnitude, and they are smaller than those predicted by previous theories for particles of this size. They become so small as to reach unphysically short timescales. We attribute these unexpectedly small prefactors to our magnetic particles being multidomain and undergoing transitions before the particles have time to reach thermal equilibrium. We show that our particles have a higher probability of transitioning the less time they have been in a state which we attribute to the magnetization spending most of its time near the barrier allowing faster transitions.

Subjects/Keywords: Arrhenius; Ferromagnetism; Magnetodynamics; Magnetostatics; Mesoscale; RTN

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APA · Chicago · MLA · Vancouver · CSE | Export to Zotero / EndNote / Reference Manager

APA (6th Edition):

Delles, J. (2019). Non-Equilibrium Two-State Switching in Mesoscale, Ferromagnetic Particles. (Doctoral Dissertation). University of Minnesota. Retrieved from http://hdl.handle.net/11299/206674

Chicago Manual of Style (16th Edition):

Delles, James. “Non-Equilibrium Two-State Switching in Mesoscale, Ferromagnetic Particles.” 2019. Doctoral Dissertation, University of Minnesota. Accessed September 29, 2020. http://hdl.handle.net/11299/206674.

MLA Handbook (7th Edition):

Delles, James. “Non-Equilibrium Two-State Switching in Mesoscale, Ferromagnetic Particles.” 2019. Web. 29 Sep 2020.

Vancouver:

Delles J. Non-Equilibrium Two-State Switching in Mesoscale, Ferromagnetic Particles. [Internet] [Doctoral dissertation]. University of Minnesota; 2019. [cited 2020 Sep 29]. Available from: http://hdl.handle.net/11299/206674.

Council of Science Editors:

Delles J. Non-Equilibrium Two-State Switching in Mesoscale, Ferromagnetic Particles. [Doctoral Dissertation]. University of Minnesota; 2019. Available from: http://hdl.handle.net/11299/206674

2. Houshang, Afshin. Synchronization Phenomena in Spin Torque and Spin Hall Nano-Oscillators.

Degree: 2017, University of Gothenburg / Göteborgs Universitet

Spin-torque oscillators (STOs) belong to a novel class of spintronic devices and exhibit a broad operating frequency and high modulation rates. STOs take advantage of several physical phenomena such as giant magnetoresistance (GMR), spin Hal effect (SHE), spin-transfer torque (STT), and tunneling magnetoresistance (TMR) to operate. In this work, it has been attempted to understand and study the excited magnetodynamical modes in three different classes of STOs i.e. nanocontact STOs (NCSTOs), spin Hall nano-oscillators (SHNOs), and hybrid magnetic tunnel junctions (MTJs). Synchronization has been considered as a primary vehicle to increase the output power and mode uniformity in NCSTOs and SHNOs. In the quest to achieve high signal quality for applications, a completely new class of devices, hybrid MTJs, has been studied. Therefore this work can be principally divided into three parts: GMR-based NCSTOs: Synchronization has been shown to be mediated by propagating spin waves (SWs). The Oersted magnetic field produced by the current going through the NCs can alter the SW propagating pattern. In this work, the synchronization behavior of multiple NCs has been studied utilizing two different orientations of NCs.The Oersted field landscape is shown to promote or impede SW propagating depending on the device geometry. Synchronization of up to five NCs, a new record, is thus achieved. It is shown that the synchronization is no longer mutual in nature but driven by the NC from which the SWs are emitted. SHNOs: The basic operation and characterization of SHNOs are demonstrated through electrical measurement and confirmed by micromagnetic simulations. Ultra-small constrictions are fabricated and shown to possess ultra-low operating currents and an improved conversion efficiency. High efficiency mutual synchronization of nine SHNOs is demonstrated. Furthermore, by tailoring the connection region, the synchronization range can be extended to 4 ┬Ám. Furthermore, for the first time the synchronization state is directly probed utilizing micro-Brillouin light scattering. Hybrid MTJs: While MTJs based oscillators utilizing a nanopilar geometry have been shown to deliver output powers much greater than GMR-based NCSTOs, they often suffer from higher linewidths. A hybrid device is fabricated to combine the high output power of nanopillar MTJs and low linewidths of NCSTOs. Realization of such devices is demonstrated and, for the first time, their magnetodynamical behavior is meticulously studied. Experimental results show evidence of both localized and propagating SW modes. Generating propagating SWs in these devices paves the way for synchronizing multiple hybrid MTJs sharing the same free layer, thus improving the oscillator performance.

Subjects/Keywords: Spintronics; driven synchronization; mutual synchronization; spin transfer torque; spin torque oscillator; spin Hall oscillator; magnetic tunnel junctions; nanocontact; nanoconstriction; nanopillar; hybrid nanocontact; magnetodynamics; spin wave bullet; propagating spin wave; Brillouin light scattering

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APA · Chicago · MLA · Vancouver · CSE | Export to Zotero / EndNote / Reference Manager

APA (6th Edition):

Houshang, A. (2017). Synchronization Phenomena in Spin Torque and Spin Hall Nano-Oscillators. (Thesis). University of Gothenburg / Göteborgs Universitet. Retrieved from http://hdl.handle.net/2077/52045

Note: this citation may be lacking information needed for this citation format:
Not specified: Masters Thesis or Doctoral Dissertation

Chicago Manual of Style (16th Edition):

Houshang, Afshin. “Synchronization Phenomena in Spin Torque and Spin Hall Nano-Oscillators.” 2017. Thesis, University of Gothenburg / Göteborgs Universitet. Accessed September 29, 2020. http://hdl.handle.net/2077/52045.

Note: this citation may be lacking information needed for this citation format:
Not specified: Masters Thesis or Doctoral Dissertation

MLA Handbook (7th Edition):

Houshang, Afshin. “Synchronization Phenomena in Spin Torque and Spin Hall Nano-Oscillators.” 2017. Web. 29 Sep 2020.

Vancouver:

Houshang A. Synchronization Phenomena in Spin Torque and Spin Hall Nano-Oscillators. [Internet] [Thesis]. University of Gothenburg / Göteborgs Universitet; 2017. [cited 2020 Sep 29]. Available from: http://hdl.handle.net/2077/52045.

Note: this citation may be lacking information needed for this citation format:
Not specified: Masters Thesis or Doctoral Dissertation

Council of Science Editors:

Houshang A. Synchronization Phenomena in Spin Torque and Spin Hall Nano-Oscillators. [Thesis]. University of Gothenburg / Göteborgs Universitet; 2017. Available from: http://hdl.handle.net/2077/52045

Note: this citation may be lacking information needed for this citation format:
Not specified: Masters Thesis or Doctoral Dissertation

.