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Dissertation_Arati2017_FINAL.pdf (4.3 MB)

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Magneto Thermal Coupling in Solid State Transport

Prakash, Arati, Prakash
http://rave.ohiolink.edu/etdc/view?acc_num=osu1514502118670282

Abstract Details

2018, Doctor of Philosophy, Ohio State University, Physics.
This dissertation encapsulates three primary studies in an effort to elucidate, via a series of experiments, a more complete understanding of the fundamental physics of electron spin-heat interactions. In the first section, we present evidence for the role of magnon-to-phonon energy relaxation as a driving force for the spin Seebeck effect (SSE) at short lengthscales, in addition to the accepted magnon spin diffusion process at longer lengthscales. Temperature-dependent spin-Seebeck effect data on Pt|YIG (Y3Fe5O12)|GGG (Gd3Ga5O12) are reported for YIG films of various thicknesses. The effect is reported as a spin-Seebeck resistivity (SSR), the inverse spin-Hall field divided by the heat flux, to circumvent uncertainties about temperature gradients inside the films. Our method for quantifying the SSE is a departure from the conventional units of thermopower, and enables universal interpretation of results from different experimental groups as well as comparisons of efficiency between various (non-thermal) spin injection techniques. Our measurements reveal that the SSR is a non-monotonic function of YIG thickness. A diffusive model for magnon transport demonstrates how these data give evidence for the existence of two distinct length scales which must be considered in thermal spin transport: 1) a spin diffusion length on the order of several microns, and 2) a magnon energy relaxation length on the order of 250 nm, which has been previously referred to in theory, but which we provide the first experimental evidence for here. In the second section, we demonstrate that antiferromagnetic NiO thin films can transmit magnons and give rise to nonzero spin Seebeck effects without a need for high magnetic fields. We report temperature-dependent spin-Seebeck measurements on Pt|YIG bilayers and Pt|NiO|YIG trilayers, NiO is an antiferromagnet at low temperatures. The thickness of the NiO layer is varied from 0 to 10 nm. In the Pt|YIG, the temperature gradient applied to the YIG stimulates dynamic spin injection into the Pt, which generates an inverse spin Hall voltage in the Pt. The presence of a NiO layer dampens the spin injection exponentially with a decay length of 2 ¿ 0.6 nm at 180 K. The decay length increases with temperature and shows a maximum of 5.5 ¿ 0.8 nm at 360 K. The temperature dependence of the amplitude of the spin-Seebeck signal without NiO shows a broad maximum of 6.5 ¿ 0.5 ¿V/K at 20 K. In the presence of NiO, the maximum shifts sharply to higher temperatures, likely correlated to the increase in decay length. This implies that NiO is most transparent to magnon propagation near the paramagnet-antiferromagnet transition. We do not see the enhancement in spin current driven into Pt reported in other papers when 1-2 nm NiO layers are sandwiched between Pt and YIG. In this chapter, additionally, we explore the possibility of novel spin-thermal superfluid systems by a synthesis and characterization study of the insulating antiferromagnet crystal KNiF3. In the third section, we examine substrate-to-film interfacial phonon drag on typical spin Seebeck heterostructures, in particular studying the effect of ferromagnetic magnons on the existing phonon-electron drag dynamics of the heterostructures. To explore this concept, we compare the thermopower of Pt films grown on ferrimagnetic YIG to that grown on paramagnetic GGG. To isolate the hypothetical drag contribution from the magnons in YIG into the adjacent Pt film, we design a thermocouple device using a hybrid sample with half Pt|GGG and half Pt|YIG(250nm)|GGG. With a uniform applied temperature gradient, the Pt acts as a differential thermocouple. The effective voltage at the isothermal ends of the Pt provides a direct measure of the difference in thermopower of the two systems, which we attribute to magnon dynamics in YIG and their interactions at the Pt|YIG interface. We conduct zero-field, longitudinal thermopower measurements and repeat the experiment using metals with low spin Hall angles, Ag and Al, in place of Pt. We find that the phonon drag peak in thermopower is killed in samples where the metallic interface is with YIG. We also investigate magneto-thermopower and YIG film thickness dependence. These measurements confirm our findings that magnons impede the phonon-electron drag interaction at the metallic interface in these heterostructures.
Joseph Heremans (Advisor)
149 p.
Condensed Matter Physics; Materials Science; Mechanical Engineering; Physics

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Citations

  • Prakash, Prakash, A. (2018). Magneto Thermal Coupling in Solid State Transport [Doctoral dissertation, Ohio State University]. OhioLINK Electronic Theses and Dissertations Center. http://rave.ohiolink.edu/etdc/view?acc_num=osu1514502118670282

    APA Style (7th edition)

  • Prakash, Prakash, Arati. Magneto Thermal Coupling in Solid State Transport. 2018. Ohio State University, Doctoral dissertation. OhioLINK Electronic Theses and Dissertations Center, http://rave.ohiolink.edu/etdc/view?acc_num=osu1514502118670282.

    MLA Style (8th edition)

  • Prakash, Prakash, Arati. "Magneto Thermal Coupling in Solid State Transport." Doctoral dissertation, Ohio State University, 2018. http://rave.ohiolink.edu/etdc/view?acc_num=osu1514502118670282

    Chicago Manual of Style (17th edition)

osu1514502118670282
487
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