Date of Award
Barry L. Zink, Ph.D.
Nonlocal spin valves, Spincaloritronics, Spintronics
The study of non-local spin valves (NLSVs) has recently proven to be a fertile area for both applied and fundamental research in nanomagnetism due to the unique ability to separate charge currents and spin currents. NLSVs may also prove essential for a new class of high-density hard disk read heads due to their favorable scalability. Recent studies have shown thermal effects created by high current densities play a significant role in the response of NLSVs. These thermal effects also provide the opportunity to create a pure spin current from thermal gradients via a mechanism call the spin dependent Seebeck effect (SDSE). Due to the challenges in control and measurement of thermal gradients in nanoscale structures, both the fundamental physics and materials dependencies of thermally-driven spin transport in nanoscale structures remains largely unexplored.
In the dissertation I present measurements of thermal and electrical spin injection in nanoscale metallic non-local spin valve (NLSV) structures. Informed by measurements of the Seebeck coefficient and thermal conductivity of representative films made using a micromachined Si-N thermal isolation platform, we use simple analytical and finite element thermal models to determine limits on the thermal gradient driving thermal spin injection and calculate the spin-dependent Seebeck coefficient that is comparable in terms of the fraction of the absolute Seebeck coefficient to previous results, despite dramatically smaller electrical spin injection signals. Since the small electrical spin signals are likely caused by interfacial effects, we conclude that thermal spin injection is less sensitive to the FM/NM interface, and possibly benefits from the presence of oxidized ferromagnet, which further stimulates interest in thermal spin injection for applications in sensors and pure spin current sources. To investigate contact resistance further we also present work comparing NLSVs with permalloy oxide contacts and devices with an alumina capping layer to prevent to formation of the magnetic oxide. The resulting devices show reduced thermal spin injection compared to initial results but overall increase electrical injection in both cases. Notably, the alumina capped devices present greater electrical injection spin resistance but lower thermal injection spin signal than the magnetic oxide devices. Performing measurements from 78 K to 300 K show an overall decrease in spin resistance signals in both injection configurations as device operation approaches room temperature. Along with reduced spin resistance a parasitic signal appears that we attribute to the Anomalous Nernst Effect (ANE), the thermoelectric analogue of the anomalous Hall effect. This ANE creates a voltage in the detection ferromagnet from a thermal gradient produced by the driving current in the injection ferromagnet. We also describe measurements that demonstrate and quantify both thermoelectric effects on electrical spin injection and purely thermal spin injection, as well as the ANE in NLSVs. Since the ANE is a result of thermal gradients only on detector ferromagnet the spin resistance signal can be enhancement or hindered depending on the device geometry.
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Alex Hojem Hojem
Received from ProQuest
Hojem, Alex, "Thermal Effects on Spin Currents in Non-Local Metallic Spin Valves" (2016). Electronic Theses and Dissertations. 1198.
Condensed Matter Physics