Advanced materials and methods for next generation spintronics
Creators: name: Siegel, Gene Phillip
Publisher: name: University of Utah
Year: 2016
Subjects: spin caloritronics
Spin injection
Spintronics

description: The modern age is filled with ever-advancing electronic devices. The contents of this dissertation continue the desire for faster, smaller, better electronics. Specifically, this dissertation addresses a field known as "spintronics", electronic devices based on an electron's spin, not just its charge. The field of spintronics originated in 1990 when Datta and Das first proposed a "spin transistor" that would function by passing a spin polarized current from a magnetic electrode into a semiconductor channel. The spins in the channel could then be manipulated by applying an electrical voltage across the gate of the device. However, it has since been found that a great amount of scattering occurs at the ferromagnet/semiconductor interface due to the large impedance mismatch that exists between the two materials.  Because of this, there were three updated versions of the spintronic transistor that were proposed to improve spin injection: one that used a ferromagnetic semiconductor electrode, one that added a tunnel barrier between the ferromagnet and semiconductor, and one that utilized a ferromagnetic tunnel barrier which would act like a spin filter. It was next proposed that it may be possible to achieve a "pure spin current", or a spin current with no concurrent electric current (i.e., no net flow of electrons). One such method that was discovered is the spin Seebeck effect, which was discovered in 2008 by Uchida et al., in which a thermal gradient in a magnetic material generates a spin current which can be injected into adjacent material as a pure spin current. The first section of this dissertation addresses this spin Seebeck effect (SSE). The goal was to create such a device that both performs better than previously reported devices and is capable of operating without the aid of an external magnetic field. We were successful in this endeavor. The trick to achieving both of these goals was found to be in the roughness of the magnetic layer. A rougher magnetic layer led to both a larger coercive field and a stronger SSE signal.  The second section of this dissertation is focused on a potential application of the SSE as a nuclear radiation detection tool. Specifically, the ability for a SSE device to detect gamma radiation was tested and found to be very sensitive with a sensitivity of ~20 nanocurie being reported. Furthermore, the SSE device was sensitive to gamma radiation even at room temperature, an advantage over current gamma radiation sensors which require cryogenic temperatures to function.  The third section of this dissertation is focused on further improving future spintronic devices through the use of two-dimensional (2D) materials, as 2D materials have a wide variety of properties which are excellently suited to spintronics applications. Molybdenum disulfide (MoS2), a popular semiconducting 2D material, was synthesized using a pulsed-laser deposition technique. We were able to grow a single monolayer of MoS2 using this technique, one of the first groups in the world to report such a feat.  The works presented herein show the potential of the spin Seebeck effect to function not only as a pure spin current injector, but also as a gamma radiation detector. Furthermore, the research conducted on MoS2 showed the possibility of large-scale, monolayer growth of 2D materials. Subsequent testing on these films has begun on finding the spin transport and detection properties of MoS2.