Spin polarization, when induced in a non-ferromagnetic material, can change the underlying material behavior especially if the spin diffusion length is of the same order as the sample dimension, for example thickness. We demonstrate thermal hysteretic behavior induced by spin polarization in Ni80Fe20 (10 nm)/Au (100 nm) bilayer freestanding sample. The thermal hysteresis behavior is uncovered using magneto thermal characterization based on self-heating 3w (three omega) method. The third harmonic voltage shows diverging behavior and thermal hysteresis during cooling and heating of the sample under an applied magnetic field, which is attributed to the spin accumulation. We demonstrate that the spin accumulation and thermal hysteresis in Au occurs due to ferromagnetic proximity polarization from Ni80Fe20 layer. The observed hysteresis behavior is also attributed to freestanding thin film structure and absence of substrate effects leading to longer spin diffusion length. These results in Au thin films may provide scientific direction to study spin dependent behavior in widely studied diamagnetic and paramagnetic materials. In addition, layered thin film structures of normal metals and ferromagnetic metals can be used to achieve magnetocaloric effect for magnetic refrigeration without using complex and rare earth magnetic materials.

Silicon (Si) is a prominent material for electronics but not a preferred one for spintronics due to its inefficiency in generation and detection of spin current large enough to be utilized for commercial device applications. The culprit is the weak spin-orbit coupling in Si. We present an experimental proof of crystallographic direction dependent spin polarization using strain gradient and spin-phonon coupling, and thermal spin current generation using spin-phonon interactions in Si thin films in spite of it having an insignificant intrinsic spin-orbit coupling. We find that strain gradient acts as an external switch that breaks the symmetry of spin relaxation behavior and modify the spin-phonon interactions. The spin-phonon interactions lead to a phonon-driven redistribution of spin potential and spin current. The phonon-driven spin current gives rise to a phononic spin-Nernst effect in Si due to interlayer coupling with Ni80Fe20 thin film. The coefficient of spin-Nernst magneto-thermopower of the Si (45.4 µV/K) is estimated to be an order of magnitude larger than heavy metals such as tungsten (W). The primary heat carrier in Si are phonons and resulting spin-Nernst effect is expected to arise due to spin dependent electron-phonon scattering, which is also supported by spin-Hall magnetoresistance and Hall resistance measurement. Observation of phononic spin-Nernst effect puts a spotlight back on Si for spintronics and spin caloritronics applications.

In recent years, current data storage and semiconductors have begun to reach their fundamental limits. In current hard disk drive based data storage technology due to the size of the bit no longer being able to be reduced. And in semiconductor technologies due to current leakage effect in small devices. Because of this, we look to explore new approaches to optimize materials used in data storages devices. First a study of Co/Pd multilayers using large current densities to permanently change the coercivity of the media due to electromigration induced surface diffusion of O2 leading to formation of CoO. Next, a study of Co/Pd multilayers using smaller current densities to reversibly change the coercivity of the media due to Rashba spin-orbit torques. Then, an in-situ TEM study of high temperature behaviors in nanocrystalline platinum thin films that paves the way for a standardized fabrication and high temperature TEM experimentation process for any thin film using our device design.

Silicon is the standard electronics semiconductor for its abundance and versatility. However, its insignificant intrinsic spin-orbit coupling leads to weak spin-charge conversion and hinders its application in spin-transport electronics. Spin-orbit coupling can be enhanced by lifting the degenerate spin bands through strain gradient and Rashba effect, and further increased with proximity effect. By creating strain gradient in Si, we broke its centrosymmetric crystal structure and created flexoelectricity and charge separation. The combination of Si inversion asymmetry and electric polarization arose Rashba spin-orbit coupling with estimated spin-Hall angle of 0.1132, same order as Pt. The intrinsic spin-Hall effect in p-Si and n-Si is proven from observation of spin-Hall magnetoresistance, emergent phase transitions and dissipationless spin current. The methods we employed to enhance, characterize and utilize spin-orbit coupling are documented in this thesis.

This thesis begins with reviewing the mechanisms for spin transport manipulation and the current understandings of spin transport in Si. We introduce the effects of strain gradient, Rashba and proximity that can lift the spin degeneracy, and the methods for incorporating the effects into Si with material structure of p-Si/MgO/NiFe/Pd using standard fabrication procedures. We validated our device setup through spin-Hall magnetoresistance measurements, and characterized the type and strength of spin-orbit coupling. Using the enhanced spin-orbit coupling, we measured its influence with magnetoresistance from NiFe, since spin current should influence the magnetization of a magnet in contact. As thermal transport was primarily carried by Si, we measured thermal properties of the device using three-omega method to study the changes in Si with spin polarization. In Si, spins relax through Elliot-Yafet mechanism, and our measured thermal properties reflect the interaction between spins and phonons. Further investigation showed signatures of phase transitions in thermal transport properties. To enhance the spin polarization, we constricted the thickness of Si to its spin diffusion length, and consequently arose metal-to-insulator transition and additional phase transitions. In a device composed of MgO and Si bilayer, we generated and detected dissipationless spin current solely through Si without a ferromagnet. In this study, we demonstrate the functionality of Si as a spin transport material with tunable spin-orbit coupling, advocating the feasibility of Si spintronics.

Magnetothermal Electrical devices are the prominent systems to generate electrical energy from heat source having small temperature gradient. The spin-Seebeck effect mediated thermoelectric energy conversion can provide an efficient alternative to traditional thermoelectric for waste heat recovery. To achieve this goal, efficient spin to charge conversion using earth-abundant materials is essential. Also, the thermal spin current from the spin-Seebeck effect has been reported to be more energy efficient than the electrical spin injection methods. However, spin detection has been the one of the bottlenecks since metals with large spin-orbit coupling is an essential requirement. Silicon is widely used in semiconductor electronics due to its abundance and versatility but having a centrosymmetric crystal structure, has insignificant intrinsic spin-orbit coupling, leading to negligible spin-charge conversion. However, in silicon, strain engineering mediated Rashba spin orbit coupling can induce interfacial spin to charge conversion arises due to an electric potential perpendicular to the interface.

The electric potential can be artificially induced, for example, using ferroelectric and piezoelectric thin films at the interface. An alternate way to induce the electric potential could be flexoelectric field. The flexoelectricity can be observed in all the material that either have or lack inversion symmetry, additionally no large gate bias is needed. Hence, the interfacial asymmetry in conjunction with strain engineering can provide an alternate pathway to achieving efficient and controllable spin-to-charge conversion for spintronics applications. In this experimental study, we report large spin to charge conversion (spin-Hall angle- 0.578) at Ni80Fe20/amorphous-Si interfaces attributed to flexoelectricity mediated Rashba spin-orbit coupling. The flexoelectricity at the interface also gave rise to flexoelectric mediated interlayer coupling. In addition to spin-charge conversion, the strained interfaces also led to almost three-fold increase in anomalous Nernst effect. This strain engineering for spin dependent thermoelectric behavior at room temperature opens a new window to the realization of spintronics and spin-caloritronics devices.

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