Semiconductor spintronics aims to integrate memory and logic functions by utilizing the electron's spin degree of freedom in addition to its charge. The building blocks for semiconductor spintronic devices are the ferromagnet (FM)/semiconductor hybrid structures, where the interface plays the key role in spin injection and detection. My graduate research focuses on exploring spin dependent properties of FM/GaAs hybrid structures by ultrafast optics and possible route for tuning of spin polarization through interface modification. In Fe/MgO/GaAs structures, the ultrathin MgO interlayer at the interface causes an unexpected sign reversal in spin dependent reflection, despite being a nonmagnetic insulator. We find that the interfacial bonding with Mg is responsible for the sign reversal. In the Fe3O4/GaAs system, we observed oscillatory spin polarization and magneto-optic Kerr effect with respect to Fe3O4 film thickness, which we attribute to the formation of spin-polarized quantum well states.

Spin-based electronics, or spintronics, seeks to utilize the electron degree of freedom in order to perform logic, computation, or information storage. Proximity based interactions between nearby systems (i.e. films, adsorbates, molecules) and candidate spintronic materials (i.e. GaAs, graphene) could lead to the realization of novel phenomena. Such effects, which rely on atomic orbital overlap, require highly controlled surfaces and interfaces which can be achieved using molecular beam epitaxy (MBE). Here, this dissertation examines the feasibility of integrating high quality single crystal ferromagnetic insulators with oxide interfaces, the semiconductor GaAs, and graphene. Graphene, a single atomic layer of sp² bonded carbon with conducting &pi orbitals that extend out of the plane, is highly surface sensitive and can be considered an ideal material for investigating novel spin-based proximity related behavior. In particular, the interactions of functional oxides or adsorbates with graphene could lead to induced exchange splitting, magnetism, and spin-orbit coupling.

High quality crystalline deposition of the ferromagnetic insulator EuO is investigated with the primary focus of realizing high quality abrupt interfaces between the functional oxide and the spintronic material of choice. In this dissertation, stiochiometric EuO films are investigated on a wide variety of substrates including the spintronic relavent materials GaAs, 2-D planes of TiO₂, and sp² bonded carbon. The integration of EuO on these materials is a key advance towards experimental observation of the exchange proximity effect.

The atomic scale control over deposition provided by MBE allows for the investigation of submonolayer adsorbates (adatoms) and their interactions with graphene. In order to understand the effect of the adsorbates on spin-based properties and phenomena, we have performed systematic in-situ deposition of adatoms onto graphene non-local spin valves. Atomic hydrogen induces magnetic moments in graphene that couple via exchange to the injected spin current. This coupling results in an exchange field which causes the spins to precess rapidly with an effectively enhanced electron g-factor. These results demonstrate the power of molecular beam epitaxy in realizing novel graphene properties and functionality through careful control over the key interfaces and proximity materials.

In this thesis, we look at controlling the magnetic properties at the nano-scale in single crystal thin film through structural modifications and external static electric fields (i.e. magnetoelectric effects). The first part of the thesis focuses on changing the magnetic properties in thin films through sample structure modifications. The other half focuses on using static electric fields to tune the magnetic properties within magnetic thin films. We show that the magnetic anisotropy can be changes through the application of static electric fields. Most interestingly though, we find that the phase transition in magnetite can be tuned with electric fields leading to an enhanced magentoelectric effect.

In this thesis, I summarize our studies investigating the spin dependent properties of graphene over the last five years and the spin transport in germanium in the last three years.

In the field of graphene spintronics, this thesis includes three major advances in these fields. First, room temperature spin transport in single layer graphene (SLG) is achieved using transparent contacts (Co/SLG) and an electron-hole asymmetry of spin transport in SLG is

observed. Second, tunneling spin injection into SLG is achieved using TiO2 seeded MgO barriers. A large non-local magnetoresistance (MR) of 130 ohms is observed at room

temperature. Third, long spin lifetime in SLG and bilayer graphene (BLG) are observed. Furthermore, strongly contrasting behavior for SLG and BLG is observed, in which SLG is dominant by Elliot-Yafet (EY) spin relaxation at low temperatures while In BLG is dominant by

Dyakonov-Perel spin relaxation at low temperatures.

In the field of spin transport in germanium (Ge), this thesis includes growth of single-crystalline, atomically smooth MgO film on Ge(001) by molecular beam epitaxy, the origin of Fermi level pinning in Ge Schottky junctions using epitaxially grown ultrathin MgO films, and

electrical spin injection and transport in germanium using both nonlocal and three terminal spin transport methods.

Graphene has proven to be an interesting and exciting material for experimental and theoretical researcher. The unique properties have motivated proposed applications including high frequency transistors, flexible touch screens, sensors, and spin logic gates, all of which will require integration of graphene with other materials. Additionally, exotic theoretical predictions involving magnetic phenomena in doped and defected graphene provide a new area of physics to explore. A central issue for both fundamental and applied physics involves understanding how charge and spin transport properties are affected by the presence of foreign materials or defects, a topic investigated here by intentionally introducing the graphene surface to transition metals, insulators, water, atomic hydrogen and lattice vacancies.

The first portion of this thesis provides the reader with a general introduction to graphene, the field of spintronics, device fabrication, measurement techniques, and sample characterization methods. Included are basic theoretical concepts of charge and spin transport in graphene. Details of the unique ultra high vacuum system (UHV) that combines in situ variable temperature electrical measurement, molecular beam epitaxy, hydrogen doping, and sputtering capabilities are provided.

The second portion discusses experimental results. Firstly, all transition metals (Ti, Fe, Pt, Au) investigated result in ¬n-type doping for sub monolayer coverage. This behavior provides evidence for the presence of a strong interfacial dipole. Secondly, metallic and insulating dopants are directly compared by in situ oxidation of Ti. Monitoring the charge transport properties throughout oxidation provides evidence of short range scattering due to insulating titanium dioxide impurities. Thirdly, the spatial distribution of impurities is found to strongly affect charge transport. Using a fixed amount of gold impurities, the formation of clusters from point like charged impurities reduces the electronic doping and scattering. Fourthly, in regards to spin transport in graphene, exposure to water is found to result in a substantial enhancement of spin signal, providing a simple method by which researchers can improve graphene spin valve device performance. Finally, spin scattering experiments provide direct evidence for the formation of paramagnetic moments in graphene exposed to atomic hydrogen or lattice vacancies, resolving a long standing controversy in the field.

Van der Waals (vdW) magnets provide an exciting opportunity for exploring two-dimensional (2D) magnetism for next generation scientific and technological advances. While previously realized in 3D ultrathin films where the magnetism is stabilized via substrate-assisted magnetic anisotropy, recent reports have shown intrinsic ferromagnetism at low temperatures (< 60 K) in isolated µm-sized flakes mechanically exfoliated from a bulk single-crystal down to a single-atomic layer. This opens up the possibility to truly study magnetism in free-standing 2D layers without direct effects from the underlying substrate and being intrinsically susceptible to surface effects such as atomic adsorbates, electrostatic gating, and proximity-induced phenomena. This dissertation examines the molecular beam epitaxy (MBE) growth and characterization of new 2D magnets, monolayers of MnSe2 and VSe2, that show ferromagnetic ordering above room temperature. Growth on different substrates and varying the substrate temperature during growth affects the growth mode and morphology of the deposited 2D magnet and also affects the measured magnetization. Direct atomic and magnetic imaging via scanning transmission electron microscopy (STEM) and scanning tunneling microscopy (STM) show stable 2D magnetic layers.

Due to the lack of dangling bonds at the surface of these 2D magnets, applying external epitaxial strain is a challenge. Later in this dissertation, the magnetic, electronic, and structural properties of vdW-layered, Fe-deficient Fe3−xGeTe2 are systematically investigated by

he application of high pressure. Fe3GeTe2 is of particular interest due to its high Curie temperature, Tc, strong perpendicular magnetic anisotropy, and tunable magnetic properties depending on the concentration of Fe and its thickness. Electrical and magneto-transport measurements show a suppression in Tc with an increasing pressure up to 20 GPa. The decrease in Tc is due to the lattice shrinkage from pressurization which leads to a weakening of the exchange interaction. These observations showcase the tunability in vdW magnets via pressure which can complement other external stimuli such as chemical doping, making them candidates for future spintronic, electronic and photonic devices.