After the first-time successful exfoliation of graphene in 2004 many more 2D materials have been studied for various applications, including spintronics, a field that exploits the spin degree of freedom of electrons as opposed to the charge in electronics. The cornerstone of fundamental spintronics is the spin current-charge current interconversion phenomena, shortly known as spin-charge conversion (SCC). 2D materials are characterized by weak van der Waals (vdW) interaction between the layers, thus, relaxing the lattice-matching requirement for the epitaxy, enabling the growth of complex vdW heterostructures. This can also offer new growth platforms not easily accessible by conventional 3D materials, and, due to the weak nature of the vdW forces, grown films can be transferred onto another substrate. Moreover, 2D materials show thickness dependent band structure and various heterostructures can be formed, opening a vast number of possibly new physics for spintronic applications that can be explored. However, most of the current research is based on exfoliated flakes that are at most tens of μm in size, limiting their possible implementation for applications. In this thesis, I present large area growth of high quality 2D materials and vdW heterostructures by molecular beam epitaxy (MBE) and study SCC effects by spintronic THz emission probed by THz time domain spectroscopy. First, CoFeB/PtSe2 heterostructures with varying the thickness of PtSe2 were studied and a transition from the inverse Rashba-Edelstein effect in a few monolayers (ML) to the inverse spin Hall effect in thicker films was observed. This is the first time a material showed such a transition. The second system was PtSe2/MoSe2 bilayer where we observed a hybridized electronic band showing an opposite spin texture to that of PtSe2. By this, we could demonstrate the possibility to reverse the sign of the inverse Rashba-Edelstein effect by inserting a single MoSe2 layer opening a new route to modulate SCC intensity and sign in vdW heterostructures with monolayer control. Those findings push us to explore the world of 2D materials even more by various means, such as electric fields, and bring 2D materials closer to spintronic device applications.

Plus d'information :https://www.spintec.fr/phd-defense-spin-charge-conversion-in-2d-materials-and-van-der-waals-heterostructures/
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