Successful PhD defense of Amine Wahada
Congratulations to Amine Wahada!
Title: “Ultrafast spin transport through oxide and nitride barriers measured by inverse Spin Hall effect”
A femtosecond laser pulse triggers an ultrafast spin current pulse in a transition metal ferromagnet/ heavy metal bilayer. The heavy metal acts as a spin detector with the ability, owing to the inverse Spin Hall effect, to convert the spin current into a transient charge current. This charge current radiates a THz pulse, making this simple bilayer an effcient spintronic THz emitter. In this thesis, we are able to measure such currents electrically on-chip. We then probe the spin current transport in thin film fabricated heterostructures where a specific layer is inserted between the ferromagnet and the heavy metal.
First, we consider the spin current transport in magnesium oxide(MgO) layer and demonstrate a short spin diffusion length of 2 Å. Surprisingly, we show that heavy metal 5d elements with less than half filled d shell hinder significantly the spin current transmission due to magnetic moment reduction of the transition metal ferromagnetic layer at the interface. Based on theoretical calculations, we prove that this effect is due to 3d-5d orbital hybridization effects and can be eliminated with the insertion of thin oxide MgO or tantalum nitride (TaN). Furthermore, we fabricated a multilayer where MgO is used to decouple multiple spintronic THz emitters. For a speciffc number of repeats, we show that its on-chip performance is almost doubled and significantly exceeds its free space radiation output.
In contrast to the short spin dissipation in MgO, we demonstrate a spin diffusion length of ∼4 nm in antiferromagnetic insulator LaFeO3. We confirm the long distance spin transport in oxides with antiferromagnetic order.